Vintage Tube Amp Spa
Controlling Un-Regulated DC Voltages one Amp at a time
when the Mains > 120VAC
From my past where I spent years of Bench Time, then learned by making many mistakes
Here you can benefit from my real Tube Amp experiences.
This page is about my journey and how I came to realize that some Classic Tube Amps have a hidden disease
The slow & silent "Power Tube Red Hot-Rod Zone"
I went on many "Deep Dives" on Vintage amps and noticed a pattern with many of the older amps from the early to mid 60's when
you plug them into 123 VAC, they can run quite hotter than necessary.
It does not always happen, but one always has to look for the signs of Amp stress and recognize it.
When I gathered my test sheets/restoration notes where I wrote down the various voltages across the amps
on their respective schematics, I saw that for each one, there was a repeating theme.
my DC Voltmeter was whispering at me, but I was not paying the proper attention.
Neither was I looking at the Power Tubes, in a very dark room when they were operating
when I was new to the Tube Hobby, who knew, I was into Tubes and had no real experience?
After 2 years, 1996-1997 I realized what was happening, as I saw Power Tube Plates Red quite quickly indeed
when I put the light out at night. The dark ember glow of Red Plates.... OMG!
A few Doctors describe their discovered diseases, by their own names.
But I was not going to call this anything with my name, just call it what it is,
Quads of expensive EL34 matched tubes, kept piling-up on me, way too soon
And the Power Tubes would actually make noises as they warmed-up and cooled down
Like small tingling sounds, such as softly tapping on a Wine Glass with a metal object.
if your Home Mains is always 120V or less, say 117-119 disregard this topic altogether
You should always check ANY Power Tubes in a dark room to look for Red Plates.
This applies for Homes that have AC voltages greater than 120VAC most or all of the time.
There is a reason why some Homes will consistently have 123-125 VAC from the Outlets.
The reasons why are the subject of a more interesting but separate subject.
Power Company Sending End & Receiving End Voltages
In electricity, Power is Generated and also Power is Lost.
Like the efficiency of a Car Motor, only a % of the Heat/Power/Torque Generated
goes to the Drivetrain. Same for Electrical Power Distribution.
The higher the sending end Voltage, the less power is lost across the transmission lines.
Simple Plost = I2 x R
As you increase the Voltage, the losses across the transmission line decrease as the
square of the Current. The Line resistances are steady, so by lowering the current you lose less power to heat.
But lowering the current means boosting the voltage to keep the Power constant.
Hence, Tube Amps that use un-regulated power supplies can be, (and I caveat this) affected by the Voltage Boost.
Especially Power Transformers with Primary Voltages in the 115VAC Range, wound to meet a standard
that was once okay, but take that amplifier from the late 50's or mid 60's, plug that into
125VAC and you will measure BOOSTED voltages on the DC and also the AC Filaments.
Hence like High Blood Pressure, it is the silent killer....
Note: The only sounds Power and Rectifier Tubes should produce, are out of your Speakers!
If the tubes make noises when warming-up or cooling down, you probably have an issue with the Amplifier.
Vintage Classic Tube Amplifier
This is the silent Tube Killer, but why Sherman, why do you say this and most Audio Forums disagree?
Why do people come at you with hate and derision when you try to explain Science?
A Wise Man once stated "Ignorance is daring"....
Answer: my Pro-Bono Webpages are my refuge. I am a Science refugee and avoid the world of beliefs and hobby dogma.
I treat all subjects with tons of detail, and mathematical fact. Even at my job I am responsible for minute technical details that can have a catastrophic result if glossed-over.
But again, as I think this is very important to the Vintage Tube Amp Equipment enthusiast (especially on old Tube Power Amps), I will go into electrical jargon, a language that is like learning a new roadmap.
While most of the folks on Audio Forums have great interest, but no idea or 'feel' for what is happening inside their old amps, I hope this will help them understand why their power tubes seem not to last more than a few month's.
So what I am to explain here, relates to all of the precious ST-70's, ST-35's, Mk4's and Mk3 Dynaco's, Scott's, Fisher's, EICOS's, Marantz's, Pioneer's, Sansui's, Kenwood's, Grundig's, McIntosh, Luxman's. etc. Any old 1950's to 1970's Tube Amp may have Hypertension.
When Paleo-Tube units are suddenly transported from 1960 to >> 2021, and operated in "their an all original condition", they are being plugged into an electrical socket that is often (not always) much too high in AC mains voltage.
The old days of no Internet and no Cell Phones, Carbon Paper,Typewriters, Ditto Machines
Those days are long gone.... (RIP), but back is the Inflation of the early 70's, :( bahhhh.
Our power grid today often measures greater than 120 VAC RMS, and this can lead to slowly frying out old Tube Amps, and also over heating power transformers and sending filaments into over-voltage.
If you are one of the lucky who have 115-120V at your home, you can stop reading this, your old Tube Amps will dom just fine.
This syndrome is focused on homes whose electrical mains in the US, territories and possessions never drop below 120VAC and often measure 123-125 VAC, here is where old Tube amps start to sweat.
I think my worst ever case of hypertension was the pair of Stromberg Carlson AR-411, at 121 VAC those amps would immediately boost their DC to almost 430 DC volts and they were not intended for more than 370 volts DC. The Power Transformer primary was wound for 115VAC and the transformer current delivery capacity was close to Imax.
The confusion that sets in when a bottom cover is pulled off and one is just staring at all those parts, who can blame people for not understanding.
Hey even Eddie VanHalen (RIP), knew about Tube amps and Variacs.... Eddie got that Brown-Sound using a VARIAC to lower the AC Voltage on his Tube Amps.
I think that there is no single Hobby that has so many interesting people, yet many totally confused by Electrical Theory and imagining things that do not exist.
People still say, write and believe that there exists 110VAC, this is ancient history folks.
The last time 110VAC was coming out of a US outlet was probably 1950 or earlier.
Not even the Motorcycle and Auto restoration folks, get into so many bizarre and unique conversations on Forums,
A Brake Pad is just that a Brake Pad, a Sway bar is just that a Sway Bar, and those that claim magic are immediately called out on the BS.
But, in the Electrical realm, people come up with some really creative thoughts, not so much Science, but lots of weird misconceptions and ensuing arguments.
You never read about someone saying that Rice Paper Tubing for Brake Lines makes Braking better, you can actually feel the brake pedal more?
And when things really get strange, Room Treatments and all of the things to make a music room sound better with your equipment.
One Shaman was selling Battery Powered Clocks to improve the Sound in a music room (Machina Dynamica fraudsters on Audiogon).
This is just too bizarre for me. Room treatments are absolutely essential, when done right.
Yet this stay-at-home hobby is good for keeping feral Men from Bars and the stray things Wives just hate, maybe that Pot Smoking friend, who always shows up Stoned :)
But you do have people claiming so much stuff on Audio Forums, that is borders on the George Carlin insanity evel.
Insanity mixed with a touch of righteousness, just the right medicine for a soldering session on a kilo buck amp, just to upgrade a few special parts that make absolutely NO SONIC DIFFERENCE and have degraded the value of the Amplifier and make absolutely ZERO sonic improvement.
Replacing old functional Screw Terminal Speaker interfaces for plastic 5-way binding posts, with zero sonic improvement or difference, but the Amplifier just lost value.
I am for Amp Molestation, but not doing stuff that makes no sense. Also people insist on replacing perfectly good RCA Female Jacks (Receptacles), so unless the RCA Receptacle is damaged, rusty or fails to make contact with the RCA Plug Center Pin, leave it alone. Drilling larger holes on Classic Amps for new interfaces is like drilling you 1965 Corvette to accept a fancy new Side View Mirror.
If it is not broken, please leave it alone. The difference between a $1.00 Capacitor and a $100 Capacitor often requires a $5000 amplifier to be able to hear a difference, enough to warrant a change.
I come from the School of, "If it is not broken, don't fix it"
This Sherman learned the hard way, years of Electrical Engineering examinations, literally thousands of hand Calculations and I graduated before students even had Laptops.
Then years on a Workbench at Motorola and GE, soldering, Oscilloscopes, Signal Generators, RF Millivoltmeters, Simpson's, Flukes, Microntas, Network Analyzers, ANRITSU Site Masters....
I used the very first 5-1/4" floppies at Motorola just as I walked out into a Kraftwerk-like World, full of computers and big hair, the 80's was full out. The Music scene was an explosion of modern synth and electronica both European but Global. This seems to coincide with the term World Music.
The Summer of 1986 and the eighties with MTV were on our SHARP Transistor Color TV. People in the 70's even built Color TV's from Heathkits, I found that quite extreme even in the 70's and 80's. even back then I thought that people who wanted to build their own Color TV's were nuts....
So yes, I did code my TI-58C to pass my Chemistry examinations, I put all of the Alkaline and Acid formulas for Ph, kPa, etc. on my TI, worked well that TI, but the HP-41 was a much better Calculator with RPN notation.
So what I am sharing here, I hope does help other Old really the older generation of Tube Amps from the late 50's to early to mid 60's. Usually Tube Amps made in the 80's onwards are just fine. I am referring to older style Fishers, Scotts, Dynacos.... Knight, Olson, McIntosh, etc.
Tube Amps matter, and there are 33+ years of reasons why I think what we do to improve old tube amps is important for the love of using classic electronics in this New Age.
Not just only for safety, but for the sake of Hiogh Fidelity, and here is my story on this silent killer; Vintage Tube Amp Hypertension.
A PRIMER ON RESIDENTIAL POWER & HOW IT RELATES TO VINTAGE TUBE EQUIPMENT
Maybe on a Hot Summer Day, when the Power Grid is totally loaded with thousands HVAC's units cranking out Cold in your neighborhood homes, our AC Power may just dip way down to 115 VAC RMS, maybe less at 117 VAC MAX in an extreme heat case of 100F degree days. That is a great stable power grid.
But I lived on an Island that had a electrical distribution that was old when I was young, now it is older... much older. Fragile is the word, the Grid is so old and over stressed, the investment required to modernize it are quite high. Without revenues, and so on. So its a tough place with 124 VAC on colder days and 122 on warmer days. Sometimes up to 125.6VAC RMS.
So here is that concept needs to be explained on how Voltage rises and falls when a Circuit loads and unloads like a Transformer, on the "Un-Regulated Power Supply", and this fact is the underlying corrective action needed to ease Tube Amp Hypertension Syndrome. Old Tube Power Supplies don't have regulation. So without it, the DC Voltage of the secondary side of the Transformer will rise, accordingly. The problem comes when the Power Transformer is not dimensioned with any "headroom" in the Power Delivery Department. So this means that when the primary goes from the 120 VAC or even 115VAC that seems to still be a standard in the primary windings along with 125VAC, my favorite tap!
Some transformer designs are inherently "passive" regulatiors with little headroom. Once a secondary winding current capacity is at nominal, any other winding that settles at an above nominal current draw. This is the effect of not having sufficient loading on a Power Transformer Secondary. The trick is to wind the Power Transformer with 20% extra capacity and with a dual primary 117 VAC and 125 VAC, but currently what is used is 115/125VAC vs. 117/125VAC designs, with dual taps on the primary to switch voltages. This is the best way to permanently treat the patient. But this is not always practical to substitute the original transformers. Therefore the only and best solution is to set the Primary Voltage with some type of Box.
Well you can make one easily if you are handy with electrical Boxes. You just get a AC Toroid that has at least 800VA of Capacity and has a secondary that the voltage is set to 55-0-55 VAC. This shaves just enough from the highish primary voltages and steps the voltage down, by just a smidge like 5 Volts... and this is perfect, and the Toroid will provide filtering and isolation from the AC Mains.
Some traditional non-toroid are Ferroresonant by design, so Transformers all have a max and min loading specification, based on a primary voltage (look at the Hammond Datasheet for FL = Full Load, NL = No Load) you get to know what the Full load voltage is. If your filaments require 6.3VAC RMS @ 3A, then put a 6.3VAC RMS @ 3A transformer winding (or section), or a discrete stand alone transformer to do the job perfectly, and with better isolation. Individual Filament Transformers also allow for better isolation to the High Voltages.
Most people think that a Transformer magically keeps and maintains the secondary voltage independent of secondary loading, FALSE. A Transformer has a loading curve and from this you know what the AC voltage is if you know the current draw. The more current that that transformer section pulls, or loads, the AC voltage across those windings will start to fall from a NO LOAD to a FULL LOAD. It can look like a horizontal line on an X-Y graph. This is how you control voltages, by not under or over loading the transformer. It is always best to load each section near or at their designed maximum. There is no fear here as when you properly load a power transformer secondary, the AC voltage developed drops down, meaning that the DC Rectified Voltage is higher than normal. If this boost is more than 5% some amps start to run HOT. While other amps seem fine with a 20% boost over the primary AC Transformer Winding. The closer it is wound to 120V the better, all the way down to 125VAC for Cathode Biased Amps that always can use an adkustment when they are played on a mains higher or lower than at your home. I like having a switch to flip between 115/125VAC.
When a circuit loads a transformer secondary, the secondary current rises rise and the secondary voltage drops a bit, that is called regulation, and the range is how it tries to regulate, but is actually a sloped load line. This is different with Regulated Power supplies that have "Active Regulation Elements" after the Rectifier does the job of converting 60/50 Hz AC to pulsating 120/100 Hz DC. The DC Voltage on a regulated supply is ROCK SOLID until the regulator capacity is exploded and the regulator shorts out. Regulators come in 6.3V, 12V, 24V, 48V, 100V, 200V, etc.
The regulated power supply uses external components (regulators) to maintain a steady DC voltage, but this is a different design than used in Vintage Tube Amps, except the Grommes that used a regulated Pentode mode Screen Voltage. It used a Tube Regulator I think with a 6L6 or a Gas Regulator, I can't recall.
What I am describing here, is not an intuitive fact, but so many people are unaware that their 1975 Dynaco ST-70 is suffering from 123-125 Volts on the Primary.
AC Transformers have two important specs out of many;
Full Load and No Load AC voltages.
Old (and still some New) Plate and Filament transformers were designed and wound for an AC primary mains voltage standard of 115VAC or 117VAC.
So when you have one of these, you need to compensate on the secondary of the transformer, to control the rectified DC voltage to the target value. Just as well the ASC Filament voltages can be higher than 10% over nominal, and tubes can wear our at a faster rate when they are powered with jacked-up filament voltage
People, the 120VAC Primary Standard (if there was ever one) wasn't any part of any manufacturer until maybe the mid to late 90's.
And people today still say "110VAC" and "220 VAC".... this is so passť.
Folks, the last time AC voltage measured 110VAC was probably when Frank Sinatra was alive and well in his early career and homes lacked Safety Grounds or Polarization HOT and RETURN orientation on their AC outlets and plugs.
So when you just plug a Vintage Tube Amp Power Transformer Primary to 120VAC - 125 VAC RMS, the secondary voltages are almost always, immediately higher, and sometimes dramatically higher.
The old Power Transformer will heat up, and stress out, and the tubes, to the point where the equipment can run super hot and the plates on the power tubes start to glow red. Sometimes you can feel a slight physical vibration from the stress on the Power Transformer. The filaments can look more like light bulbs than the deep orange glow.
Now there are exceptions, but believe me, 8/10 old amps that have been on my workbench break out into a hard sweat when connected to 120 VAC.
And being the "tube-whisperer" I am, I can see and feel the pain, just by visual and audible inspection.
I can tell when an old amp is struggling, and can hear it in the way it plays music. I will measure the DC voltage on the first power supply capacitor and like a Doctor taking your Blood Pressure, I can tell if there is trouble ahead.
Stressed-out Tube Amps and Preamps lack "air", they can sound mechanical, lifeless, dry and often stiff.
There can also be 120Hz ripple Buzzzzz on the speakers a few decibels higher than you would like as the Power Transformer is delivering more voltage than needed.
I know descriptions of sound are weird, but this is the best I can to convey audio into words.
Basic Transformer Formula #1
'subscripts' [p = primary], [s = secondary]
Vp/Vs = Is/Ip
Vp= Primary Voltage
Vs = Secondary Voltage
Is = Secondary Current
Ip = Primary Current
Here we have the most basic of Transformer formulas, it describes an "inverse" relationship between Voltage and Current.
The primary and the secondary AC voltages through a Power Transformer always have an inverse relationship between their Voltages and Currents.
And in case you did not realize, and many folks do not, Transformers do not work with DC, Transformers work only with AC.
Why, just pick up a book, Google it and start to read. Read about Nikola Tesla, Westinghouse...
When a reporter asked Elon Musk on how He learned about Rocket Science; Elon responded "I read alot of books"
The only slight twist in the application of magnetic inductance and reluctance are power supply DC smoothing Inductors (a.k.a. Chokes).
Inductors pass DC current while "choking" or smoothing out rectifier AC 120Hz ripple. This is the Law of Superposition, one of the most basic concepts in understanding circuits, look it up, it explains lots of things.
So in simple layman's terms, the way that a Transformer operates, it converts smaller voltages into higher voltages, and vice-versa, and the same for currents.
That is why they are called Transformers, they TRANSFORM VOLTAGES AND CURRENTS.
Another very important learning item is to beat the heck out of Voltage and Current theory and really understand each one to the point where you can take a White Board and explain them.
So with Transformers, we have a combination of voltages and currents flowing in the Primary and the Secondary.
The convention is that the Primary is the side that connects to the AC socket, and the Secondary is the side that connects to your Power Supply Rectifier, and Tube Filaments.
This formula, when applied, produces the results that explain how Vintage Tube Amp Hypertension comes into being.
Just do some basic math, enter three out of the four variables and solve for the missing one. Then after writing these values down, increase the primary Voltage from 120VAC to 123.5 VAC and calculate the AC voltage on the High Voltage Winding and also the Filament Windings, bingo, you just have the first concept of how a Transformer "transforms" Voltage and Current.
Now with a Vp of 120VAC, as in a modern Tube Amp Transformer, Secondary HV Voltages are typically higher, per design and the Filaments are orders of magnitude lower.
Just as a practical example "Solid State" Amps go the other way, these amps use Secondary Vs. Voltages in the 20 to 60 Volt range, even as high as 100Volts. Transistors rarely need very high DC voltages across their P-N-P, N-P-N, or N/P doped Source to Drain Silicon of FETS and MOSFETS.
So when we increase primary AC voltage into an un-regulated and rectified AC to DC Power Supply, this creates a permanent "BOOST" and is always measurable on the Secondary, and often, this is not good news for the Amp and the Tubes.
The solution to this problem is simple, but almost none of the classic manufacturers did this simple fix, just wind the Power Transformer primary for the "worst-case" primary voltage of 125 VAC.
This way the equipment can be plugged into any duplex outlet and be happy.
This is how I DESIGN all of my equipment and power supplies. I baseline the operation on the worst case scenario.
Then I compensate when I have a Plate and Filament Transformer that has a 115/117 VAC primary, so that the rest of the circuits can work when slammed with 125 VAC. It can be done, it just takes some extra math and empirical testing to "park" the Un-Regulated Power Supply in the "neutral zone", Spock states: This is logical.
The exception here, is when the Output of the Rectifier is connected to a filtered input of a Regulator Circuit. The AC wall voltage can go up and down quite a bit, while the Regulator Circuit keeps the DC output rock-steady.
BUT 98% of old Tube Amps do not have DC Regulation. In rare cases as in the Grommes and Leslies, the Pentode Screen voltage is regulated through a Gas Regulator Tube, this lowers distortion, and is a cool thing.
Look it up, I think the amp is a Grommes 260 and the Leslies are the 122/147 Models, they use Tube Gas regulators.
Now about wire gauge.
The "gauge" or the wire thickness of Plate and Filament Transformers for Tube Amps, is usually made of Primary wires that are "thinner" and Secondary Filament wires that are "thicker", generally speaking. The High Voltage section is typically thin wire.
Practical example: How do you think Power Companies send massive amounts of MVA Power through those Short, Medium or Long Power Transmission Lines that cross our lands.
The power company generates AC voltage at approx. 12.5 Kilovolts (Nuclear, Coal, Oil, Hydro, etc.) by spinning turbine generators.
Then they rely on huge massive Power Transformers to "Boost" the Sending End Voltage to basically "Lower the Current" across the Transmission System.
Hey our Transformer formula in action again!
This is how you pay for Power, you receive over the wires that are no thicker than your arm, that carry 250 Kilovolts, then get stepped-down to the split-phase 240 Volts AC that then splits out in your ASC breaker panel into two 120VAC rails. This is another great subject to learn about. How to properly understand the role of each wire that connects to your Duplex Outlet. You would be amazed at how many people will take a Vintage Tube amp, damage the unit by installing an IEC socket and then connecting the Safety Ground to the chassis.
Historical note: AC vs. DC was a huge fight between Edison (DC) and Tesla/Westinghouse (AC). Edison preached DC Power Transmission, Tesla was a fan of AC Power Transmission, and we all know who won that battle. The reasons why are key to understanding Electricity and knowing cool things.
Now, we will discuss the Filament windings, then the High Voltage windings (often called Sections).
The typical Tube Amp Power Transformer has a HV Section from 200 to 800 Volts (it can be 2 wire or 3 wire known as Center Tapped) AC, 6.3 Volts AC (also 2 or 3 wires), and 5 Volts AC (same, these can also have a third wire).
The center tap wire on a 6.3 VAC filament section is of great benefit for Tube Amps and Preamps. It allows the 6.3 and or 12.6 VAC Filament Section to be connected to DC Ground. This allows the reduction of Common Mode noise and also, if necessary allows a simple means to "float the filaments" with DC voltage on Totem Pole Tube Gain Circuits. This is yet another little piece of knowledge, remember the Law of Superposition? Hard at work again here. Look it up, it is a cool piece of information.
Manufacturers also make have some different sections if they design for DC on the tube filaments, and this is more common on Tube Preamps, to have a 12.6 Volt AC section, or maybe a 20VAC section (as in the Scott LC-21). These can be rectified and produce DC for the Preamp Tube Filaments, it lowers noise. But... at the price or the polarization of the filaments, causing them to wear out 10% faster. This is another curious aspect of Tube Filaments operating on DC current. Look it up, another small piece of Tube wisdom. A philosopher once said "seek and you shall find", in my Spanish "el que busca, encuentra".
Enough on Filaments, only to close this out by stating that you want to avoid over dimensioning Tube Filament Transformer sections.
If you need 3 amps of AC filament current at 6.3 VAC, get a 3 ampere, 6.3 VAC section. Headroom works against parking your tubes at the proper quiescent AC filament voltage. If you believe that a 6 ampere, 6.3 VAC filament section will work better with a 3 ampere load, think again. This is a very important DIY tip that took me years to learn through the school of hard-knocks!
Now lets talk about the High Voltage part, that is probably where most of the Hypertension issues manifest themselves.
The HV DC current (milliamperes of AC) are produced from across the High Voltage Plate Winding section on the secondary and will almost always have somewhat thin wires, as the gauges required for 250 to 500 Volts AC at milliamps of current are thinner than Filament wires, and usually the case rather than the exception. Unless the Secondary HV winding is providing more than 350 milliamps or greater, unusual for any Tube Amp, save for the Dynaco MK6 with defunct 8417 Power Tubes R.I.P.
Now, we move on to winding ratios and the formula, also important as it allows one to determine an un-labeled Power Transformer winding ratio, and also the impedances of any "mystery" Output transformer, such as a rare Output Transformer with "no labeling" that you are salivating to use on a DIY application. Output Transformers are the most critical component in a Push Pull Transformer Coupled amp, but that is a subject I discuss in other parts of my website.
Basic Transformer Formula #2
Vp/Vs = N1/N2
Where V = Voltage, N = Turns
Note: A "Turn" is counted as one 360 degree winding across a laminated transformer or toroid magnetic core.
We also have an impedance "version" of this type of formula that allows one to use a Sine Wave Signal Generator, plus an Oscilloscope, to empirically work-out the "Primary and Secondary" impedances of Output Transformers when they are not published or labeled. The Power Rating is calculated from the size of the core itself, that is more involved, and I will not go into that interesting tangent.
Basic Transformer Formula #3
But I diverge here, as an Electrical Engineer I have these formulas burned into my mind, after all, 35 years of Engineering does that, it makes us Geeks start from the basics in College, and then, apply the heck out of these in the real world.
The Tube Whisperer believes that any Wise man, armed with the knowledge of Mathematics, can always understand Matter, Space and Time through Science.
Now we discuss the High Voltage secondary Power Transformer section further and how this can get boosted and hurt your Tubes.
In most Tube Amp Power transformers, the secondary HV plate winding voltages are greater than the primary voltage, or equal to (e.g.. maybe for voltage doubler applications?) look this up, there are also ways to triple the voltages. The penalty is that it halves or thirds the current capacity...
Hence and this is all a mathematical thing, when you take a ratio of Primary to Secondary windings and work some numbers, the design center voltages on a Power Transformer will have two values given;
No Load = the resulting AC voltage measured when the transformer section is not connected to anything.
Full Load = the resulting AC voltage measured when the transformer section is delivering the full rated current.
I previously stated that the key to good and safe "Un-regulated" Tube Power Supply design, is to always load Tube Filaments @ no less than 80% of the Full Load Current rating, or ideally, at the published Full Load specification. DIY tip!
This is a Audio Rodent belief that if you use a 6 amp Transformer on a 1 amp Filament Load is better.
Here the unlikely reality that goes against the "tweaker mentality" and gets well meaning DIY enthusiasts into Tube Hypertension, if you paid for 3 amps AC, you need to use the 3 amps AC. More here, is never better for Tube Filaments.
Now this is not the case on the High Voltage section, here we need to consider 'headroom' as part of the normal load and operation, this goes with the Tweaker Mentality, part of the laws of chance and assumptions.
Now loading a HV AC Transformer secondary to 100% will result is a Power Supply that does not sound good, there is no margin, and the DC High Voltage will experience sags. Voltage sagging is something we want to avoid in HiFi Tube Amps, but may be a interesting "tone-affecting" feature in a Tube Guitar/Bass amp.
The HV section that creates the 200 to 525 Volts DC after it is rectified, is usually happy at 65 to 75% of full load. It is never a wise design and load the HV High Voltage Transformer Section at full load, this means Heat, Trouble and Voltage Sag.
Again the High Voltage section requires "headroom" to properly give your amplifier or preamplifier the necessary steady state DC voltages that the tubes require and keep the Power Transformer from overheating. Keeps the Plate Voltages steady from wandering when the tubes crank up Also Sprach Zarathustra!!
Now if you end up with too much DC voltage, you have either "over-dimensioned" the Power Transformer Secondary HV section capacity, or need to drop some of the extra DC volts across a DC Choke, or Power Resistor or both.
If you have to drop more than 30 to 50 Volts, you probably need to re-think your HV design. But I digress, as I am talking about Tube Amp Design here, and I want to explain Tube Amp Hypertension, and this is not really about how to size a Power Transformer. I am working on my first Tube Amp Book, to be published in 2022-2023 to take people who want to get into building amps to the level they need to enjoy good results.
My knowledge is so useful, to be able to follow what happens in Vintage Amps operating at 125 VAC that I included this here as something good to know. When you decide to build DIY, this is all critical information.
So.. about the Filaments again...
The AC voltage to your precious and expensive NOS Telefunken or Bugle Boy Tube Filaments, should settle near the 6.3VAC or 12.6VAC, and stay at or within a +/- 5% value, you are fine, and your $wallet$ is safe.
This is quite straightforward for modern current production units and almost always taken care of by the designer.
But, on a Vintage Power Transformer, it can, when a 115VAC Primary is connected to 120-125VAC significantly boost all of the AC secondary voltages, not only to the tube filaments, but the HV section as well, causing overheating, super hot tubes, filaments that look like small light bulbs, tubes making noises as they heat-up into "over-heat" steady state. These are all the tell-tale signs of discolored artwork lettering on Tubes.
If your tube amp filaments are providing illumination to a dark room, something is off and wrong.
Folks these are filaments, not Light Bulbs, although they are very similar in their fundamental design.
When over-heated, some Power Tube lettering will change color from Red to Orange.
As in JJ Tube lettering, and I love how JJ's sound BTW, they are probably the best value in tubes today.
The other acid test of Vintage Tube Amp Hypertension diagnosis is to allow your Vintage Tube Amplifier or Preamplifier a warm-up time of 15 to 30 minutes.
Then, try to hold the palm of your "bare hand" on the top of the Power Transformer.
If you cannot hold your hand there for at least 10 seconds, without pain, you are killing your tubes and making the tube sellers very, very happy people.
You are probably in that Frequent Buyer Program, funded by Audio Forums and have a great solid "return customer" profile and likely complaining that tubes just don't last.
Not to even mention those tired and worn out Electrolytic Capacitors that you just cannot get yourself to update as you fear that your equipment will lose value, ridiculous indeed.
That expensive $45.00 Metal Can FP type Electrolytic replacement capacitor that you cannot afford, and is sorely needed by that "Un-Molested" equipment.
We now have a safety situation of a 35 to 75 and even 80 year old Power Supply capacitor that is dry of Electrolyte. And this Capacitor is now operating at or above it's maximum DC voltage rating, and drawing DC current, and may blow a fuse, if you are lucky. Any Metal Can cap should never get even the slightest warm. If that Metal can cap is warm, it is leaking and drawing DC from the HV Power supply.
Unbeknownst to most, there is also Vintage Equipment "High Voltage Electrolytic and Coupling Capacitor "current creep" that does melt Power Transformer Secondary HV windings before any fuse will blow.
So make sure that you stop belief systems, and break out a calculator and a notepad. Stop believing those Forum rodents and shy away from EBay sellers who flaunt the "Unmolested equipment" narrative, they are, literally, playing with Fire.
Using "Un-Molested" Vintage Tube gear is a sure way to accomplish and achieve failure, while never, ever enjoying the full sonic potential of a 100% restored Vintage Amplifier or Preamplifier.
Imagine the Car restoration people driving their old cars with the most minimal change, they would be driving an Un-Molested 57' Chevy, happy that the car is all original except for the Battery. Then one day, the brakes fail and they crash. Was the value of this vehicle different when restored? Absolutely.
But the Vintage Tube Amp hobby is full of delusion, and the manifestation of this has a name, and this name is "Un-molested".
Old stuff sounds old, old parts are beyond lifespan, and old solder connections can behave like Low Pass Filters - (LPF), cutting off precious high frequencies, delivering that vintage muddy, old, idiosyncratic tube sound that hundreds of Dynaco PAS users seem to enjoy? I just really don't get this.
I can totally understand paying for a 100% original PAS2/3 Preamplifier to put on a shelf and not use it. To own as an antique, not to be operated. This is where "Un-Molested" makes total sense. In museums and large collections of non-operations electronics. This is a cool thing, just don't dare to plug them in a turn them on.
Every time I see those EBay photos of Vintage equipment, old capacitors, old carbon resistors and the Green Selenium Rectifiers, I just shake my head. I just cannot understand how people will pay $800 for an Un-Molested Preamp when a fully and lovingly restored unit loses all value, no bids!
The bizarre world of belief and perception hard at work for collectors and well meaning but confused buyers.
You cannot even start to believe me until you actually take any 100% stock original amp and test it.
Yes, Test Equipment, that inconvenient part of the perception game, that calls on fools at the worst possible moments when smoke gets in your eyes, tube amp smoke that is, not the song. Although it may happen simultaneously!
Ever wonder how that "Un-Molested" group of Vintage Hi-Fi lovers, may never hear the full potential of their vintage gear, as they lust after that fuzzy, soft, rolled-off sound. They even start to get used to bad sound, and when they hear a restored amp with new tubes, they claim it sounds "strident", "bright", "like Solid State", well this is called High Fidelity.
In the realm of "Un-Molested" Vintage Equipment we have here the host of pre-existing Vintage Tube Amp symptoms:
>Huge channel to channel imbalances, old Balance Controls are pretty much shot, an old $1.00 part holding the music hostage to a belief.
>Really bad Volume controls that are way out, and I mean way off. The resistive carbon wafers often measure 30% different. Just use a dual channel oscilloscope to compare Left/Right Channels, sobering Oscilloscope traces.
>Huge differences in out of tolerance Capacitors and Resistors. The ignored Dynaco PAS Channel Mode and Width Selector Switch. Here we lose 50% of the glory of the PAS2/3. The Carbon Resistors can be tens of Kilo-ohms out, and these resistors are critical in value, and work in tandem and collaboration with the "Un-Obtainable" 750K PAS Balance Control. When these resistors measure differently, the Dynaco will not image and the channel separation is dramatically degraded.
>Then we have unhappy audio session visitors, and people, just too polite to tell these owners that the equipment sounds like shit.
Now there is one "band-aid" solution to all of what I state above, to lay out some hard cash for a high quality 10 ampere VARIAC to operate your pristine old, un-molested Marantz 9's.
If you really believe, that your Vintage Amps sound better left "un-molested", be my guest, and feed it what it needs from a Variac, start at 110 to 115 VAC and enjoy the fuzz and all of the Vintage Amp beliefs and update your Insurance Policy.
I will always issue challenges to the "Un-Molested" cult., always.
Dare them to connect their gear to a Signal Generator and an Oscilloscope, and witness what is really happening to the music.
When audio beliefs do not allow one to touch anything, and keep Selenium rectifiers and Dry/Leaky Power Supply and Coupling capacitors that they hope will "reform", then a VARIAC is the only thing that can save your vintage pieces from reality, and your house from a Fire.
This way, you can safely dial down the AC RMS voltage from your AC wall outlet down to 115VAC or 117VAC and make your vintage amp play, but never safer. Never, ever leave any original un-molested vintage equipment alone, and powered up. Only get up for the Bathroom or another Soda.
Now.. once your equipment gets the magic wand, you will immediately notice that the amp gets better Bass, the noise floor drops down, and the tubes don't make the noises of slow death, the filaments glow with joy and that Tweeter hiss, is now far less.
You will also note a drop in the pesky 120 Hz Power supply buzz, it drops down to where you actually need to put your ear up against the speakers. You will NEVER EVER have a noise free Tube Amp. But you can lower the noise floor to such levels that are imperceptible from your listening position (sofa or favorite chair/beanbag).
But when you don't upgrade your old gear, and connect it to 2021 AC Wall Voltages, your precious Tubes are living with "Vintage Amp Hypertension Denial" their "voltage pressures" are in the Yellow to Red ranges and the current rises, taking out what little life they had to start with.
If you take your RMS AC Voltmeter and carefully measure the AC loaded voltage across the filaments, and expect to read 6.3VAC, you may be shocked (no pun intended) that your meter reads 7.5 to 8.0 Volts AC RMS.
One sure audible sign of overloaded filaments is, that loud musical passages sound overwhelmingly HUGE.
As if someone is turning the Bass control to Max when the music goes to from Mezzo Piano to Forte and back down, this is a non-linearity effect created when there are more free electrons emitted by the filament being sucked up by the plate when the music swings hard.
Filament voltage control is a critical ASPECT of High Fidelity in Tube Audio Circuits. Ignore and just order more tubes... at your own $$.
Also Vintage Amp Hypertension will increase DC voltages to the Power Supply Capacitor sections and these can 20% to 30% higher, ouch.
Just read about that stock ST-70 FP Electrolytic Capacitor "first section" and please understand it.
And never dare to power up a Vintage Amp with No Tubes in the sockets, the DC voltage will be dramatically higher and unless your first capacitor is rated at 600 Volts DC.
Also never dare to feed a Tube Rectifier Power supply with anything less than 60% of the AC Mains Voltage, doing this will cut the life of the rectifier as the 5V Filament is starved and the High Voltage is asking for electrons it does not have available. The silence of the tubes..... the Whisperer listens.
This my readers, this is "Tube Hypertension-101" explained in my best Layman's terms for those who fear Calculators and Formulas, or may have a case of "Vintage Equipment Nervosa".
These and many of the reasons why you keep having to replace tubes every 3 to 6 months are laid bare and raw.
I am a Tube Whisperer, and do hear when any amp loses that "magic" and sounds flat.
When Tubes die out and fade away, or when brand new EL34's immediately get Red Plates right out of the box they came out of, something is off.
This my amigos, is the bane of the flat sound of new tubes in un-molested vintage gear. And when you leave these cults, and become an independent thinker, your High Fidelity experience will reach a new phase, a bright sunny day, better sound and spending more money of source material, better streaming sources and interesting tweaks for your music room.
So what can we do?
Well I am one who just will pull out and replace, if possible, the original old Power Transformer and use a new one that can connect to 125VAC and not even break into a sweat. Or I can try to modify the Power Supply, but with limited success.
If you absolutely prefer to keep the Vintage Power Transformer wound with a 115/117VAC primary, invest in a high quality Variac, or build yourself a Steroidal Balanced Power Transformer Box. If you are on the cheap side, and I have nothing against cheapskates, they also have rights, they can buy a Bucking Power Transformer box on EBay to lower the Primary Voltage down to 117VAC.
Parking Tube Amps and Preamps for optimal operation at 125VAC is my fundamental design criteria, then everything else falls into place!
This is especially true for and of Cathode Biased Tube Amplifiers, as these have no way to adjust the current through the Power Tubes.
So what are the benefits of a serious Vintage Amp upgrade?
>No transformer mechanical buzzing
>None of that "lost magic" as your tubes now Bias-up with no fuss, and play well.
>Extra Cash, to help you score that nice Bordeaux Red for your next music lovers session
>Primary voltage control to 115/117 VAC, saving your tubes, and improving musicality.
After all this is why we love tubes, they are imperfect musical devices, able to make music in so many different ways, never twice the same.
But Tubes do need care and feeding, else you just replace tubes often and complain that Tube amps are too much of a hassle.
Tubes are like Orchids, they don't live long if you don't understand them.
BELOW ARE MY PERSONAL DESIGN GOALS FOR UN-REGULATED TUBE POWER SUPPLIES
The maximum AC Filament Voltages that I recommend should never exceed (+/- 10%), so let's do some math so it becomes clear what I am taking the effort to explain here.
6.3 VAC RMS x 0.1 = 0.63 VAC RMS
+Max Filament Voltage is 6.3 VAC RMS + 0.63 VAC RMS = 6.93 VAC RMS (Maximum AC Steady State)
So... 7.0 Volts AC RMS is the absolute "LIMIT" on any 6.3 VAC Tube Filament, exceed at your peril $$.
-Min 6.3 VAC RMS - 0.63 VAC RMS = 5.67 VAC RMS (Minimum AC Steady State)
At 6.0 Volts AC RMS your tube life will extend by 20%
Less than 6 Volts and your tube will suffer from sleeping sickness (more about this in 1960's Mainframe Computers and Tubes that got sleeping sickness, a problem we don't see since computers went Solid State).
I really suggest and prefer this specification to be no more than (+/- 5%)
6.3 VAC RMS x 0.05 = 0.315 VAC RMS
+Max 6.3 VAC RMS + 0.315 VAC RMS = 6.615 VAC RMS (AC Steady State Filament Voltage)
-Min 6.3 VAC RMS - 0.315 VAC RMS = 5.985 VAC RMS (AC Steady State Filament Voltage)
Perfection for 6.3 Volt Tubes is to operate the filaments between [6.0VAC < Wally-World < 6.6VAC]
So now that we get the mathematical results for the Filaments, let's now examine what Vintage Power Transformer Tube Hypertension does to your Plate Voltage with an actual Case Study.
How this has a dramatic effect on all Tubes, all Power Supply Capacitors and Components, the Cathode Resistors and in general, the well being of any Vintage Tube amplifier or Preamplifier (a.k.a Dynaco ST70, EICO, Scott, Fisher,...... the list is very, very long) is sobering.
Let's make this one easy and free of Math, I can explain the concept better using a real amp.
SHERMANAUDIO CASE STUDY: THE DYNACO ST-35 "MANUFACTURED HYPERTENSION"
The first capacitor Section of the stock Dynaco ST-35 amp has a published specification based on a 120VAC Voltage.
So you may say, what's the big deal, 120VAC, easy.
The most I ever measure at home is 123.5 VAC, but this is where the deception hides, so what's the problem Sherman, explain this to me, are you obsessed?
I pull out of the Dynaco Manual a clip of the ST-35 DC Voltage Chart.
This chart is referenced to 120VAC on the PA774 primary, and if your amp is lucky, and your AC wall voltage is 118VAC or 120VAC and below.
And even at 120VAC your friendly factory vintage Dynaco Power Transformer PA774 is already sweating for one key issue, Dynaco over-dimensioned the HV Secondary, but why?
I really don't know why, but have a strong hunch it has to do with the 7189 Tube. I digress for a moment here about the 7189.
When this Tube was unleashed on the market, it was probably common for people to believe that a 7189 amplifier could use a 6BQ5/EL-84 as a substitute, wrong.
The truth is that an amp designed for 6BQ5/EL-84 can use a 7189, but this is a one way street. If you have an amp designed for 7189, a 6BQ5/EL-84 will perish, and in ways that fried quite a few Output Transformers. The Olson AM-224 is a case study of fried OPT's. I have two of them, one, has fried OPT's, the other actually made it's way to me before someone put EL84's into it.
So the ST-35 has the 7189 thing going and I suppose that original PA774 design was made to take advantage of the 7189 tube, and make the EL84 suffer enough pain to wear them out, but no quite kill them off immediately.
The manual makes NO MENTION of this, and I have un-masked Dynaco here. So all of you ST-35 owners out there, take heed, you have been Dynaco-ed!!
Now, make that PA774 primary voltage 123.5 VAC, a very common AC voltage that we have at homes across our nation. Now remember that transformer formula:
Well on the PA774, we are starting with a AC Secondary of 330-0-330 VAC, and this was a voltage selection intended to push the limits of the Maximum Plate Voltages of a standard EL-84/6BQ5, beyond the max, it was really wound for 7189's. and I will show why.
In reality, the Dynaco people most likely wanted people to enjoy this precious design using 7189 Tubes, that were readily available in 1968, but today if you need a 7189, you have to buy the Russian version 6n14pEB or pay $100 each for real 7189's.
Warning: the 7189A is not a 7189, it depends on how the tube socket was wired. The specs of the tubes are the same, but the pinout is slightly different so check before plugging in. Some manufacturers used the spare pins on a 7189 socket for solder tie points rare but possible.
I highly recommend that any ST-35 owner only use the Russian Tube, real EL84's just burn out too fast in the stock ST-35, but they do sound great as they pass away.
I have a small tube cemetery full of Mullard 5-20/EL34 and ST-35/EL84, and these are like spent Rifle Cartridges, when I was learning to shoot without knowledge back in the year 1996-2005, a time of Tube learning, Red Plates, Motor boating and lots of hard $lessons.
If we examine the ST-35 voltage chart below, and look at the bottom line, these are, from Left to Right the DC Voltages that should be measured with a VTVM (high impedance tube voltmeter), else what your trusty modern VOM will measure will likely be 5 volts lower due to meter circuit loading.
Just look at the Plate Voltage on Pin #7, 370 Volts DC!! Ouch. You need to subtract the Cathode Voltage from the Plate Measured voltage to calculate the actual Plate Voltage. So 370V - 13.5V = 356.5 Volts DC if the Dynaco ST-35 factory plate voltage value. Go to the EL-84 spec sheet, the Max Plate Voltage for an EL84/6BQ5 is 300 Volts. Dynaco is parking the 6BQ5's at 356.5 Volts a whopping 56.5 Volts over the published specifications.
This to me screams 7189 all over the place, and why I recommend using EL84M/7189 or the Russian 7189, makes sense, and holds up to the punishment.
These voltages are proof that the ST-35 was not an EL-84 amp but a 7189 amp with an EL84 disguise.
The (3) three Power Supply Capacitor HV voltages are suspect, and last one, is the Common Cathode Resistor voltage reading on a stock ST-35 FP Capacitor, now unobtainable, 13.5 VDC, more on this here...>>
If you take a stock ST-35, with the Stock or even a New Dynakit PA774, and carefully with one hand in your pocket, and the amp belly-up, with meter ground Lead clipped to DC Ground, tell me what you measure?
My "Cover" ST-35 was measuring +420 VDC on the first PSU Capacitor, yes folks not +380 VDC, so what was happening here?
Well, for one, the secondary windings of the stock PA774 and the new replacement PA774's are wound to 120VAC and a ridiculous 330-0-330 VAC.
I am connected to 123 VAC, sound familiar, measure familiar, Hypertension? Yes.
This is not a conspiracy to sell more tubes and lower distortion, it is just bad Dynaco design, intended to push the bias on the Tubes to achieve an unreasonable Power and Distortion specification, the Solid State tube amp complex.
Dynaco seemed to have been competing with Solid State, in the Tube heyday. So the solution was to crank up the voltages and currents on the tubes, to compete with the "lower transistor distortion specifications", happened in the best of families.
Yes my friends, Hype is not modern, it creeps along the ages.
I am not sure why Dynaco wound the HV secondary up to this extra 30 Volts AC per side, but I have solved this problem. I replaced the PA774 by using a Hammond 272FX with 300-0-300 VAC.
But will the 272FX fit the stock ST-35? Unlikely, Dynaco chose to make this amp small, to give it Tube Amp Curb appeal.
I haven't one on hand to try, heck I may ask Hammond to make one that fits.... (a future Transformer Project in the backburner).
I even asked one of the PA774 sellers, if I could order a PA774 with a 125VAC primary.
The answer was "so you have a voltage problem?"...... (sigh), the answer was No I don't, everyone else does!
All we need is for the folks who so lovingly re-create Dynaco Power Transformers to FIX these problems, not just throw them over the fence just duplicate the original windings.
STOP, please STOP, wind the primaries for 125VAC, save the planet, save the tubes, save the Music.
On a side note, the same issue happens with the Mullard 5-20 Amplifier, the stock Mains Transformer is way over dimensioned in the Mullard Book and a real EL34 Slayer. You can read about this saga of mine in my "Hand Made Amps" section and how I learned about "Tube Amp Hypertension" the hard $$ way.
Refer to ST-35 "Stock" Voltage Chart Below:
So... if the +385VDC is now reading +410-420VDC, the rest of the ST-35 amplifier is also boosted?
ST-35 Published Voltage Table
Hence even the Cathode Resistor DC voltage read 17 Volts on my VOM, and this my fellow Tube lovers is a Vintage Tube Amplifier Hypertension diagnosis.
The Maximum Plate Voltage and Plate dissipation of an EL84/6BQ5 tube is published in the RCA Tube Manual:
Max Plate Voltage = 300 Volts DC
Max Plate Dissipation = 12 Watts
These are the nominal values, and some of the specs can be exceeded, but never the Max Plate Dissipation, never, but if you like Red Plates, be my guest
Pin #7 on the 6BQ5/EL-84 Plate connection and the voltages in this chart above, are referenced to Ground, this makes a difference in the math when you calculate the Plate Dissipation, you have to subtract the Cathode potential voltage from the Ground referenced Plate Voltage, or you need to place the negative of your VOM on the EL-84 Cathode Pin and the Positive lead on the Plate Voltage Pin. This here is a common mistake made by even seasoned Technicians, believe me, I made the same mistakes. Its all part of being honest and willing to grow and accept/admit mistakes.
Those who never admit to making a mistake, live their lives within a mistake.
So we are informed by Dynaco, in DC Voltage this chart, that the actual plate voltage on the EL84/6BQ5 is by inspection:
+370 VDC - +13.5 VDC (Cathode Resistor Voltage Drop) = +356.5 VDC.
Does +356.5 VDC = +300 VDC from the EL84 Max Plate Voltage spec sheet? No.
Hummmm, we have some of that funky vintage "spec game" tube stuff going on here.
The ST-35 Cathode Resistor is 95 Ohms, so we can now calculate from the ST-35 Voltage Chart the Cathode Current from the table.
Ik = 13.5 VDC / 95 Ohms = 142.1 milliamps (this is 4 tubes as the resistor is common to all)
So we divide 142.1 milliamps / 4 Tubes = 35.5 Milliamps per EL-84/6BQ5 Power Tube.
So when we calculate the quiescent Plate Dissipation on the ST-35 Power Tubes:
P = V * I = 356.5 Volts DC * 0.0355 A = 12.65 Watts, another OUCH from the folks at Dynaco, at least they dared to publish these measurements.
Now does 12.65 Watts = 12 Watts? No.
The stock Dynaco ST-35 is a EL84/6BQ5 tube gobbler, and really requires the 7189, so your amp will likely eat up your EL-84's in a matter of weeks or a month.
From the RCA-30 Tube Manual the 7189 Tube specs are:
250Volts AC and 48 milliamps and this equals 12 Watts.
But we don't have 250 Volts in the ST-35 we have 356.5 Volts and 35.3 Milliamps and exceed the Plate Dissipation by 0.65 Watts.
The 6BQ5/EL84 max values are at 300 Volts DC while the 7189 can handle up to 400 Volts DC on the plates, a different tube, way more robust than the EL-84/6BQ5.
This made amps, designed for 7189, literally go into Power Tube runaway current when a lowly innocent EL84/6BQ5 was substituted by your friendly Tube Substitution manual and even the Instruction Manuals!
Sending customers to the Tube Stores on a regular basis if their 7189 amps even survived.
The EL-84 will sound glorious in a 7189 amp, then the EL84's will die or the Output Transformer Primary will melt.
After 10 minutes the EL-84/6BQ5 Plates will glow red, the music will sound flat, and maybe the amp will start to Hum if you are lucky.
The ST-35 amplifier was really designed to work optimally with the 7189 Tube as it came from the factory, and can work with EL84/6BQ5 with the Transformer Primary dialed-down to 115VAC with a Variac.
To park this amp is a "safe zone" you need to calculate the properly corrected DC Plate Voltage.
So if we have 12 Watts Plate dissipation MAX, and we like the music to sound correct, lets park the tubes at 35.5 milliamps.
Therefore the proper maximum Plate Voltage for the EL84 is 12 Watts = Vp * 0.035
so Vp = 12 Watts /0.0355 amps = 338 Volts DC.
Does 356.5 Volts DC = 338 Volts DC? No
Granted, the EL-84 can always take a higher plate voltage than the published specifications just as long as we DO NOT exceed the Plate Power Dissipation of 12 Watts.
ST-35 Case Study Conclusion:
Dynaco execution had Audio performance high on the feature list, but the compromise was the parts vs. cost conundrum.
Dynaco chose to use Diodes on the ST-35, and this was a wimpy decision, including the fact that the original Dynaco Plate and Filament Power Transformers ran HOT back then, and today, you cannot hold your hand on them for more than a few seconds after the pain reaches your brain.
Dynaco kept costs low, they kept prices low, margins high and the results today really require the use of a high quality VARIAC to operate these ST-35 amps with 6BQ5/EL-84's in stock vintage form.
On a separate subject if you compare the prices of these kits nowadays in today's dollars, they were not a bargain in any sense back then. But today most folks haven't moved their price compass, and believe that a $500 component, or a $1000 Tube are exorbitantly expensive. These are actually bargain prices.
But you ask someone today to pay $1000 for a Tube Amp and they would rather pay $1000 for an iPhone or a Macbook Air, so fair enough.
But I do have one thing I must state about Dynaco, they wound among the absolute finest Output Transformers in the market, their frequency response is still breathtaking today and the winding techniques difficult to duplicate. These were at times wound by hand, by people who blurred the boundaries of science, technique and art.
My impression of the stock ST-35 Amplifier was always to be worried Power Supply Diode "Plate Voltage" slam, slowly hitting the ST-35 Tubes with H.V. before they warmed up, creating Plate Electron Stripping. This caused the tubes to die a faster death.
This is probably akin to our "Sugar addicted" society that consumes ridiculous amounts of Carbs, Sugars and Junk Food and complain that they can't lose weight and exercise like mad.
I always thought that a nice 5AR4 Rectifier Tube with the slow turn on, would have been a better long term solution, but the cost...... $$$, I chose the 5AR5 and Hammons 272FX, the ST-35 sounds simply spectacular, relaxed, musical, even bordering on mystical. And this is with the new Z-565 Output Transformers from Dynakit Parts. These are amazing! I would order 20 pairs if I had the funds... to make some ST-35's for sale...
The final piece.... Diode Tube Slam.
Yes Solid State rectification is great, but it is another bandit in the Vintage Amp Hypertension issue another silent "Tube Killer".
So, I decided to pull out the Dynakit PA-774 Power Transformer and sell it as I needed a 5V filament section. The Hammond 272FX was the perfect choice, although I am not sure it will fit a stock ST-35, I sold my ST-35 years ago when I was cash strapped (darn).
What I did next, could go against common convention, but I decided to replace the ST-35 Diodes in the Power Supply with a Tube Rectifier and OMG, wow!
Yes a 5AR4 Tube Rectifier upgrade, slow turn on, wonderfully tight Bass.
A good basic secondary ST-35 Voltage is 300-0-300 VAC with a primary of 125 VAC.
I also use a C354 Choke in place of the 50 Ohm Resistor, why not?
The 272FX 125VAC tap is the worry free selection, and I use a SPDT switch to go between 115 VAC and 125 VASC, and the safer one to choose and park the amp in a comfortable place is the 125 VAC. The amp will work at 115 VAC but only when your Mains is 115-117VAC. At 120 VAC the 125 VAC primary is the best choice.
I always like to park Cathode Resistor (self-bias) about 5% below the schematic or published DC voltages, it is the safe zone.
The 272FX offers this unique 115/125V Primary that is just awesome of Hammond to listen to the DIY Tube community and respond with this option.
I have written many pages on the realities of Un-Regulated Tube Power Supplies, so I will not go into this here, you can find these rants throughout my website.
Here is my Dynaco cover ST-35, and it sounds MARVELOUS.
The plate voltage is 15 volts below the published spec and I am thrilled, the Hammond 272FX does not get HOT like the PA774.
The bias board allows me to park the quiescent current through the EL84's at different levels, and I will be experimenting for the optimal place.
Here is the result, and I am loving this with my Dynaco PAS3x, that I also updated and is sounding fantastic.
UNREGULATED TUBE AMP DC - GOOD TO KNOW STUFF
YOU MAY NOT HAVE ANY ISSUES WITH HIGH MAINS
BUT...... LIKE HIGH BLOOD PRESSURE, HAVE YOU TAKEN SOME READINGS LATELY?
2011 was a year that has me in a life/career transition. Having moved away from the Caribbean in late 2010 after 15 years the roots are still not in place. My shop area is still in a transition and projects have been delayed.
But I can share thought's and ideas......
SUBJECT: VINTAGE AMPLIFIER AC VOLTAGE CORRECTION or ADAPTATION
Voltage correcting transformers are a must to effectively manage the tube life and performance of Vintage Amps.
Not many people understand how un-regulated power supplies work. This is a key understanding in owning Vintage Tube amps.
It is a fact that any odorous and dusty old tube amp will work best when connected to a 115 to 118 VAC mains. Power Transformers of 40-50 years ago were manufactured based on a 117 VAC primary voltage, so the turns ratio was in fact making the correct secondary voltages when connected to 117 VAC.
If you want to compare your voltage readings on that old ST-70, ST-30, A500, A700, Fisher 400 or Marantz 7 you needs to have a Variac and set the primary voltage to the one that is published on the schematic.
The problem we face today is that Primary AC voltages in the range of 115 to 118 VAC are not very common today. Unless one happens to live very far from the power station, in the Styx, you will probably not measure 125 VAC with your VOM, but 120V or 119V.
Now connect that old Dynaco ST-70 to 125 VAC and you will immediately exceed the DC voltage rating of the first section of that Un-Molested stock Metal Can Quad Electrolytic FP capacitor, not to mention AC and DC voltages sometimes 20 to 25 % above nominal, I have seen 30% in some old Stromberg Carlson amps I own.
In order to appreciate what I am saying here, one must understand that Power Companies always prefers to err on the high side when delivering commercial power.
Meaning that less Power Loss = I2 x R, means less power lost on the electrical lines, more profit, less loss as heat when we buy juice from them for our homes.
But ..... today's 125 VAC mains does not work well with most old tube amps, unless some method of voltage control is used, or the design itself provides a 120 VAC tap.
Most vintage tube amp designers happened to make their quiescent points in the upper half of the input AC voltage range, designing some headroom for the rise in AC mains voltages.
But not all did, and we shorten the life of our tubes when we connect old equipment directly to the power grid without measuring with a DC VOM meter.
If you don't believe me, you need to measure the voltages and currents on your vintage tube amp and compare them to the original values shown on the schematic to understand what "High Mains" AC does to your precious tubes.
Also the AC Filament voltages will often rise above the 10% maximum allowed and dramatically shorten tube life. When you operate a 6.3Volt Filament at 7.5Volts, ouch!
Most current production tube amps do not have this problem, as their transformer primaries are wound for 120 VAC and the voltages settle down quite nicely.
But amps built from the late 40's up to the mid 70's have some issues as their Power Transformers primaries were using the 115 VAC standard, obsolete in today's un-regulated power supply designs but still commonly used.
Regulated power supplies correct for this, but 99.99% of Vacuum Tube amps do not implement regulation. Some Tube Preamps do and in fact benefit from this. But regulating 485 VDC is not an economical thing to do, and would probably double the list price of any Tube Power amplifier. Also the complexity and size of a regulated 500 VDC power supply is $$.
High AC Mains will make filaments wear out sooner. You know when this is happening, the tubes act like light bulbs and actually provide illumination is a dark room. When tube filaments are lighting up your room, they are actually screaming for help !
Remember that 6.3V and 12.6 V (DC or AC RMS) are the nominal voltages that the tube filaments require. The specifications will allow 10% over, meaning 6.3 VAC + 0.63 = 6.93 VAC.
But..... operate your tubes at 6.93 VAC and you shorten tube life.
This is a well known and scientifically proven fact that has been common knowledge since the late 50's. Wise designers operate tube filaments at the nominal value or 3% under.
There are a class of tubes that will suffer dramatically from high filament voltages, DHT Triodes.
These tubes have filaments that do not work well with high filament voltages.
The filaments on these tubes will draw proportionally more current as the voltage rises unlike a resistor whose behavior is defined by Ohms Law. In old radio's (I mean 1930's) 2A3 tubes actually had a rheostat in series with the tube filaments that allowed filament voltage adjustment over a specified range to adjust gain.
ONE DIY SOLUTION TO CLASSIC TUBE AMP HYPERTENSION
Build a "home brew" - Balanced Power Toroid - in a box, with a fuse and switch. Use a toroid that has 55-0-55 VAC on the secondary and the common but outdated 115 VAC on the primary. This solution will shave off those extra volts and dramatically extend tube and amp life. Parts Express sells some toroid 800 VA that will work to power a tube amp.
Or you can use a 60-0-60 VAC toroid and 125 VAC primary. In any case the goal is to produce no more than 120 VAC to your amplifier, preferably 117 VAC.
In my modded Transcendent Balanced Power unit I replaced the 60-0-60 VAC Toroid with a 55-0-55 VAC keeping the same 115 VAC primary.
Un-loaded and connected to 125 VAC, the voltage I measure is 121 VAC-122 VAC.
As you load the transformer with equipment, the voltage will drop a few %.
The more you load the transformer secondary, the lower the settled voltage.
For instance, if you have a Dynaco Stereo 70 that will draw about 1.5 amps when powered up, this means about 180 VA of AC Power.
You can use a 500 VA Model so that it will load at less than 50% of it's total capacity, I would go with an 800 VA for extra headroom in case you want to connect a PAS3 Preamp as well.
The trick with voltage correcting transformers is not to load then past 50%.
1000 VA load at 500 VA (4 amps AC RMS continuous)
800 VA load at 400 VA (3 amps AC RMS continuous)
The 500 VA model is far more practical as you can also connect a Tube Preamp.
There are 625 VA, 800 VA and finally the 1000 VA models to choose from.
The benefits of Balanced Power operation are also realized here.
Any vintage tube amp will run cooler with lower AC Mains, the amp will sound better and you will be happier knowing that your vintage amp and precious tubes will last longer.
The side benefits of Balanced Power are lower radiated electrical fields from the AC Secondary cabling and quieter performance.
The link below will provide Toroid's for your project:
Here is the schematic of my "modded" Balanced Power unit, You can build one in any metal box that can be machined for the parts:
Order any one of the Toroid's in the 500VA to 1000VA range, depending on your application and build this into a small project box.
You will need to machine a mounting for a 120 VAC mono receptacle, usually a Greenlee Hole Punch, or your local machine shop can punch a correct diameter hole.
If you want to power a complete vintage tube amp/preamp/etc. system with an 8 Amp fuse, you need to order the 1000 VA Avel Lindberg Model [Y236954].
Maybe you can request that Transcendent Sound sell you the Balanced Power kit without the Toroid and you simply order the model that I show on the schematic. I haven't tried this yet, and my suggestions to Transcendent Sound were not well received. The suggestion was to use a bucking transformer on the primary. I don't see this as a solution but a fix to a problem with their design.
When I built the Balanced Power unit from Transcendent Sound I connected my amps to the unit (I had not even bothered to measure the secondary AC RMS voltage) and noticed that my amps were GLOWING BRIGHT and the plates of the 6550 tubes were cherry red.
I measured the loaded secondary voltage on the stock design and my meter read 131 VAC.... yikes!!!
My plans in 2012 are to build a few "Vintage Tube Amp Savers" in stock for sale with the option to have an Analog or Digital meter on the front panel.
Here is the Balanced Power Unit, built inside of a discarded and gutted MASTR III GE Repeater Power Supply.
The courtesy AC Duplex was just perfect and so were the fuse holders.
Here is the inside of the unit. Avel-Lindberg 800 VA Toroid 115VAC Primary, 55-0-55 VAC Secondary, just perfect for Vintage Amps.
AC Output is around 121 VAC No Load and 115 VAC Full Load, but typically operates at 117 VAC for most of the time when the AC Mains
is around 123 VAC. No bucking transformers, just great innovation here and recycling old equipment.
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