Power transformer effect on tone?

[Vesperado]

What in the construction or property of a transformer contributes to tone in a tube amplifier?

Sag is contributed to current supply of the HT. Voltage affects the performance of the output stage. Bias voltage affects the amount of signal arriving at the anode.

Having tested several power transformers I have observed not a little audible effect on the overall tone. I would like to hear from you all what contributes to why I am hearing tonal variation between them.

I know inductance can choke freqq. and how that a certain wind can produce more inductance over another slightly different wind. I also know variation in laminate material and core size produce various losses.

Basically, I keep hearing that Power Transformers have NO BEARING on tone, but from my own observations something is going on.

So please, enlighten me!

[LOUDthud]

There are several factors that affect overall tone. When a transformer is swapped out, you need to confirm that the output tubes are biased the same and the Voltages are the same throughout the amp. One often overlooked Voltage is the Heater Voltage. Don't believe me ? Try this experiment. Connect the heaters in an amp to an external 6.3VAC transformer and connect a Variac to just that transformer. As you change the Heater Voltage you'll hear the gain of the amp change by a couple of dB.

Another often overlooked factor is the FEEL of the amp and how that affects your playing. To eliminate that factor, use one of those pedals that will record your guitar's output and play it back.

I built a clone of a Trainwreck Express on an old Traynor chassis. The B+ was high at around 440V. As an experiment, I connected the Heater supply to a 6.3V transformer and used two Variacs. There is definetly a sweet spot when the B+ is lowered to just below 400V. You can hear and feel the difference.

[Roe]

There are several factors that affect overall tone. When a transformer is swapped out, you need to confirm that the output tubes are biased the same and the Voltages are the same throughout the amp. One often overlooked Voltage is the Heater Voltage. Don't believe me ? Try this experiment. Connect the heaters in an amp to an external 6.3VAC transformer and connect a Variac to just that transformer. As you change the Heater Voltage you'll hear the gain of the amp change by a couple of dB.

Another often overlooked factor is the FEEL of the amp and how that affects your playing. To eliminate that factor, use one of those pedals that will record your guitar's output and play it back.

I built a clone of a Trainwreck Express on an old Traynor chassis. The B+ was high at around 440V. As an experiment, I connected the Heater supply to a 6.3V transformer and used two Variacs. There is definetly a sweet spot when the B+ is lowered to just below 400V. You can hear and feel the difference.

I built two very similar expresses with different PTs, one has 395v on the plates, another 420+v. the former is sweeter and more compressed and middy, the latter a slightly more brutal and unforgiving.

With higher voltages you normally get a quicker response, less compression, later distortion and more brightness

[Stevem]

As the current Output of any given PT starts to reach its limit then the tone of the amp will start to change.

The point where the power supply voltage starts to sag is where you have maxed out the current available from the PT.

This is also the point where the filament voltage will sag with the resulting lowering of the gain out of each stage.

Depending on what you want out of the amp this change will be good to your ears and to others it will not.

There’s also a big change in play feel at this point, but your just asking about tone.

[R.G.]

What in the construction or property of a transformer contributes to tone in a tube amplifier?

You might like to read a series I started on the basics of transformers, here:
https://ampgarage.com/forum/viewtopic.p ... cs#p467963
Transformers can be modeled very accurately with the idea of an ideal, perfect transformer hidden inside the real-world transformer and various imperfection components added in series or parallel with the primaries and secondaries.

You are getting good comments from the other posters about things that can cause changes in sound when the transformer voltage changes, for both the high voltage supply and the heater voltage.

Sag is contributed to current supply of the HT. Voltage affects the performance of the output stage.

More accurately, sag results when the amount of current pulled from the PT->rectifiers->filter capacitor changes the plate supply voltage. The PT and rectifiers both have what amounts to an internal series resistance that causes the voltage on the first filter cap to sag lower by limiting how much they recharge the first cap when the rectifiers turn on. The first filter cap value has an effect, because for a given capacitor size, the average high voltage sags because of increased ripple voltage. You can "fix" that by using a bigger filter cap, if your vacuum rectifier can stand the current pulses. All of these affect how much the plate voltage sags, and that as noted causes audible changes in the tone.

Bias voltage affects the amount of signal arriving at the anode.

That's not how I'd state that. Bias voltage affects the overlap in conduction of the two output polarities. It can change crossover distortion products, which CAN be heard. I don't know how the amount of signal arriving at the anode (presumably of the output tubes?) would be affected by that.

Having tested several power transformers I have observed not a little audible effect on the overall tone. I would like to hear from you all what contributes to why I am hearing tonal variation between them.

To play just a little dumb, everything contributes to this. plate voltage, filter capacitor size, transformer internal resistance, rectifier resistance, bias voltage change as the PT is more heavily loaded, and many more things. It's really a little too simplistic to blame all of these on the PT.

I know inductance can choke freqq. and how that a certain wind can produce more inductance over another slightly different wind. I also know variation in laminate material and core size produce various losses.

The PT's leakage inductances cause a changes in the sag in the plate voltage on the first filter cap. It's a time-domain effect caused by the rectifiers turning on a bit earlier and off a bit later on each half cycle. You get a lower average plate supply voltage, but with less ripple on it. Yes, special winding techniques do make for different leakage inductances. But in today's world, no PTs are wound with the special lower leakage techniques, ever, as a result of the economics. Economics also dictates that very seldom is an odd core material or bigger core size used. Sure, they could theoretically be used, but there's almost no difference in ordinary transformer iron, and hasn't been for decades. Core size is picked to allow the absolute minimum amount of iron and copper to be used to ---just barely--- make the desired voltage and current. There are variations that are theoretically possible, but the iron hand of economics has squeezed out all the fancy stuff. It's one aspect of MBA Disease's race to the bottom.

Basically, I keep hearing that Power Transformers have NO BEARING on tone, but from my own observations something is going on.

You're right, something is going on. But it's not as simple as one transformer versus another. The devil is very much in the details, and things other than "tone" dictate what is available to you as alternate transformers..

[LOUDthud]

Economics also dictates that very seldom is an odd core material or bigger core size used. Sure, they could theoretically be used, but there's almost no difference in ordinary transformer iron, and hasn't been for decades.

Don't 50Hz transformers have a slightly bigger core ? In the USA, the vast majority of Fender amps have only a 120V primary. Can we assume they are intended for 60Hz only ? How much of a temperature change would you expect going from 60Hz to 50Hz ?

Famously, Neal Young was hearing differences in his amps when they were operated on 50Hz while on tour. Was it just the ripple frequency coming through when the amps were clipping ? I think he changed over to using something like a UPS so the amps would be operated at 60Hz no matter where he played.

[nuke]

Tone is entirely subjective.

However, there are good and less good power transformers. I have a couple of New Sensor Corp Fender replacement transformers, described as 50-watt bassman, tweed bassman, super reverb, etc direct replacements.

However, the core stack is a lot shorter, fits the hole right and has the right number of wires. It is missing the electrostatic shield. (the copper tape strap around the core). They also seem to "float" more, meaning less regulated. Unloaded, they put out more voltage than the originals and loaded, they put out a bit less. My theory is they save a bit on skinny cores and use thinner wire and an extra wind or two. (RG knows a ton more about transformers than I do).

They are "good enough" as repair parts as they'll make it go again. Not ideal though.

[R.G.]

Don't 50Hz transformers have a slightly bigger core ? In the USA, the vast majority of Fender amps have only a 120V primary. Can we assume they are intended for 60Hz only ? How much of a temperature change would you expect going from 60Hz to 50Hz ?

50hz cores are bigger, and it's a direct result of the lower frequency. For a given core, the ability to stay out of saturation means that the voltage and frequency are inversely proportional. For a 60Hz core that's running between positive and negative saturation by a given voltage will begin to saturate if you feed it the same voltage at 50Hz. This is all pretty linear/proportional, so the same core that's fine at, for instance 120Vac 60Hz will be driven to the same extent of saturation with a 50Hz voltage of 120 * 50/60 = 100Vac.

Transformer designers will design a 50/60Hz rated PT for 50Hz 120Vac. This will work fine at 60Hz 120Vac, with perhaps a hair lower loss by staying a little further from saturation and having more area to convect/radiate heat out of. In the USA market, my guess is that they are all designed for 60Hz, the economics demon squeezing them into not using one penny more iron or copper. But if you had to do a 50/60Hz transformer, you design it for the lower frequency.

The real criteria is how much you let your iron saturate. The common transformer irons saturate softly, so there is some play in how far you can let them go into saturation. It then becomes a heat transfer design question, and that's probably settled with a number of prototypes and duels at sunrise between the engineers and accountants.

I personally would never run a 60Hz transformer on 50Hz. It might survive fine, might have different voltages (saturation means not all the turns-ratio voltage gets from primary to secondary) and would certainly have more primary current and heating. I'd say that all USA Fenders are designed for 60Hz.

Famously, Neal Young was hearing differences in his amps when they were operated on 50Hz while on tour. Was it just the ripple frequency coming through when the amps were clipping ? I think he changed over to using something like a UPS so the amps would be operated at 60Hz no matter where he played.

I think what he was hearing was real. His amps were/are probably USA or Canada models, and likely designed for 60Hz. Running them on 50Hz, even with a step down transformer would make the plate voltages a bit lower, they'd run a bit hotter, and any ripple leaking in would be 100Hz, not 120, and that would sound noticeably different. It would probably have been cheaper to trade out the PTs for 50Hz rated ones.

[Roe]

As the current Output of any given PT starts to reach its limit then the tone of the amp will start to change.

The point where the power supply voltage starts to sag is where you have maxed out the current available from the PT.

This is also the point where the filament voltage will sag with the resulting lowering of the gain out of each stage.
...
There’s also a big change in play feel at this point, but your just asking about tone.

+1. Often the power supply affects the tactile feel and response more than the tone directly.

[nuke]

I keep getting whacked by the "too many request" problem on the board.

The "less good" power transformers I've got in my pile of unused parts are notably smaller cores and lack a flux-band and probably the electrostatic shield. They also seem to "regulate" more poorly than the heavier part numbers they replace, meaning the voltage is higher unloaded and drops more as load is imposed. Clearly, they're cost cutters.

My suspicion is that they skimped on the iron and put a few extra turns of skinnier wiring on the secondaries to shave a few pennies on the per-unit costs. I think that way, they could be something passable to replace the original part number at a lower cost.

But I think the less steady voltage, the lack of shielding and a flux band, would affect tone with the modulation effects of less stable voltage, and the lack of e-shield allowing more line-noise in and the lack of a flux-band allows more radiated hum into the amp.

[B Ingram]

Sag is contributed to current supply of the HT. Voltage affects the performance of the output stage. ...

Sag is contributed to current supply of the HT. Voltage affects the performance of the output stage.

More accurately, sag results when the amount of current pulled from the PT->rectifiers->filter capacitor changes the plate supply voltage. The PT and rectifiers both have what amounts to an internal series resistance that causes the voltage on the first filter cap to sag lower by limiting how much they recharge the first cap when the rectifiers turn on. The first filter cap value has an effect, because for a given capacitor size, the average high voltage sags because of increased ripple voltage. ...

I owned some vintage Fender amps, and some vintage Vox amps.

I measured the winding resistance of the Power Transformers of both brands, and found they are typically quite a bit higher than PTs offered as "vintage replacements" today. The higher winding resistance lowers the DC Volts output, and contributes somewhat to "more sag" when pushed.

I suspect the manufacturers of the "modern replacement PT" believe they're doing the customer a favor by winding with fatter wire & lower-resistance to reduce self-heating of their power transformers. But they're making the PT "not the same" as the original PT.

-----------------------
Separate, but Related:

I find builders of all stripes complaining their B+ voltage is higher than they expected. they don't realize their PT has lower winding resistance.

BUT! They also don't realize that in the span of 3 years, Fender may have used 3 different PTs, with 3 different B+ voltages. And none of those voltages are accurately represented on the schematic! Mainly because Fender (at least) didn't strive for slavish accuracy, as much as "all the right parts shown in all the right places" (though some technically-knowledgeable collectors of 1950s Fender amps prove the factory sometimes sent out amps whose circuits did not exactly match any available schematic...)

[R.G.]

I owned some vintage Fender amps, and some vintage Vox amps.
I measured the winding resistance of the Power Transformers of both brands, and found they are typically quite a bit higher than PTs offered as "vintage replacements" today. The higher winding resistance lowers the DC Volts output, and contributes somewhat to "more sag" when pushed.

I suspect the manufacturers of the "modern replacement PT" believe they're doing the customer a favor by winding with fatter wire & lower-resistance to reduce self-heating of their power transformers. But they're making the PT "not the same" as the original PT.

Fortunately, this is easy to fix/adjust if the replacement transformer has a lower equivalent series resistance - just put resistors in series. But you need to know what the original resistances were to do this, or accept that adjusting things to get a tone is part of the process of getting "that tone", whatever you believe "that tone" to be.

Issues like this make chasing "original" very difficult for people who don't have the technical skills to rummage around inside an amp safely, and the education/training to know what to measure and do. And it helps perpetuate the mythos of magic parts. As Stevie Wonder sang, “When you believe in things that you don't understand, then you suffer.”

And as you note:

BUT! They also don't realize that in the span of 3 years, Fender may have used 3 different PTs, with 3 different B+ voltages. And none of those voltages are accurately represented on the schematic! Mainly because Fender (at least) didn't strive for slavish accuracy, as much as "all the right parts shown in all the right places" (though some technically-knowledgeable collectors of 1950s Fender amps prove the factory sometimes sent out amps whose circuits did not exactly match any available schematic...)

This happens a lot. An amp manufacturer will source parts that they can get to complete amps, and will spend a considerable effort to get the least expensive part to make the amps work, at least if they're a volume manufacturer. Hand- and boutique-builders can avoid this if they have a big enough profit margin and time available to get the next few amps out the door.
Also, before Fender had acquired their mojo and become vintage and original, there was only the need to get amps made, tested and shipped. Ditto Marshall, ditto Vox. There was no "original" to be. These are the amps that our myths and legends have deemed to be vintage magic - even if Fender, Marshall, Vox, etc, etc, happily changed suppliers for 10% lower cost per transformer. Or, they would happily have changed to a batch of 10% more expensive PTs that could arrive by next week to keep the line running until the prior version could get there in a month and run the costs back down.

So yes, you're right - all vintage [whatevers] are not the same, and even vintage {whatevers} from the same year, etc. may not be the same. A run of 1965 (for instance) amps may have been one set of parts of a given batch right up until a Thursday in June when they ran out of X suppliers OTs and had to break into the crates of OTs from supplier Y.

People in general do not understand the competing forces in manufacturing. It's true that they don't make them like they used to; and maybe they never did.

[B Ingram]

I agree entirely!

On another forum, someone was very concerned that their 1962 Fender amp didn't exactly match the voltage numbers on the schematic. They wanted to modify their amp to "get the voltages right."

I tried to be as persuasive as possible to tell them, "Your amp is right, the schematic is wrong. And there were other transformers used, so there are other 'right answers' that also don't appear on the schematic."

As you say, "chasing original" is a tricky game (and may not even be worthwhile in some cases).

[martin manning]

It's true that they don't make them like they used to; and maybe they never did.

Totally stealing this.

[nuke]

On "getting the power numbers right" my go-to test is to put a vintage amp on a variac, and dial the line voltage until the filaments under load (amp in operating condition) hit the exact filament voltage. (ie, 6.3VAC RMS). Then measure the input voltage presented to mains. That will usually tell you what the original power transformer was wound to expect.

I have found that generally, transformer makers back in the old days, tried to hit filament voltages correctly.

My original tweed Bassman manufactured in 1960, with the original transformers, dials in at 111VAC. I run that amp on a line-voltage bucker I built to keep it relatively sane. Right into the wall at today's 122-125 vac, pushes the B+ way on up there.

The Magnatone 260A I recently restored (date code 1959) dials in at 118vac, and it seems pretty safe.

My 1979 Deluxe reverb says 117 vac on it and dials in at 119vac. It also has a 5U4GB which keeps the B+ a little tamer.

When Fender switched to Schumaker transformers, they tend to be very consistent in voltages.

But what's there is what's there and when in doubt, I've found dialing in the filament supply tells you what all the other windings were actually wound for.

Marshall and Vox and other things that natively ran on European line voltages, seems all over the place. Just never know what they'll be.