Characteristics of Guitar Amplifier Output Transformers, and criteria for design

[cerrem]

I was also amazed when I calculated it

See for yourself: original VOX AC30 from the 60's



Check Bright Channel volume control and its coupling cap, at low volume you have C1 500pF and VR2 500k .
Use this handy online calculator:RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin

Using those old series nominal values you get some 639Hz cutoff (at mild 6dB/octave, so you *still* have some Bass and Low Mids, just quite attenuated) , with modern normalized values it would be 470pF and 470k = 720 Hz. In practice same thing.

And that is with volume set low; when set to "10" capacitor also sees R9 and R7 to ground (depending on Normal Channel Volume setting, but which can be assumed is set to 0 if unused) so cutoff frequency starts at some 1200/1400 Hz

I BET Chris Jennings did not fire up his slide rule to calculate a cutoff frequency, but most probably had somebody playing his prototype LOUD and tried different capacitors until he found one which cleaned the distorted sound a lot .... tried and true design technique

FWIW revered Trainwreck amps are *basically* "a Fender amp with an extra tube for gain/sustain and a VOX type strong Bass cutoff for clarity"



Notice 0.002 coupling cap and 180k grid resistor at the third triode, which is also a cold cathode clipper, cutting below 440Hz.
The rest of the circuit is again a basic Fender with tweaked (Marshallish) Tone Control values.

Like at Mc Donalds, where they can offer a couple dozen "different burgers" using basic 6 or 7 "components".
Actually less, because they *always* need to use the bun and at least 1 patty

Wow... You have a copy of a schematic I had written on many years ago.... That is my hand writing for the OT secondary colors ..... I may still have the original copy with my pen writing in BLUE ink... I must have done it when I was Kenny Fisher's shop back in the 80's ....

[J M Fahey]

Wow... You have a copy of a schematic I had written on many years ago.... That is my hand writing for the OT secondary colors ..... I may still have the original copy with my pen writing in BLUE ink... I must have done it when I was Kenny Fisher's shop back in the 80's ....

Small World indeed

[R.G.]

Another step sideways.

My all time favorite power amplifier for guitar was one I constructed myself using the power and output transformers from a Magnavox hifi (not stereo! console from the late 50s or early 60s that I found in a vintage/antique furniture shop. It ran a single 6AQ5/EL84 single ended. I bet it did all of 4W fully cranked.

But "fully cranked" was so very, very sweet a sound. It had almost all of the adjectives I've heard describing smooth overdrive, touch sensitivity, creamy distortion and so on. It just wasn't all that loud.

Couple that with one of the 100/500/1000W Class D amps and you get a breadbox sized giant killer. The available power makes it a real challenge to build speakers that will take the output. There are smaller output Class D's of course, if you insist on lower power. The speaker/amp interaction can easily enough be handled by putting something like a real speaker you like as a load on the actual low powered amplifier.

I'm guessing that these kinds of amps will become ever more popular with working musicians in the near future.

[J M Fahey]

Cool.
But remember you still need to drive a suitable speaker as a load for the first tube amp and tap *that* to drive the big one, Class D or even AB.

The complex impedance, phase shifts, etc. offered by the speaker interacting with the tube amp are an important part of the signature sound; if you use a resistor there, youīll find it lacking "something".

Or at least use a very good reactive load, such as Aikenīs.

[nickb]

One question I have is, when looking at the loadline and specifying the primary voltage and current, should I use RMS or peak numbers?

I think that what you are concerned about is the heating effect (of the copper in the transformer) so RMS is appropriate. Now you get to make another design choice. Should you use current at max output or something, perhaps more realistically, go for something lower since you're probably not going flat out 100% of the time?

[Mike Sulzer]

I think that what you are concerned about is the heating effect so RMS is appropriate. Now you get to make another design choice. Should you use current at max output or something, perhaps more realistically, go for something lower since you're probably not going flat out 100% of the time?

If the purpose of a load line analysis is to maximize the RMS power, then a part of doing that involves paying attention to what happens to peak voltage and current for both positive and negative deviations from the quiescent point.

[SoulFetish]

If the purpose of a load line analysis is to maximize the RMS power, then a part of doing that involves paying attention to what happens to peak voltage and current for both positive and negative deviations from the quiescent point.

Well, that's an interesting point. I suppose, the primary reason I asked the question was for the purpose of properly sizing diameter of copper winding in the primary for handling the heat, as well as appropriately choosing an appropriate core stack area. Here is some load line data for my operating conditions:
(courtesy of https://bmamps.com/ivds.html)




Power out watts ≈ 17.1
Max Power Dissipation watts(squre wave, per tube) ≈ 11.47
Max Dissipation watts (sine wave, per tube) ≈ 10.53

The circuit is designed to push the amp into output stage into clipping early, and will likely be operated with some level of overdrive during operation. So, I would like to design it to handle 25-30W. According to the M19 data posted above, for 30W power, I calculated a minimum core stack area (mm²) of about 1469.7 (which I probably would just give to a manufacturer to fit with available EI stock, as I have flexibility in mounting space). Does this seem correct?
I'm just wondering how I should size the primary copper diameter (knowing that I need to account for wire diameter and insulation for a given winding window).

[Chuck H]

Or at least use a very good reactive load, such as Aikenīs.

I used that Aiken design (or rather, an 8R impedance adjusted version) with a large-ish 50R rheostat for my own attenuator. So it's still resistive well below the true impedance spikes of a speaker, but it's really, really good. I've never felt the need for a "bright" switch or other tweaks at any level. So +1 on the Aiken load.