Solid state amp with OT?

[überfuzz]

I've been toying with the idea of building a solid state amp with an OT. Before I look dig deeper I'd like to hear your ideas on this. Is there any such designs I should check?

[Leo_Gnardo]

I've been toying with the idea of building a solid state amp with an OT. Before I look dig deeper I'd like to hear your ideas on this. Is there any such designs I should check?

McIntosh built lots of very high quality SS hi fi amps with output autoformers so you could drive wide ranging loads and derive full power. And Peavey, a couple of 50-ish watt guitar amps also with autoformers late 70's, an idea they didn't continue. I'm unaware of any SS amps that use transformer outputs, but that doesn't mean there aren't any.

[J M Fahey]

There´s an 80`s Marshall with 4/8/16 autoformer, Peavey had one of those Pedal Steel amps with a 2/4 or 4/8 autoformer,I bet Enzo remembers model number, I also make them (as special order) with 2/4/8 or 4/8/16 autoformers for guys who plan to carry my heads anywhere and use whatever´s there.

No big deal, generally you design the autoformer to get 20 or 28V (whatever the amp puts out at max power) and multiply by 1.4 or 0.7 to get double or half impedance.

[Enzo]

Any PV solid state guitar amp in the 80s that used the 400B/G power amp board potentially had one. Actually the same board was either a 400B or 400G, same schematic. There was no actual 400B/G. The B used in bass amps had no transformer, but did have the compressor. While the G ones had the transformer but no compressor. Musician Mark 3 was an example.

On that was the PV part DG117 Autoformer. It was a 4/8 ohm.

Their original Renown (not the Renown 400) had one as well, but it was a 2/4 ohm. Part number was 70518344.

For all I know Peavey parts may still have one or both of those. Then again it has been 35 years since those came out.

The Renown is a large amp, like some of the steel amps, but I never thought of it as a steel amp. I don;t recall specifically a steel amp with one, but there sure could be.

[Enzo]

But now let us ask uberfuzz the real question. Did you intend to put a matching transformer on an amp circuit? Or did you intend to make some sort of push pull transformer output like a tube amp but with transistors driving it?

[52 Bill]

The original Gallien-Kreuger amps like Santana used at Woodstock had output transformers.

[g1]

Here are a couple schematics with autoformers for reference.
As Enzo mentioned, if you want to do push-pull transformer out like a tube amp, that is a different animal.

[R.G.]

There's a problem lurking here. You'll need to be careful to put the OT outside the feedback loop. Transformers have big phase shift issues at low and high frequencies, so if they're inside the feed back loop of the amp, they will severely limit the amount of feedback, and SS amps of modern design don't take kindly to that. A simple autotransformer outside the feedback loop is probably OK, but that doesn't much qualify as an OT in the normal sense of the word, to me at least, just as to Enzo and g1.

It is possible to design a SS amp for an OT, there are many designs for this in the literature from the late 50s. An interesting variant might be to "fake" a pair of output tubes with a pair of high voltage MOSFETs with lots of source degeneration to pull the transconductance down to something similar to a tube.

There's a lot here that's left as an exercise for the reader...

[g1]

It is possible to design a SS amp for an OT, there are many designs for this in the literature from the late 50s.

As far as I know, these are the extremely rare cases where it's not safe to run an SS amp unloaded.

[printer2]

Rauland PR1501, 15PA amp uses an output transformer and some 2N3055's, not P-P though. TOA A901-A (10W) also has a real transformer also.
http://www.toacanada.com/assets/files/A-901A_manual.pdf

Doubt that is what you are looking for though.

[Tom Phillips]

I've been toying with the idea of building a solid state amp with an OT. Before I look dig deeper I'd like to hear your ideas on this. Is there any such designs I should check?

I've been following this thread since the start and überfuzz hasn't been back since the first post 4 days ago. I think a significant missing bit of information is why he is considering a SS amp with an OT. I.e. what he hopes to achieve, what performance improvement he expects or what problem he wishes to solve. Can you let us know überfuzz?
Cheers,
Tom

[J M Fahey]

I made lots of 40 and 60W 12V battery powered amplifiers , with MTP3055 MosFets , real drain to drain output transformer, and said MosFets gate driven from inverting Op Amps, basically like Music man amps, only Mosfets were both "driver transistor and power tube" at the same time.

Only way to get 40 or 60W RMS straight from 12V without switching converters (way back then, we are talking more than 20 years ago, ferrite cores were very hard to find in Argentina).

Today I just use the proper SMPS plus a conventional split supply amp. much easier and cleaner.

Those MosFets were very nonlinear.

[printer2]

I think the point of using an output transformer with transistors would be to have them operate more tube like, under the same conditions. The output impedance will be more akin to what tube amps provide and you would would have the same voltage inductive effects of the OT. You would have to run low levels of NFB as tube amps do around the transformer. I thought of doing an OT SS amp just to compare it to a tube amp of the same power, a conventional SS amp with current feedback, a conventional amp with higher power supply and a resistor in the output to kill the dampening factor, a switching amp with the same resistor... Yeah right. In another life when I have more free time. Disappointed finding out the OT's in the two amps I listed are not P-P. Guess I can get rid of them now.

[J M Fahey]

I think the point of using an output transformer with transistors would be to have them operate more tube like, under the same conditions. The output impedance will be more akin to what tube amps provide and you would would have the same voltage inductive effects of the OT. You would have to run low levels of NFB as tube amps do around the transformer. I thought of doing an OT SS amp just to compare it to a tube amp of the same power, a conventional SS amp with current feedback, a conventional amp with higher power supply and a resistor in the output to kill the dampening factor, a switching amp with the same resistor... Yeah right. In another life when I have more free time. Disappointed finding out the OT's in the two amps I listed are not P-P. Guess I can get rid of them now.

I know where you are going, I have been there.
Already built a tube type power amp,with MosFet drains driving a center tapper OT connected to +V and overloaded it usng a guitar preap.

Close, but no cigar.

To say bthe truth neither SS nor Tube but what you get from a Music Man amp (or similar Peaveys), you get the high impedance drive sound, a little overshoot ... because power Mosfets have the flyback killing diodes built-in, a quite flat topped clipped signal ... and not much more.
Exactly what you get in a Musicman, if you ever care to overdrive and scope one (I did).
I have screen captures somewhere, and very different to conventional (Fender/Marshall) waveforms.

You are fully missing 2 important artifacts which flavour Tube sound:

1) grid rectification and subsequent compression.
MosFet gate is fully insulated through a thin layer of *glass*

You also lose waveform kinking (which many confuse with crosover distortion) which also adds extra flavour.

2) screen grid sag and subsequent compression

As you see you lose 3 important conventional pentode flavouring mechanisms so in practice this more expensive, complicated and hard to bias output circuit is not significantly better than a conventional SS amp with mixed feedback.

Sorry .

I was really pissed off when I found this

as you see

[printer2]

Thought of the rectification, sag not so much. Can lead to a lot of wasted time but it does beat being bored.