Transistor transformer coupled output?

[glebert]

Since it seems like we will never get any more information about the amp from the OP's friend, how about a dumb question that is tangentially related? Cool? Great!

I have long had a theory (totally unsupported by any real technical thought) that the OTs of tube amps add a bit of "snap" to the sound of the guitar during string attack or other playing dynamics. My thought (as much as there is one) is that the interactions between the windings and the core can create a bit of a "crack the whip" effect for fast transients. Technically I guess you would call this ringing or overshoot/undershoot. It would probably be undesireable in a hi fi, but we are talking guitar amps. I was always curious if a driver stage transformer (or output autoformer) would give a bit of that kind of flavor.

[R.G.]

I would have to think that through, but at first reading, I don't think so.
That is based on two things, the first being that there really isn't all that much speed in guitar signals. It sounds like the signals are trebly to us, but guitar pickups cut off in the 4kHz to 5kHz region from self capacitance and their high inductance. The signal path through the preamp to the power amp typically has low pass elements to it as well, so unless something is adding "snap" accidentally, it's not driving the transformer very quickly.

The second thing is that I can't think of any winding-core interaction to add transients to snap. True you can get fast transients if you load up an inductor and then try to stop the current quickly - V = L* di/dt and all that. But in an OT, the windings are "snubbed" by the secondary loads. That is not true if you run with unloaded secondaries, which can cause the amp to oscillate. This causes the output tubes to try to bang the OT from full on to full off on both half primaries, and the primary and leakage inductances to make inductor-flyback pulses of up even thousands of volts; that's what the plate diodes in many amps are for, to catch these pulses before they puncture the wire insulation. But with speaker loads, it would have to be a very rare signal to get inductive snap.

The totem-pole stacked class B transistor circuits with their interstage trannies work always loaded, but I can see that with massive drive or too low a frequency running the turn-off polarity on the transformer primary out of stored field, you might get transients. But I can't see it happening with normal signals.

I have often thought that the interstage transformer driven solid state amps had a sound intermediate between tube amps and all-transistor amps. The roundly-denounced Thomas Vox amps, for instance, do sound different to both tube amps and full SS amps to my ears. I put this down to interstage transformer distortion and not using enough feedback to cover it up - or to eliminate the other distortions and cover them up, too.

Just speculating.

[TomCarlos]

A friend is working on an old solid-state Silvertone 1425 guitar amplifier. This amp has a transformer on the output before it goes to the speakers.

I believe this is called a 'transformer-coupled' amplifier. I have never seen this before in a transistor amp.

What is the purpose of the transformer, and what are the advantages/disadvantages?

Thanks!

Lookie Lookie.... I found two Peavey Output Transformers in my parts boneyard. They came from two similar amps, Peavey Renown Solo Series. The part number is 705-18344. Check the schematic and see if you can find the specs for these. If anyone is willing to make a Donation directly to the MEF site using the link, say $20 each, I am willing to cover shipping. Please send me a Private message if you have an interest in one or both.

DC resistance measurements on both items....

Black to Green - ~.4 ohms
Black to Blue- ~.4 ohms
Black to White- ~.5 ohms

[Jazz P Bass]

I would think transformer transient response would be rather poor.
I am thinking overshoot & undershoot.

A continuous square wave on the input, while observing the amplifier output should put it to the test.

[J M Fahey]

I would think transformer transient response would be rather poor.
I am thinking overshoot & undershoot.

A continuous square wave on the input, while observing the amplifier output should put it to the test.

WHY?
Only if incredibly bad, I donīt expect that from any Peavey product.

Transients will not be worse than in any Tube OT; actually less because they are:

*autoformers, way simpler than any OT.

To boot, relatively very few turns needed; "nothing" compared with any Tube transformer primary (thousands of turns with very fine wire):
This is equivalent to an OT secondary only.
Primaries are plagued by parasitic capacitance, not here.

As of specs: power and impedance comes from on which amp they were used.

Practical use?: consider these one high quality version of Weber Maxi/Load matcher transformer,

[johncatoline13]

Hey Tom, retired e.e. ohio state. I want the peavey auto former. I have a visa debit. Otw, I can just send you $40. Please reply with address. I'll just send you cash. John catoline 330-786-8160. Thanks!

[TomCarlos]

Hey Tom, retired e.e. ohio state. I want the peavey auto former. I have a visa debit. Otw, I can just send you $40. Please reply with address. I'll just send you cash. John catoline 330-786-8160. Thanks!

Thanks John.... I will follow up with you privately.