Transistor transformer coupled output?

[Axtman]

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!

[The Dude]

There are several reasons it was done- impedance matching, AC voltage step up, isolation, etc. Early transistors could not handle the supply voltage of modern versions. So, supply voltage was low and output stepped up via the transformer. I suppose another advantage is that there can't be DC on the output.

[glebert]

Seems like the 1425 amp model is also known as the 200XL. I can't find a schematic under either name. I don't see any Silvertone solid state amps with output transformers, but there seem to be some with transformer to do phase splitting before the output transistors. I know Sunn did that with some amps too.

Peavey had an amp called the 400G that had what they called an "autoformer" after the power transistors to do impedance matching IIRC.

[J M Fahey]

I also suspect it is a *driver* transformer (which were common in the 60s), not an *output* one.

An OT is as large and heavy as the PT; a driver one is like a small filament transformer.

Pictures always help-

[mozz]

Millions of small transistor radios had output and driver transformers.

[Axtman]

I don't have pictures. I will ask my friend. He said he bypassed the transformer and plugged the speakers in directly, and the amp worked. So I suspect, as you mentioned, that it is some sort of impedance matching arrangement.

[J M Fahey]

Millions of small transistor radios had output and driver transformers.

Is Silvertone 1425 a 4 AA battery powered transistor radio?

[mozz]

Maybe this schematic is close?
silvertone_1423.pdf

[Axtman]

This is not the exact amp but what it looks like.

[Tom Phillips]

That Silvertone 1423 schematic posted by Mozz shows a "Driver" transformer rather than an output transformer.
It is possible to use output transformers or a setup using an auto transformer which is a tapped coil approach. Macintosh did that and I have seen it in a Bogen Model MT-250.
The purpose is to maintain the full output power of a solid state amp at various impedance loads but it adds a lot of expense and weight to the amp.
Attached is an interesting white paper about output Autoformers. Although amplifier schematic shows the auto transformer as a simple tapped coil, the reality, shown in figure 9 in the paper, discloses that the actual design is quite a bit more complicated to obtain the high quality performance.


Coderman Autoformer White Paper.pdf

[mozz]

Chances are the Silvertone only has a driver transformer. They were made for Sears, cheaply.

[Helmholtz]

Chances are the Silvertone only has a driver transformer. They were made for Sears, cheaply.

See post #6.

[mozz]

Must have missed that post. Does sound odd, wonder if it was step up or step down? Would bypassing it cause either to overheat the output transistors or have a lot less output?

[glebert]

For some reason there are no pictures of the chassis or even the backs of these amps to see what the transformer looks like. I did find this picture of the (probably similar power amp) 200BXL which has a second transformer that definitely looks bigger than a driver.

[Helmholtz]

Any transformer having a multi-tapped winding can be used as an autotransformer within its voltage, current and frequency limits.
E.g. an OT of sufficient wattage having a secondary with 4 Ohm and 8 Ohm taps can be used between a (tube or SS)) amp having an output impedance of 4 Ohm and an 8 Ohm speaker. This will cause a few percents of power loss and maybe a little bass drop.
An even better solution is a dedicated impedance matching autotransformer.
I like to use an impedance matching (rated for 150W) autotransformer to connect different impedance speaker cabinets to my '63 RI Vibroverb.