Transistor transformer coupled output?

[g1]

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?

In general, I think the reasoning is to allow users with higher than rated load impedance to get full power out of the amp. For example, an amp (solid state) rated for a 2 ohm load will deliver substantially less power to an 8 ohm load. Many users would want to get full power into the higher impedance.

However, for real old stuff, it could be the other way around, as you suggest. If the amp was rated only down to 8 ohms, and you wanted to run multiple caps, it may need an OT to avoid overheating into a 2 or 4 ohm load.

[glebert]

FWIW, found this schematic for the 200BXL (1431), guess in the pic above it really is just a driver transformer. Still can't confirm it uses the same power amp as the 200XL (1425) though they seem to be the same era. It seems like it would be impossible to bypass this driver transformer and have it work. Did people ever add autoformers onto amps as mods to be able to drive lower impedance loads? Also, we are probably way passed the point of useful discussion for the OP unless they can actually get gutshots of the amp in question.

[Greg Robinson]

I like to use an impedance matching (rated for 150W) autotransformer to connect different impedance speaker cabinets to my '63 RI Vibroverb.

I have an old Peavey Automatch II - a 400W autotransformer with 2/4/8/16Ω taps and an isolated 70/100V winding. Very handy, would like to get my hands on a second so I can do stereo.

[mozz]

In general, I think the reasoning is to allow users with higher than rated load impedance to get full power out of the amp. For example, an amp (solid state) rated for a 2 ohm load will deliver substantially less power to an 8 ohm load. Many users would want to get full power into the higher impedance.

However, for real old stuff, it could be the other way around, as you suggest. If the amp was rated only down to 8 ohms, and you wanted to run multiple caps, it may need an OT to avoid overheating into a 2 or 4 ohm load.

Yes I can understand how it works, but why would they use it in the design? I would think using the correct impedance speaker would be cheaper than using a transformer with a cheaper speaker. Maybe they got a deal on 16 ohmers but wanted to drive it with 4? That all being said, back then could you design a output stage specifically for a certain impedance?

[Tom Phillips]

Yes I can understand how it works, but why would they use it in the design?...

My guess that, since it was designed in the early years of solid state guitar amp, the designers were trying various overall circuit approaches with lots of influence from their traditional training related to tube circuit design.

[Helmholtz]

Apart from delivering full rated power at different speaker impedances, the output transformer also protects the speakers from DCV.
Reason is that the OT is a DC short to the amp.

[Tom Phillips]

... the output transformer also protects the speakers from DCV...

That's a big benefit in the early days of SS guitar amps.

[loudthud]

I have one or two of the Peavey impedance matching auto-transformers. They're monsters that can handle 300 or 400 Watts. There were some Peavey guitar amps that had an internal auto-transformer. Look for an amp with a heatsink and an impedance switch.

[nevetslab]

The Interstage Transformer was also used in the Vox Beatle Super Reverb V1143, schematic of power amp attached

VOX Beatle V1143 Pwr Amp Schematic.pdf

[R.G.]

The Interstage Transformer was also used in the Vox Beatle Super Reverb V1143, schematic of power amp attached

Actually, all Thomas Vox amps used an interstage transformer to drive the outputs. This includes all Beatle, Royal Guardsman, Buckingham, Viscount, Berkeley, Cambridge, Westminister, Sovereign... all of them. These were from an earlier era when complementary devices were simply unavailable, so there had to be some kind of phase inversion/isolation transformer used.

[J M Fahey]

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.

That transformer shown between the heatsinks is a large-ish driver one.
Compare it to the one suggested by RG Keen (or was it Steve Ahola?) for VOX amps.

An OT, specially for a Bass amp, would be almost as large as he black one on chassis left.
At least 75% of its size.

Just found it:
http://www.geofex.com/Article_Folder...ransformer.pdf

It uses a 24VA mains transformer as a core donor, not a wimp by any means.

[52 Bill]

Early Kustom amps used large driver transformers as well.

The only solid state amps that I remember having traditional output transformers besides Pignose amps, were the original Gallien and Kreuger amps. They were the ones that Santana used at Woodstock.

[R.G.]

A bit of history. The transformer interstage driver for class B transistor amps was only common for a short time. My fascination with Thomas Vox stuff made me chase this design for years. I filtered through electronics books and manuals, even patents to try to find out how this all worked. Think of it as another way I wasted my awake time doing circuits and not practicing guitar.

The design shares the issues with all single ended amp designs. One polarity of the output has to be stored in the M-field in the iron. The transformer driver can only pull one direction actively. The other polarity is done by the driver turning off primary current and letting the stored field out on the secondaries. Since one of the output polarities has to be entirely supplied by the field in the iron, you have to make the idle field energy in the iron big enough so it can drive the outputs as long as you need it; it's a low end limited design, because driving low frequency loads requires the longest sustained energy from the iron field. Run out of stored field, and the drive to the output transistor collapses. In most audio power amplifier designs, that means the base drive gets weak.

It's been a kind of idiot's delight to study for a long time. SE transformers are HARD to design well.

[DrGonz78]

This amp brings back memories to a 1464 repair. Yup interstage driver transformer. Amps look identical.

[DrGonz78]

Here’s a link to the old thread.