Ry Cooder Green Man

[Helmholtz]

and it's this preamp that is generating the "harmonics" and "richness". .

Added harmonics is just another way of saying it generates distortion.

Even a clean booster increases harmonic content generated by the amps own (preamp) stages.

[Chuck H]

Added harmonics is just another way of saying it generates distortion.

Even a clean booster increases harmonic content generated by the amps own (preamp) stages.

Sure. But in a non master volume amp that means loud. Ry Cooder testifies that getting harmonics at lower volumes is what the Green Man does for him. And it can be seen in the images that he runs the volume on the Green man low. This is why I think it's the Green Man making the harmonics and he's not overdriving the host amps with it.

[Helmholtz]

This is why I think it's the Green Man making the harmonics and he's not overdriving the host amps with it.

Probably.

Also it's a tube preamp and each tube generates some harmonics even considerably below clipping.

Also the output transformers will have some effect (probably mainly acting as band-pass filters).

Apart from facilitating easy phase reversal, the transformers can avoid ground loops.

[Chuck H]

If I had to guess...

It's a single tube being used for a 5E3 type instrument channel preamp circuit feeding a pair of triodes run as power tubes through phase inverter type transformers. NOTES:

For the preamp I think the typical tweed volume control is not the units volume control. Rather, it's set at some fixed level allowing the tone control to be effective at any setting.

The volume control may be between the preamp and power amp (per se) but more likely it's a dual ganged wire wound thing used to attenuate the transformer outputs.

There are likely voltage dividers between the preamp and output to reduce what would otherwise be too much voltage gain. Adjusted to get something most similar to tweed amp performance at the output as this setup can achieve.

This is as close as I can guess based on reading everything that's been posted and what I've seen elsewhere on line regarding the performance of this device.

EDIT: The preamp tube is likely a 12ay7 as per the 5E3 schematic. Just guessing based on the mind set of the people that were involved in the units creation. The second (power) tube would probably be a 12au7 or a 12at7.

[Helmholtz]

Driving the OT shouldn't require "real" power (depending on type). It's mostly about proper primary impedance.

While a PP stage would allow for a smaller OT, it needs a CT and 2 triodes.

An alternative would be a SE reverb driver circuit, could even use a typical reverb transformer, though available power and transformer size might be overkill.
One half of a 12AT7 should do.
Capacitor coupling would avoid DC current and allow for smaller transformer size.
Many options depending on available OTs.

The volume control may be between the preamp and power amp (per se) but more likely it's a dual ganged wire wound thing used to attenuate the transformer outputs.

With 4 outputs that seems unlikely.

Also there's no need for wire-wounds as you're only dealing with signal level not power.
Each secondary should be terminated by a load resistor (i.e. 8R) that gives desired reflected primary impedance.
The output pots then could be regular carbon types of maybe 1k.

Are quadruple ganged pots available?

[Mick Bailey]

Quad ganged pots used to be available, but I haven't seen any in catalogues for many years. The last commercial use I'd seen was in the Korg MS-20 synth in the frequency to voltage converter.

Edit: Unless I'm missing something, the unit pictured has two outputs and Ry Cooder is quoted as saying the guitar goes in mono and comes out stereo.

[lasikukko]

Maybe it uses a 6SJ7 pentode, Ry used Gibson GA20 style preamp driving Hiwatt power amp earlier. And a double triode driving the transformers perhaps?