C3g:GM70:GM70 schematic advice

[louischia]

attached is my schematic. im not good with my calculations. advice needed for my schematic designs and component values. Thank you!

B1+ = 280V
B2+ = 800V

interstage driving 1st GM70 = 3:1
interstage driving 2nd GM70 = 1:1

[djn]

I can't speak to your scheem as I don't know that much, but are you looking for more wpc with the two GM70s? If so, why not PP?

[louischia]

cos i prefer A1 SET

[tinitus]

being a 3 stage design, is there any sense in the GM70 driver

GM70 should be easy to drive, thus a 2stage design ought to be possible


maybe first stage and its interstage trafo could easily be replaced by a line driver trafo with gain
and being SE, the GM70 driver intertage could be coupled for gain as well

[kevinkr]

Where people get the idea that the GM70 is particularly easy to drive I'll never know, it's not really - at least if done properly.. I'm not a big fan of the quasi-Sakuma approach and recommend ditching the driver GM70 - I'm not sure what it brings you other than some theoretical distortion canceling. Better I think to make things as linear as possible on a stage by stage basis.

You can use a single stage driver based on such tubes as the D3A (the choice I made in my GM70 project), 7788 or C3G.. Choke, transformer or CCS coupling are all options.

The GM70 requires 3A @ 20V for filament heating, not to mention north of 100W plate dissipation in most cases, and I can't imagine using more than one of these per channel in an SE amp. Depending on operating conditions chosen, 20 - 25W is achievable with a single stage driver and the GM70 running in class A1.

[Wavebourn]

Here you go: how GM-70 tubes were used in Russian amplifiers originally.

[kevinkr]

Originally Posted by Wavebourn

Here you go: how GM-70 tubes were used in Russian amplifiers originally.

Hi Wavebourne,
Now that is what I call really complex.. Quite beyond anything I have ever contemplated. Wondering what it sounds like, have you heard one of these?

What sort of power output? It seems designed to deliver very high power.

I noticed a phase shift oscillator in there driving a small power amplifier, very odd. Is this a projector amplifier or something similar? Unfortunately not speaking any Russian and the considerable complexity of the schematic make it hard to decipher. There is both a lot of voltage gain, and the driver stages are robust to say the least.

[Wavebourn]

Originally Posted by kevinkr

Hi Wavebourne,
Now that is what I call really complex.. Quite beyond anything I have ever contemplated. Wondering what it sounds like, have you heard one of these?

What sort of power output, several hundred watts?

They were used on stadiums, for example.
Can you imagine a bunch of aluminum bucket horns on poles driven by such an amp? How well would they sound?

Output power was 600W. As you may see, GM-70 tubes were driven by 6L6 cathode followers that were driven by 6L6 push-pull stage.

Originally Posted by kevinkr

I noticed a phase shift oscillator in there driving a small power amplifier, very odd. Is this a projector amplifier or something similar?

The amp had all measurement equipment inside.

Usually they were used in a reserve combination, when one was working, while other was fully serviced and ready for the duty.

[Wavebourn]

Here is the picture:

[kevinkr]

Wow!

Certainly explains why the GM70 graphite plate version at least is so ubiquitous and inexpensive. I assume a lot of public venues and soccer stadiums had at least one of these back in the old USSR.

Super cool.. I bet those pa horns sounded none to good, and the amplifier is a bit big for anyone other than Dick Burwin to fit into his listening room.. Guess we'll never know..