Geloso G1 1020 A from PA to Guitar Amp (The Power Section Mod)

[kagliostro]

The original schematic of the amp is this



I was advised to put some mods in place



I decided to renounce to the Local NFB on the Power Tubes

but I'm still undecided if adopt the other mod eliminating something that seems to be there to avoid self-oscillations, a mod that I would mainly do to reduce the number of components on a board that is somewhat small in size

Adopting that elimination mod this is what I've draw



I've a doubt, as I don't know well the Concertina PI, is that 1M resistor connected between pin #7 and the junction of the RK resistors to be eliminated ? is that part of the autooscillation prevention as seems to me   

One other thing, the Power Tube Grid Stoppers are disegual, one is 1K and the other is 47K .... difference it will be better I maintain or will be better to put those resistors of an equal value, 2 x 1K or 2 x 47K ?


The .01 coupling cap between the Volume wiper and pin #7 of the PI is to be added (as advised to me) or it can be shun ? Same thing for the 470K to be added as Grid Stopper at pin #7 of the PI ....

Many Thanks

Franco

[sluckey]

I've a doubt, as I don't know well the Concertina PI, is that 1M resistor connected between pin #7 and the junction of the RK resistors to be eliminated ? is that part of the autooscillation prevention as seems to me   

The 1M is necessary to establish bias.

One other thing, the Power Tube Grid Stoppers are disegual, one is 1K and the other is 47K .... difference it will be better I maintain or will be better to put those resistors of an equal value, 2 x 1K or 2 x 47K ?

I would use equal resistors. 1.5K, 5.6K, and 8.2K are common values used with EL84s.

The .01 coupling cap between the Volume wiper and pin #7 of the PI is to be added (as advised to me) or it can be shun ? Same thing for the 470K to be added as Grid Stopper at pin #7 of the PI ....

That cap is necessary to prevent the volume pot from upsetting the grid bias. There is a high dc voltage on the concertina cathode and the grid is bootstrapped through the 1M, so there will be a big dc voltage on the grid as well. I've never used the 470K grid stopper but some newer designs consider it as an improvement.

[kagliostro]

Many Thanks Steve

I really was thinking wrong about the 1M resistor, didn't considered the bias of the triode (me stupid)

Also the .01 cap was useless in my mind but now I understand well

If I've read about the Concertina I really forgot near all   

OK for all the Grid Stopper, this is the easiest part to understand

Many Thanks Again

Franco