McGohan M20 Amp

[sluckey]

Any pre PI MV will allow you to crank up the preamp for front end overdrive. Then dial the volume down with the MV.

[angelodp]

Ian, when you say "from the book" I am looking at Valve Wizard, is it the floating paraphase you are referencing?

No. It's just a plain and simple paraphase inverter, exactly like the one at the top of this Wizard page...

http://www.valvewizard.co.uk/paraphase.html

Yup ..

I guess I do not understand how this amp is similar to what you are referencing. This Mcgohan appears to be using the plates from the dual triode to create the phase inversion. How is that similar to the Wizard diagram?

[angelodp]

Any pre PI MV will allow you to crank up the preamp for front end overdrive. Then dial the volume down with the MV.

Does the 470k just before the PI look like a reasonable spot? Might you have any references from one of your builds for that style MV?

Thanks A

[sluckey]

I guess I do not understand how this amp is similar to what you are referencing. This Mcgohan appears to be using the plates from the dual triode to create the phase inversion. How is that similar to the Wizard diagram?

The wizard did not draw the first triode. Maybe my note on his drawing will help you see the two schematics are practically identical.

Does the 470k just before the PI look like a reasonable spot?

Yes. Just replace that 470K with a 500K-A pot.

[angelodp]

Hmmm, just tried the MV there and not sure I love it. Seem to be getting blocking distortion... from the preamp section as I try to dial in a nice breakup. This being a PA system, this could be a futile effort.

Thanks Steve for your input.

A

PS yes your drawing was very helpful

[angelodp]

Ok so back to this amp. Just to be clear this schematic is not exactly what I have. The amp has two inputs with 6AU6-A's as the pre-amp stages. I have converted one of the preamp stages (the 6AU6) to triode operation. The screen is to the anode with a 100R, suppressor G to the cathode, and I have a 330k Plate R, 1.5k w/4.7uf on the K. I am managing to get a very useable guitar tone with Treble and Bass controls quite effective. I do have an air-brake on the speaker at 5 (the setting just before bedroom), and I can achieve pretty cool breakup and overdrive tones. I do think that these 6AU6-A tubes are great in triode mode. I have to be careful not to get the guitar to close to the chassis as the tubes are prone to microphonic.

I think I am close to a Clubman configuration.

Anode voltage on Triode change was off, very low. I changed up the Anode resistor to 33k, closer to the clubman (10-18K which seems weird), and changed up dropping string, now have 185v on anode of Triode (6AU6-A), changed up K to 2.2k with the 4.7uf. This has re-arranged the woofy bass to a much tighter feel. I am running the amp with a Big-Shot into the two inputs. One side wet the other dirty, kinda interesting as a parallel amp.