McGohan M20 Amp

[angelodp]

HI, Looking for the correct schematic for this McGohan amp. It varies somewhat from the M20B schematic that is readily available. The M20B has 6SC7 tubes (triodes) and one 6SQ7 in the preamp section, whereas the M20 has 6AU6-A tubes (pentodes). The PI and power and Rectifier tubes are otherwise the same. Iron is in good shape and it lights up with no tubes (no shorts on limiter). It's a PA amp and might be interesting for guitar.

[Stevem]

Do you have pictures of the guts you can post?

Does it not look stock on the inside?

[angelodp]

yup - looks like the dropping R's got cooked, I pulled them and also getting ready to put 3 prong safety plug and rewire the on/off. The on/off is weird, when you pull it lights up the 6.3v light but the AC is not flowing and when it's off the AC is conducting??? Need to re-do all that with the safety three prong and reassess. Have to buzz the PT as well.

PT seems fine and now have IEC. Investigating further.

[angelodp]

Steve, how would you proceed with this pentode input? Is it worth having a listen as is? Put in an input jack, speaker jack, power caps..... have a listen. Can I set up the input per the usual with the 1M to ground?

A

[TUBEDUDE]

The P.I. shown is a pentode Labelled as a 6SL7GT, but the 6SL7 is a dual triode.

[didit]

Hello --

Actually that PI circuit is classic "from the book" paraphase dual triode, just strangely rendered and therefore at first glance looks to be an oddly misused pentode.

Best .. Ian

[TUBEDUDE]

Doh! Thanx Ian, I just glanced and assumed.

[angelodp]

So this amp has 6AU6-A preamp tubes, and yes has the 6SN7GT in the PI. Since I do not find a schematic with the actual compliment of preamp tubes, I can look at creating a schematic. I am close to just giving it a try as I have added the input jack and speaker jack and soon will have the new power caps in.

[angelodp]

So I found a 5V4 in the stash, and it lit up. Then in go the other tubes since no shorts and adequate voltage and wham, its a guitar amp. Treble and Bass do have a nice response and the volume is substantial into a nice breakup. So just the addition of new caps and input & output jacks did the deed. I did up the power cathode cap slightly to 300 ohms.

[angelodp]

2 un-used 6AU6 tubes. Maybe tremolo like the Silvertone and a one tube Reverb? Looking into it.

[angelodp]

Can a paraphase inverter take the PPIMV setup?

[angelodp]

Hello --

Actually that PI circuit is classic "from the book" paraphase dual triode, just strangely rendered and therefore at first glance looks to be an oddly misused pentode.

Best .. Ian

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

[sluckey]

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

[didit]

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 ..

[angelodp]

Sluckey... from a thread over on EL34....

"You have a paraphase inverter. It's not gonna like any of the post PI MVs. About all you can do is put a standard volume pot in front of the PI."

On this amp I am working on, would an addition of a volume (Master) pot right in front of the PI allow for breakup at a reasonable volume.... this amp is very loud. I did try an airbrake with some success.