• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

McGohan MG-8 Mod suggestions?

Hi all,
I have a McGohan M8 single ended tube amp that I would like suggestions for recommended modifications to make it sound great as a guitar amplifier. Of course it’s a pretty simple circuit, not too far off from a tweed fender champ.

I have enclosed some photos of the amp and a schematic.

A couple of specific questions:
1. Should I eliminate the phono input?
2. If so could I use the phono volume pot for something else?
3. I plan to replace the filter caps and the connecting electrolytic caps, should I look at replacing anything else?
4. Should I consider changing the output tube from a 6l6 to a 6v6?

Thanks in advance for any assistance you can offer. I’m excited about this fun little project.

Justin
 

Attachments

  • McGohan M8 schematic.jpg
    McGohan M8 schematic.jpg
    771.7 KB · Views: 103
  • 7FEE7CA2-1B0C-47A6-BCC9-89E6759EDDBD.jpeg
    7FEE7CA2-1B0C-47A6-BCC9-89E6759EDDBD.jpeg
    99 KB · Views: 107
  • B899156B-07D0-4BFB-96C5-777A3D32940C.jpeg
    B899156B-07D0-4BFB-96C5-777A3D32940C.jpeg
    74.7 KB · Views: 78
  • 410EA44D-1B46-455C-903D-12F684F9B3D8.jpeg
    410EA44D-1B46-455C-903D-12F684F9B3D8.jpeg
    119.3 KB · Views: 92

PRR

Member
Joined 2003
Paid Member
6V6 in that socket will fry. What is wrong with 6L6? I've seen guitarists use them.

The plan shows no grid resistor on V1b. Unlikely to be wired that way.

It also shows separate cathodes inside V1, but 6SC7 has only one cathode pin.

> not too far off from a tweed fender champ

In fact this IS the kind of amplifier the Champ derives from. Major working difference is gridleak bias instead of cathode bias on the first two stages. Fender did this a little but I think high-output pickups forced a change to cathode bias.

But the shared cathode on 6SC7 does not encourage cathode bias. (The shared cathode in two sequential stages is prone to regeneration.)

You could do far worse than, bit-by-bit, changing to be more like a Champ. The obvious split-cathode twin triode is 6SL7.

The "phono" is really your iPod input for play-along.
 
McGohan revised schematic

Thanks for the reply.
You are correct on the grid leak resister in V1b I missed it on the first uploaded schematic. Thanks I've revised and re-uploaded a corrected version.

I put this schematic together based on a schematic I found from a McGohan MG-7 which did have a 6sc7 - however the MG-8 doesn't. It has a 12ax7.

Based on that, what are the advantages to changing the circuit to cathode biased (you mentioned that Fender made this change because of the high output pickups)? if it makes sense to make this mod and I follow an old champ circuit for values for the resisters and caps for the cathode bias, will I need to consider anything else in this change besides removing the grid leak resisters?

the only reason I mentioned the 6V6 was because I'd read that that was a mod that some do for more warmth. But I'm good with staying with the 6L6.

thanks again for your time and input.
 

Attachments

  • McGohan M8 schematic rev 1.jpg
    McGohan M8 schematic rev 1.jpg
    777.7 KB · Views: 49