Metropoulos Forum

Several years ago I built a JTM45 combo and overall it's not that different from my Plexi build. I decided to mod one of them to a higher gain circuit. The 45 was the lucky winner.

I know the Dirty Shirley is loosely based on the 45, so I started looking at what would need to be changed to do a conversion. All in all it wasn't that much, so I went with that.

Basically I swapped out or added a few components in the front end and made a couple of small changes to the PI. Then converted it to a 2 hole instead of a 4 hole and cascaded V1 to create a gain and master instead of two independent volumes as it was originally.

I double checked everything and put in the 5881s and hit the power switch. So far, so good. When I took it off of standby, it squealed really loudly. Cats were scattering all over the place!

The amp was very quiet before the conversion but now it's uncontrollable. I've got some before, during and after pics I'll post tomorrow if anyone can see anything I'm missing.

Try unsoldering the nfb wire from the output jack and see if the squeal has now stopped. If it does stop, simply swap the two plate leads at pin 3 of the power tubes. If it still squeals with the nfb disconnected, it's probably being caused by the extra gain that you created in the circuit. You may need to try shielded wire, snubber caps and possibly your lead dress to help stop the oscillation.

Thanks. I didn't think I'd need to swap the the plate leads, but it is a possibility. I did use shielded wire for the input and MV connections to V1.

It squeals with the gain and MV on 0 and nothing plugged into the input.

I tried to upload pics but keep getting an error.

I'll try your suggestion and let you know.

Thanks again!

Please let us know if lifting the nfb wire fixes the issue.

That didn't do it. I'll try again to post pics and see what you think of the lead dress. I initially ran the shielded wire from the MV over the top of the board but running it under didn't help either.

I think the pic issue is the file type. I'll change them to jpg and try again.

Sorry that didn't help. You may find some well place snubber caps will help to cure the oscillation that is causing the squeal. Depending on how much gain the circuit is making now, you may also need to adjust the voltage dividers between stages or tweak some component values to lower the gain that is being made. Studying a few schematics of some other high gain amplifiers may give you an idea of the best location for snubber caps and component values. Ceriatone has layouts available online for all of their modded Marshall circuits that may give you some ideas.

Thanks, I'll check that out.

I changed the file type but now it says the board has reached it's upload limit.

I believe that after the forum got hacked several years ago, it will no longer let you upload photos directly. You must upload them to a hosting site and then link to them here.

Makes sense, thanks.

I don't think the issue is really gain related. All in all it's not that high gain of an amp and I added the structure switch and it's in the lowest gain setting.

I removed the wire from the gain POT (not MV as I posted before) and it still had the issue.

You may be able to upload some photos over at the "Marshall Forum". Many of the old members here visit there quite frequently and I'm sure someone can offer some helpful advice.

Thanks for all of your help and suggestions, I really appreciate it.I do belong to the Marshall forum as well so I can try that.

I filmed a quick video to show what the amp looks like and the noise.

https://www.youtube.com/watch?v=Qa-kU_GjPLk

I didn't see anything with the wiring that jumped out at me. It is hard to see all of the connection points and wiring runs without a good close up shot though. That noise is a bit different than what I was expecting though. A noise like that has popped up in my builds when I have made a wiring mistake. I'm guessing that you have gone through all of the connections and wiring after doing your modifications?

One of the guys on the Marshall forum found it right away. The orange and green wires were reversed on the turrets. They were correct on the tube sockets but not the turrets. That fixed it.

Thank you again for all of your help.

Excellent! A picture is definitely worth a thousand words! Glad you got it straightened out.

For sure!

Replacing the PPIMV was kind of an afterthought and I didn't even think about it. I just kept checking the mod parts and wiring.

Going to use it at rehearsal tomorrow night. Looking forward to hearing it in a band situation.