Mk IVB Restoration and Troubleshooting

Hi guys,

Last year I purchased my dream amp, a used (obvi) Mark IVB wide head. It was in real rough shape, and you can see from the pics where it started and where I got it to in terms of restoring it.

Before


After


I took it to my amp tech when I got it to not only put it back in the original headshell but give it a thorough look-over and fix any problems it might have.
He had to replace a preamp tube, resolder a couple pots and re-seat the reverb leads, and after another playtest everything seemed fine. I haven't had a chance to gig with it yet because I'm still putting together a stereo rig that includes it and my Marshall Silver Jubilee, so it's just been living in my office and I've been practicing through it at low volumes (the lowest it seems capable of) since I've got it. So tweed power, triode and Class A are all selected, and I have the master volume basically as low as I can get it to go with it still making sound. (I'm not sure if it's natural for this amp to be so tricky to dial in a low-volume level, my Marshall is a 25/50w head so the Mark IV has quite a bit more muscle to flex than what I'm used to).

Anyways, it's recently developed an issue where after I've been playing for a while the volume level will start to rise and fall gradually, and seemingly randomly. I'm not sure if this is a tube issue, or if the amp is somehow switching itself between full power and tweed power or something like that. Anyways, I figure I probably have to bring it back in to my tech, but since pretty much everyone is closed these days, I figured I'd ask the community here for any advice in the mean time.

Here's a link to the first vid that I recorded with it during quarantine just for fun.
https://www.youtube.com/watch?v=74_CdaNmitU

Thanks so much for the help,

-Josh

Looks great

Nice restauration and playing!

Concerning the volume rise...my guess: it's a faulty powertube or a dirty tube socket...

Since you're playing in class A mode, you're only using the outer 2 tubes so you can troublehoot by swapping tubes but be aware that the outer tubes aren't EL34's, if they are, don't put the EL34's in the inner sockets, just leave them out and test with the tubes in the outer sockets and don't switch to simul-class...

To be sure that it's not he power section, you can use the send form the mark IV into your PC and load up a good cabsim (the wall of sound by Two Notes is currently free) and a CAB IR. If it's not changing in volume anymore, then the powertubes or the output transformer is the culprit.

I use my Mark IVa like this but I use ReAxis, a genious plugin by Mercuriall since it has a power amp sim and and a cab sim so the Mark IV's preamp comes to life into the digital realm.

Kindest of Regards,
Tijl

Thank! That's awesome.