Mark IIB Volume Swell

[zebpedersen]

My Mark 1981 Mark IIB has developed an annoying volume swell which has never been present before. I've only had the chance to test it at pretty low volumes, but it's never happened to me using the amp before...

I've taken a known good 12AX7 tube and swapped it out into all positions and played a couple of minutes in each, and the problem still comes back. I've also run the amp in 60w mode and switched the inside tubes for the outside ones, again to no avail. In addition to this I've tested two different cables and with/without the footswitch connected. No changes.

Amp is 1981 Mark IIB, graphic EQ (switched off for testing), reverb, 60/100W.

I'll test it tomorrow at a higher volume, and if the problem persists I'll see if I can record a YouTube video to demonstrate it.

For now, anyone got any ideas? I'm no expert, but the amp looks very tidy inside.

 

[RustyChops]

I had the same problem with my '83 iib. I tried all new tubes etc but the problem remained.
I took it to Boogiebabies who fixed it up. ( but then I live near to him..) I'm not sure if was just the caps or not but had them replaced anyway..he cleaned up the whole board , pots etc and replaced a few incidentals.

Search for Boogiebabies on this forum and send him an email..perhaps he can steer you in the right direction to troubleshoot.

good luck,
rusty chops

 

[zebpedersen]

Thanks man, good to know it's in the 'not too serious' category. If you turn the amp up a little louder than normal then it's fine, which isn't the worst thing in the world, I guess! I'll get it to a tech at some stage.

 

[Boogiebabies]

If you have a clear top relay, carefully take the cover off and clean the contact with De-Oxit.