Problem with a '91 Mark One Reissue Mk1C

[telecasterbloke]

Just bought this amp in the UK second hand and am really loving the tone - for the first time, I'm able to recreate Rory Gallagher's Irish Tour tone almost to the letter.

Except that is when the volume starts to rise. Theres a really nasty wolf note/harmonic that appears to be about an octave below where I'm playing.

Tube wise it's got a crop of Sylvania 12AX7s with a 12AT7 in the first stage (Phillips) and these look like new old stock. No noises when i tap them. Output tubes were a mixed bag, one Mesa, One Sovtek, and a pair of JJ 6L6's which I've swapped out for GT6L6Rs. This didn't cure the problem.

The speaker doesn't appear to be at fault.

Any ideas?

Cheers, great site full of great info by the way!
Gary

 

[Boogiebabies]

I hate to say this, but it sounds like it's time for a filter cap job.

 

[telecasterbloke]

Is that bad then? Are they they big ones on the power supply board?

 

[Boogiebabies]

Yes.

 

[telecasterbloke]

Is that a "yes" that filter caps are a bad job to have done or "yes" they're the ones on the power supply. Or both? Sorry if I'm being dumb....

 

[Boogiebabies]

"Yes, Yes" - Jeff Goldblum


The big blue ones need to be changed. It might not be a bad idea to see if they are bulging or leaking yellow goo before you fire it up again.
When they go, they can explode.

Just to be 100% sure, replace all the tubes.
Also, in rare occasions a cathode cap, or posssibly a coupling cap may have gone bad.
It's usually the filter caps failing.