Mark IV- Triode setting doesn't work- any help?

[babow2]

Hey all, I just picked up a used Mark IV- B (early B) head. It has Mesa 6L6's in all four slots, but also came with a pair of Mesa EL-34's. the amp is very clean and everything has worked just fine. I normally play in Pentode, simulclass, midgain, tweed settings. None of my knobs are set at extreme high or low ranges and the master vol. is around 3. So, i am not pushing anything particularly drastic as far as settings. I owned a Mark IV for about 10 years until about 5 years ago, and never had any problems with that amp.

When I switch to triode. I get a howling, feedback/scream after playing about 10 seconds. I changedthe outside tubes to the EL34's- same result. The amp works fine in Pentode, but switching to Triode yields an awful, piercing microphonic type howl.

I have been really happy with the amp set in Pentode, so I don't miss anything not using Triode, but I can't help but wonder if it is indicative of an easy problem (tubes worn) or perhaps a precursor to a larger problem that I should have checked.

Any insight from you Mesa wizards would be appreciated.
Thanks,
Brent

 

[Boogiebabies]

If it's not a bad switch it may be a bad resistor in the triode wiring. I would call Mike B. or George Mueller at Mesa and ask the same question.

 

[babow2]

Thanks for the info. I can call or drop by and ask. I live 2 miles from the factory. I figured someone (like you) might know the likely answer and whether I could deal with it myself.

Again, thanks.

 

[GD_NC]

I know this post is a little old, but I had a very similar issue with my Mark IV. I thought updating the post might be a good reference for others. My problem was the exact opposite of Babow2's problem. The Mark IV works fine in triode but hums loudly, pops and crackles in pentode mode. I took it to my local MB tech who determined that it was a bad pentode/triode switch as Boogiebabies suggested. A new switch is on order from Mesa. Hopefully that will take care of the problem.

 

[babow2]

Just to close this out, here is what happened. They needed to replace an ISO screen resistor in the Class AB tube sockets. I guess the amp was originally tubed with 430 tubes and came to me with 420 tubes. Rich at MB said the new ISO resistor made the tube sockets more "flexible" for handling different tubes, and that was the problem.

Lo, and behold, problem solved. They also did the standard "general maintenance clean up" on the amp.

All for $76.

thanks for all the input.

 

[kmanick]

you're so lucky that Mesa is local to you 8)
I'd love to send my IV to Mike B. for a once over but I don't want to ship it all away across the country if I don't need to.

Just to close this out, here is what happened. They needed to replace an ISO screen resistor in the Class AB tube sockets. I guess the amp was originally tubed with 430 tubes and came to me with 420 tubes. Rich at MB said the new ISO resistor made the tube sockets more "flexible" for handling different tubes, and that was the problem.

Lo, and behold, problem solved. They also did the standard "general maintenance clean up" on the amp.

All for $76.

thanks for all the input.