My Mark IV has a sqeeeeeeeeling problem

[d7guitar]

It starts after approx 5 minutes of being on. :shock: It sounds just like microphonic feedback. It was first noticed during a gig. I thought the soundman couldn't get an unruly microphone under control when our singer said. hey, it's your AMP!!!! And, it only squeels when the Lead or Lead-Eq is in use. Now during gigs, i switch to the RHY1 or RHY2 channels between songs so i don't squeal the audience to death. I have changed the tubes, all of them (input and output). I swapped them out with good used ones. That didn't work so i bought all brand new tubes. Still doing it. Also when i switch from Lead channel to either RHY channel, or back to Lead from RHY, i get a piercing high pitch "pop". Very annoying. Changing the amount of gain doesn't make a difference. I bought this amp new approx 12 years ago. Still using the same settings as then. First problem i ever had with this amp. One thing i noticed is that the output tubes glow a VERY BRIGHT purple and are too hot to touch in less that a minute... is that normal??? Maybe i am just over thinking this?
Anyone have ideas? :roll:

 

[Fronzil]

bump for curiosity

 

[stephen sawall]

You covered what I was thinking (preamp tube) .... bump.

 

[d7guitar]

Yes, i have been reading this forum, and BTW it's an awesome forum, and previous posts led me to trying all new tubes. I haven't seen anyone sharing a similar problem either so i decided to join and ask the community. From what i've read, contacting Mesa seems to be a pretty good experience so i'll call them on Monday to see if they have ideas i can try.

 

[d7guitar]

Okay, I started looking through my old original Mesa Boogie 12ax7 tubes. I found a SPAX7 tube and remembered that it was originally in tube V1. Before swapping it i tried, once again, tapping the current tube in slot V1 and it made the sound. So i swapped it with the SPAX7. So far so good. I've had it on for over an hour now and no squealing. I have rehearsal Sunday night so i will know for sure by then. It makes the sound with a regular 12ax7 or a 12at7. The SPAX seems good for now????
i will update everyone later.

 

[stephen sawall]

Hope it works for you ....

.... and welcome to the forum. :mrgreen:

 

[d7guitar]

Okay, the amp friggin rocked tonight!!! No squealing, no noise, just bad *** guitar tones. Played for two and a half hours I loved it. Then it just shut off. The 3A slow blow fuse went. Didn't have a spare so i put a 5 ampere in it just to check if it would come on. It did and i shut it right back off. Off to the store for fuses now. Hopefully it was just coincidental??

 

[babow2]

From your description, this may not be it, but.....
I had a similar high pitch squealing on my Mark IV. It turned out to be a pentode/triode switch problem. Actually, to be specific, there was a screen resistor on one of the tube sockets that needed to be replaced, which was affecting the pentode/triode switch. I only got the squealing in one of those two modes. i can't recall which off hand.
So, if you do get the squealing again, try switching the pentode/triode switch to the other setting and see if that changes anything.

Again, from the symptoms you describe, it sounds like there is something else going on, but I thought I would mention it.

Also, i wouldn't take as "coincidence" the fuse going just after you made a change that seemed to solve the squealing problem. I would assume there is a connection.