Is this squealing due to a microphonic tube?

[stevehollx]

Got a Mark IV second hand earlier this week. The lead channel has this nasty squeal when I have the lead gain past 3 (vol 1 is at 7, lead master at 2, master at 2).

I don't hear it on the clean channel. I forgot to check if it occurs with R2.

If I turn the guitar volume off completely, the squeal goes away. If the tubes are microphonic, you'll still squeal with the guitar volume off, right?

It's strange, because if I mute my strings it goes away. But the instant I take my hand off, it is squeal city. I've noticed it's mostly with the G string.

Now I know it's not an issue of having the gain too high or with my guitar, since the Mark III I have doesn't do this.

I'm trying to figure out what the issue is. I retubed the amp with the tubes from the III and the problem occurs. When I fired up the amp after retubing it, I noticed I had NO gain, and then checked the tubes and noticed the V4 tube was blown. I may have knocked it when racking the head. So I swapped that out with one of the tubes that was in it before, and the squealing was back. It could be the tube I replaced it with is the initial culprit, though.

I'm also thinking maybe V3 got jacked when V4 blew, and I should swap that one out as well. I'll probably just buy a new set of 5 and re-tube the whole preamp again.

I just don't get how it could be a tube if I don't hear the issue with the guitar volume down, though. Any ideas?

 

[boogietone]

I had V1 in MKIII go microphonic. I lightly tapped the tubes with my finger without a guitar plugged in and V1 was very loud. So I swapped V1 with one of the other preamp tubes and it went away. Sounds incredible now like a MKIII should. That tube will get replaced this weekend though.
Later

 

[Dutchman]

Tap the preamp tubes with a pencil and see if they start ringing. If so replace, it certainly does sound like a bad tube though.

The new Tungsol 12AX7's are my favorite new tubes. The old Mullards are still the best sounding IMHO.

 

[boogietone]

Tungsol's are what I'm using for the preamp. I really like them. I have STR454 for power tubes as well.

 

[MesaDCLP]

Make sure you are in Harmonics and not midgain on the lead switching...

Hope that helps.