squealing preamp tube... which one?

[griff3]

Hey All, does anyone know of a way to determine which preamp tube is squealing in my Mark V?

It's just started doing this on Channel 3 with the gain dimed (or close to it) and I've heard that sound before and I know it's a preamp tube. Obviously I could swap them all, but would rather not.

There aren't a huge amount of hours on the tubes, but I haven't been using it much lately and it's just been moved around a lot so I suspect one of them is just shot.

Thanks for your help.
Griff

 

[Jackie]

Um, you have the Gain DIMED, what do you expect :lol:

I'm just joking. Who am I to judge how you set up your amp.

Anyway, I checked the manual and the gain stages and tube roles and there could be a way to diagnose it...
CH3 uses V1, V3, V4, V5, V6.

I doubt it's V1 because it shares gain stages with CH1 and CH2, which you say you don't have trouble with.

It's probably not V3 either, same reason.

One half of V4 (V4A) is the Reverb send buffer, while one half of V5 (V5B) is the return. If the reverb is acting up in any way, it could be V4 or V5.

One half of V6 (V6B) is the Effects return buffer. If the FX loop is acting up in anyway (volume loss when engaged), it could be V6.

Lastly, it could be the PI. Run the amp from the Slave out into a PA system and if the PI is faulty, it shouldn't squeal because the signal doesn't get that far.

NOTE that I don't own this amp and what I just told you was just me making deductions from the amp's manual and my info may be completely useless but I think it's worth a shot. If there is any error in my thinking I kindly ask the others here who have more knowledge and experience with this amp to correct me (false info is the worst).
If all else fails, just get a known working 12AX7 and swap every one you'll find the culprit sooner or later.

 

[griff3]

Thanks. After some digging I found the same info in the manual (should have thought of that first I guess :shock: )

As it turned out, it was only with one specific guitar (a strat) and was pickup related, not tube related. Thanks for the help anyway.

Griff