Caliber 50 screams in overdrive channel

[Franklin]

Hi everyone,
I have a problem with my Caliber 50 Combo: It works perfectly in the clear channel but when I switch to the overdrive channel, there is nothing more but a gruesome high-tone-screaming like a feedback.
I found in the manual that this is a "result of a deteriorated 12AX7 preamp tubes. This is most often the first or second tube on the right, as seen from the rear."
So I got a 5-set of new (Mesa!)peamp-tubes and changed them, but the problem is still there.
Before I give the amp to someone who knows what to do (and perhaps wants lots of money for that) I wanted to ask for your experiences and ideas.

Thank you

 

[LHanson]

I'm no amp tech, but I'll dive in.....

The "lead channel" of a .50 turns on a handful of circuits, according to the schematic.

The lead switch turns on 4 LED switches:
If you look, there are 4- LDR1, LDR2, LDR3, and LDR4.

LDR1 pretty obviously turns on the second half of V1. LDR 2 and 3 turn on some tone circuits, and LDR4 turns on the Lead Master Volume.

I would suppose that if one of the LED switches for the LDR 2 or 3 went bad, a uncontrolled feedback might happen. I have no idea how to fix it, however.

A few questions: Does the Lead Master control work? Have you tried turning down the first volume to see what happens? What gain and tone settings are you using? Does it do it at any master volume setting?

 

[Franklin]

Thanks for your reply - and sorry for my bad (German-)English.

The Lead Master Control works, yes.
The volume does not change that bad sound, except on 1 and 0 - but even then I can hear that kind of feedback (very quietly, of course).
If I use the tone controls, the feedback changes to higher or darker sounds.
By the way: "Behind" the feedback the guitar can be heard, faintly in the background.

Do you think it would make sense to desolder the LDRs and check them ?

 

[LHanson]

I think it's time for professional help, actually.

One other thought: Have you moved the amplifier? Could it be some sort of interference or magnetic field nearby?

 

[DW]

Since you have determined it's not the pre-amp tubes... how loud are you playing? Are the pickups in your guitar well potted? Has the amp worked fine before and just started having this problem? Have you played that guitar through other amps with similar amounts of pre-amp gain and had no problem?

I say this a lot but my Mesa Mark III exposes pickups that are not potted well quicker than most any other amp I've owned and the result is a whistling. squealing feedback noise.

 

[Franklin]

Surely no interference or magnetic field. It doesn´t matter where the amp stands and with which guitar it is played.
One day I switched to the Lead channel and there was just that noise. Before that everything worked fine.

 

[Franklin]

For anyone who has a similar problem:
Finally I brought my caliber to an amp-tech and he found out that the problem came from the driver tube 12 AX 7 A. I changed the pre-amp tubes but kept the (very) old driver tube inside...
He also changed the EL 84´s and now the amp sounds really fine !