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View Full Version : Diagnosis for my Mesa?


Tommy_G
10-07-2012, 10:43 PM
My Mesa Heartbreaker Head was working perfectly (and with perfect tone) hours before I packed it up and drove 3 hours to a gig.

I set up the rig at the gig location and the output level of Ch2 was quickly getting weak relative to its balance with Ch1. Turned off the amp, turned it back on...no sound from Ch2.

Swapped in my backup Tungsol RI into the first valve position of the Ch2 preamp. Nothing. Went to a music store and bought 2 more Tungsol RI's. Nothing.

Took out my "desperation tube", a strong example of a Sovtek WA. Now I got all kinds of volume, but the breakup on 10:00 was where it normally is at 2:00 on the gain. Thinking something may have magically fixed itself, I put back in one of my backup Tungsol RI's. Nothing.

Then, simply to salvage a semblance of decent tone for my gig, I pulled my NOS Raytheon from the cathode follower and put it in the first gain stage of Ch2, and put a spare Groovetubes Chinese into the cathode follower position.

Good tone, but again, the onset of preamp breakup was wayyy too early.

Any ideas on what could be going on where the Tungsols no longer work, but super strong tubes do work in the first valve position of the Ch2 preamp (actually V3 on this amp) but with over the top gain?

For instance, is it more likely that a tube stage cathode bypass resistor or capacitor is shot, or more likely that the resistor going to ground ahead of the input/grid of the tube is no longer in spec (sometimes called Rg) ? Or something else? Is it even possible that you can have 4 bad Tungsol RI's in a row?

The last known "oopsy" with this amp was when I overdrove the crap out of the front end of Ch2 with a OD pedal. Could that have blown something out? I did smell some magic smoke at the time, but I popped open the amp and couldn't identify anything that was obvious on the circuit board as being burnt.. the amp has been working ok for the past 2 months since that event happened but I did replace the first valve stage of Ch2 since then.

phsyconoodler
10-08-2012, 01:34 AM
Mesa amps have to have chinese or JJ style ubes in the cathode follower positions.Most other tubes,Russian especially can't handle the voltage.
I really don't have an answer why your RI Tungsols didn't work other than they are bad.
You may also have a tube socket pin issue???