Finally i finished this repair.
Hum has been caused by a missing contact with ground rail on one of the master potentiometers.
Gain pot on red channel is 500K. I left it in place to do not confuse amp's owner with different amount of gain.
Initial issue seems to be a EH tube in the V3 stage...
Update!
I got a reply from Mesa support.
They said something like:
We don't have time to answer on these technical questions.
First models of rectifiers have good drive and clean is not such good as drive.
If you want good clean - buy newer models of rectifier.
(this is what I understood from...
I wish you to get well and soon.
I already sent an e-mail to the Mesa support.
They asked me for a serial number to provide me a schematic.
I asked to forward my message to the technical stuff.
Still don't have a reply, but will wait some more days.
Hello,
Sorry for delay. I was ill and spent a week in my bed.
Now I'm back!
I excluded the V3a and V3b Stages from this amp. I soldered these stages on a separate tube socket and connected wires to a PCB.
(Ground, Power and Input)
This was made to exclude any impact from a PCB side.
Also I...
I'm back.
I tried two more tubes to see if it helps to resolve this problem.
Measurements:
All 3 tubes are working with hard enough clipping.
Sylvania distorts hardly than others.
The other strange thing that for all 3 tubes Ug2 > Uk2 about 4...10V Difference
But in the original schematic...
Sorry for my English.
I'm not a native English speaker.
I desoldered V1 V2 V3 sockets and cleaned them in solvent.
Also I cleaned all tracers and solder joints with solvent.
After that I dried PCB and sockets for about 2 hours.
Then I soldered all sockets back without using of additional Flux...
I checked all LDRs separately.
Also i tried to run V3a without 1uF capacitor (simply desoldered it to prevent any impact on gain)
LDR3a-b works well. when I shorting its leads with a screwdriver - I can hear a Gain increase.
Internal Plate resistance is about 62K in parallel with anode...
My friend asked me to help with his Triple Rectifier 150 W dual channel Amp.
recently he had an issue with a V3 tube.
Symptoms - volume drop on all channels. After replacing - am worked fine.
One month later he had another issue with a send-return loop switching.
Repaired - One dead optocoupler...