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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...

Thank you for your advice. Already posted a thread there: http://music-electronics-forum.com/t40525/ Maybe I will finally fix this issue.

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...