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Triode setting hum


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My DT25 was giving me no volume output so I took to a music dealer repair shop to check it out. I've been playing with the unit for about 1 week after not using it for several months. This shop/tech has serviced the amp in the past,  used to be a L6 dealer, and has the L6 repair documentation. I live in Panama and this dealer no longer carries L6 products. There are no other dealers  here so I can't get any further assistance from L6 US which I understand.


The tubes had not been replaced in 2 1/2 years so we did that with tubes which I bought according to L6's specs. After that the amp was now giving output but it would stop intermittently during play after a few hours or sometimes there was no output when turning the unit on. Doing further troubleshooting they traced the problem to a relay unit which they replaced and now they have been testing succesfully for several days. There is one remaining issue with the Pentode/Triode switch. When switching to Triode mode the amp produces an unexpected hum. 


Does anyone know what the issue may be? Shipping this 30 lbs unit round trip to an authorized repair dealer in another country is obviously out of the question. (Might as well buy a new amp)



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Now this is just a blind guess, but if one relay already was broken, it might be another relay causing this. If one output tube gets switched to triode, but the other tube either is stuck in pentode mode, then the asymmetry in the push/pull stage could lead to hum.

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