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I have a Traynor Ironhorse which is new to me. When I first got it, I noticed after about 15 - 20 minutes I would get a loud hum from the amp. Sometimes it was intermittent. I put new EL34's in it when I got it, and it seemed fine for a while. So played it today, and the hum came back. Decided to swap preamp tubes with a known good (brand new) 12AX7 one at a time - hum remained. Switched all the preamp tubes with some older ones I had - same. Put the original EL34's back - hum. I should mention that I did my testing with both pedals straight in and eliminated from the signal chain.

Finally, I got out an extension cord and plugged it into an electrical outlet on a different circuit. Hum gone now (played it for over an hour, usually starts within 15 minutes). Pedalboard reconnected, no hum. What drives me crazy is I have three other tube amps on the original circuit, never the slightest bit of noise (they're not all on at the same time). Anyway, it now sounds awesome and is very quiet. I'm not knowledgeable about how these things work but maybe this post will find someone else pulling their hair out and they can try a different circuit.
 

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It is possible you have a bad or loose ground inside the amp. To me, that's the most common reason for hum after tubes going bad.
 

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If you have an outlet tester, check that the outlet is wired up correctly.
 

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If you go back to the original outlet, is the hum still there? It’s possible in your tube swapping that you’ve removed some dirt or corrosion from the pins or sockets. Or the voltage may be different on the outlet.
 

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Tried plugging one of the silent amps into the socket the humming amp was/is in and see if it starts to hum? I had an amp that would only hum when I had another one on nearby. Regardless of what outlet or whether straight in, or through my board.
 

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something else to try ...
plug in the noisy amp to the clean circuit
plug another amp into your dirty circuit
then run a multi meter set on AC between the 2 amps chassis ( ground to ground )
if any voltage shows , the humming amp may have reversed ground wiring.
 
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