• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Big Big hum ???

Hi, i was testing some mod i done to my amp and suddenly the only thing i heard was a big hum. What could it be ? Can my output transformer burn out ???

I took some voltage and found out that one of my decoupling cap fail. Instead of having 270v i was reading 50v. I change the cap and the voltage was up again. But there was this big HUM. Can the voltage peak that burn out the cap destroy the output transformer too ?

Thanks for the help....


diyAudio Moderator Emeritus
2004-01-27 8:55 am
You don't mention the type of transformer/rectifier arrangement you are using for B+. If you do have a center-tapped secondary with full-wave rectification using two diodes (as ooposed tp a single untapped winding and a diode bridge), then it's possible one half of the winding could have burnt out as a result of a partial short-circuit caused by the bad cap. This would give you half-wave rectification and 50Hz/60Hz hum.

Remove the rec tube, and check for correct voltages at the anode connections on the socket. If one is missing, then the mains Tx could be faulty--check all connections to secondaries. If OK, try another rec tube--could have suffered when the cap went. If still hum, check all other smoothing caps or chokes in the supply--they could be bad.

Worth checking for heater/cathode shorts in all tubes except rec too! Unlikely the O/P Tx is bad, but possibly Mains Tx could be.:mad: