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Output Transformers and Hum
Posted: Sun May 16, 2010 3:52 pm
by syscokid
Even though all the sound and great tone is there, can a defective OT cause a hum that increases in volume as the Master Volume is increased (in my 82' JCM800 50w)?
Help,
Mr. Electronically Challenged
Re: Output Transformers and Hum
Posted: Tue May 18, 2010 9:43 am
by syscokid
BTW...
There's a Mercury Magnetics O50JM in my JCM, that was installed in the amp about 3 years ago.
Re: Output Transformers and Hum
Posted: Tue May 18, 2010 6:29 pm
by Mars Hall
Bad tube? The problem 9 out of 10 times with a tube amp.
Re: Output Transformers and Hum
Posted: Thu May 20, 2010 11:54 pm
by syscokid
Thanks, Mars Hall for a reply. You're right, a bad tube could cause an annoying hum. But, what I'm seeking here is a little bit of knowledge on trouble shooting a bad OT. Maybe the OT in my JCM is fine, but I would still like to know if it's possible if any OT can appear to sound fine, yet produce a hum because of some part of it's function has failed.
Re: Output Transformers and Hum
Posted: Fri May 21, 2010 12:50 am
by flemingmras
No...a hum that decreases in volume when the master is turned down indicates that the hum is coming from a stage upstream of the master pot. Anything in the power section that causes hum would hum at a loud volume regardless of where the volume pots are set to.
Make sure your grid wires on your preamp tubes (wires going to pins 2 and 7) are elevated as far away from the chassis as possible. This helps IMMENSELY to reduce unwanted hum in the preamp. You need not worry about the phase inverter grid wires though as it operates differentially (i.e. in push-pull mode) so common mode rejection should cancel out any hum induced into the PI circuit.