• 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.

amp buzz in one channel on inrush / power on


One channel of my amplifier has a significant buzz/hum when initially turned on, then subsides after 1-2 seconds.

What can be the cause of this?

Please help me find the problem... if it is even a problem :)

both channels do it, however one is significantly louder than the other.

What should I look for?


2013-04-06 11:52 am
Do you have an oscilloscope? A way to look after the problem is watching the voltage across an artificial load to see what happens there and after that digging more in the amp.
Do you have a schematic of your amp? How is the power supply built? Is there any softstart?
There was a soft-start on a relay, but I disconnected it as it became unreliable (occasionally needing a tap with a screwdriver to engage). I intend to rebuild that with a manual switch instead. I understand it's due to inrush current and started the same time I disconnected the soft-start, but my only concern is that the one channel is significantly louder, and would like to diagnose this if it is a problem before proceeding to re-implementation of the soft-start switch, I'd rather not ignore it by masking it. Also no oscilloscope on hand.

Can tube filaments cause this, maybe I'll start by swapping L to R tubes see if there's a change to the other channel.

BTW all diodes and capacitors seem to balance well enough without dismantling for accurate measurements...for now.