Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on August 02, 2021, 06:59:54 pm

Title: Diagnosing hum
Post by: fossilshark on August 02, 2021, 06:59:54 pm

Hi all, I have an amp here with a nasty hum. The hum is not present when the master volume is all the way down, but when there is signal going through the amp the hum is always consistent and no controls affect it. Also the amp takes a really long time to cut volume after switching off standby.

I suspect the filter caps, but how can I be sure? All plate voltages measure normal as well as the bias supply, how do I begin diagnosis?

Thank you for your patience, I am trying to learn.

Title: Re: Diagnosing hum
Post by: jordan86 on August 02, 2021, 09:29:00 pm

This may be a helpful place to start.
http://www.geofex.com/ampdbug/ampdebug.htm

Title: Re: Diagnosing hum
Post by: Willabe on August 03, 2021, 04:29:55 pm

Can you post some pics of the inside/guts and flip side/iron set placement?

Is this a new build? Did it always have a bad hum?

How did you wire your grounding?

http://www.valvewizard.co.uk/Grounding.html