Relays and hum

[drewl]

while reading the post on the crumble clone hum, Enzo you stated sometning about the relay currents causing hum.
I have a build which has some hum from the power trans, which gets louder when I switch in another 1/2 triode.....
I've done most of the usual tricks, but what can I do to see if the relay is contributing to the problem?

I think the main problem is the power trans is going bad....it hums without the standby on....
I've built 2 others like it which are dead quiet....the hum isn't noticeable when playing with a band, but hell...I,m a perfectionist!

[Enzo]

One quick way to see if relay coil currents - and fields for that matter - are causing trouble is to remove the relay. The relay connects a couple points together when it energises, so tack on a couple jumper wires to make those connections and disconnect the relay coil. Now the amp will function as if the relay were energized but without any relay coil activity.

If you need to still switch the relay functions, then tack in a small toggle switch to do the job.

ANother way would be to disconnect the relay coil from the circuit entirely and drive it from a battery for testing.

RElay coil currents tend to be more ripply than say preamp supplies. If the ground return path for a relay coil shares copper with the preamp circuit grounds, then that ripple will be impressed upon the signal.