Hum on Tube Amp - Can't find source

@ervikingo There are a number of AC line problems that can cause diodes to radiate noise which can get into the audio circuitry. Chief among these is the 5th harmonic of the line frequency (in the US, which uses 60Hz, this would be 300Hz).

Harmonics of the line frequency occur when power transformers upstream from your house are loaded too heavily. If they are loaded past 50% of their ratings is where you get into trouble.

If this is happening sometimes the power company will be willing to replace the transformer if they can find which one is doing it- the 5th harmonic can cause their equipment to run hotter and be less reliable. So it might be worth it to have someone from the power utility to come by and do some tests.

Another way the amps could be in trouble is if the safety ground wire and the neutral wire were swapped somewhere. I've seen this problem happen in the US and it can cause a lot of buzz and hum!

For those interested in monitoring their AC power quality, this is a cool little logger, which can also read some real-time values.

Simple Logger® II Model L261

Hi ervikingo:

Are you using a dedicated line?

Try to twist to single wires between them and connect from main breaker to your amps. This is one way to avoid some frequencies to interfer.

You can also remove ground fir a while from your socket.

Ask for the guy if you are not familiar dealing with electricity.

Good luck!

@dpop thanks to you and all others staying engaged with my dilema. I’ve been at this for weeks.

"After powering up the amps, how long before the hum appears? Does the hum then remain constant for the rest of the time after it first surfaces, or does it come and go?"

It takes 1 to 2 minutes to manifest. Once it starts, it doesn’t go away

"...on all tubes at the same time?" That I can see on all EL34/KT77

I’m going to upload to youtube and post a link

@wsrrsw It ain't Hammer Time buddy!   Noise is present even without interconnects