Hum on Tube Amp - Can't find source

Ok. One more improbable guess. Are the mono blocks near one another or the pre amp? I’m thinking power supply interference. As in the PSU on an Amp or the Pre acting up. The hum problem exists using other circuits so it’s cause is something in your equipment.

Got a rubber mallet? Did you try tapping?

“Give a small boy a hammer and he will find that everything he encounters needs pounding.” Abraham Kaolan

@ervikingo 

I'd love to look at the videos you posted, but I don't have Dropbox, and don't want to sign up for even a trial membership. Can you think of any other way to post them? 

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? 

the changing glow on the tubes. 

...on all tubes at the same time?

Once the noise starts you can't bias them as the voltage cycles.

I'm not 100% sure, but I don't think that's normal. Of the one stereo tube amp I've had in the past with adjustable bias, I never witnessed anything like that when biasing the tubes. Once the tubes warmed up, the bias values were stable. I've worked on many high power FM transmitters with tubes in the past, and never saw tube bias fluctuate very much there either. Once set, it remained there for a lengthy period of time.

I'd like to thank you @ervikingo for staying engaged in the conversation. Many come to this board, throw out a question or problem they're experiencing, and never show any interest, or offer any feedback after that.  

@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!