Hum problem

Hi Michael,

To clarify, on my 70/70 there is a slight mechanical buzz (which I suspect to be transformer-related) for a few seconds just **before** the B+ relay closes, about 20 seconds or so after the amp is turned on. (I assume the relay is closing, i.e., completing a circuit path, rather than opening when it presumably applies the high voltage B+ to the tubes at that time). Also, the relay closure itself is essentially instantaneous on my amp, at least since VAC worked the amp over a few years ago when I sent it in for an unrelated repair. Prior to that the relay would clatter for a couple of seconds when closing, which it should not do if in top condition.

Best regards,
-- Al

Thanks- sounds like the relay is a secondary problem. The hum is nothing to do with the relay though. Its a process of the amplifier's operation. If you've already played with the tubes, then the filter caps are still on the table.

Thanks Ralph. Good to know.

I have  a few questions.

If your amp did not hum at the techs shop then it's not the amp.
1. Is your amp in a rack or free standing on the floor.
    ( if in a rack take it out of the rack  / unplug everything except the speakers )
2. You lifted ground, how did you do that . Switch on the amp or removing the ground leg off of the power cord. You need to lift the electrical ground of the power cable to the amp.Using a 3 to 2 adapter  does the trick put you have to pull off any metal loop that would touch the center screw that holds the wall plate on the wall outlet plug.
Take your power DODD transformer out,plug straight into wall jack.
3. Have you tried other speakers to make sure it's not your speakers crossovers.
If the ground is till there then it's the amp for sure.
 If you are clean then plug in the Preamp  with nothing in the input section,turn it on. ( do not put the frame of the preamp in contact with the amp frame ).
4. LED lights of any kind on the audio AC wall socket all the way to the circuit breakers ( notoriously buzzy and it goes down the ground leg)
Same applies to dimmers of any kind.
5. If you put a meter on your AC wall jack ,what is the reading between the + leg to ground ( should be 120V ac ) then - to ground ( that should be 0 ac volts ).
If you have dimmers on that AC line turn them low to midway and then check your ground voltage again.If it goes higher than previous test with them off you might have found your problem.Even completely off LED light circuits still burn power that is  leaked by the dimmers ( hence LED light glow when in the off position )

It makes no sense that the amp is quiet with another system and installed in your set up it starts buzzing.
Anyhow a lot of good thoughts in the upper threads but there is no mention on how your systems is installed in the room ( rack or no rack ) Components touching other components. I fight those issues daily with amp racks more so in permanent installation steel frames and rails.

Good luck.

I had this with an Audible Illusions modulus 3a pre. The tech fellow I hired could find no explanation. I used a DAC direct to power amp for about 3 Mos. One day I tried the preamp and the hum was gone. My neighbor was big into ham radio. He had moved. So rf?