Need help with hum

[sliberty]

My UniWreck (an EXpress born from a salvaged Univox head) is coming togather. I believe I have eliminated the oscillations I experienced early on. I may have some tone stack concerns (mid control not working as expected for example), but thats for later.

Now I am working on a hum issue. I believe it is 120hz hum (approx b flat on guitar A string).

It hums without a guitar cable plugged in, and it hums the same with controls zero'd or up at normal levels. I am not sure if this is relevant, but I noticed that pulling just the 1st or both the 1st and 2nd 12AX7's didn't effect the hum at all, BUT if I pull all 3 - 12AX7's, the hum diminishes big time - almost completely gone.

For what it is worth, I didn't use the simulated center tap on the heaters - I used the real heater center tap that Moose's transformer provides. Maybe I should try the 100 ohm resistors instead???

Any and all ideas are welcome.

Thanks,
Steve

[UR12]

My UniWreck (an EXpress born from a salvaged Univox head) is coming togather. I believe I have eliminated the oscillations I experienced early on. I may have some tone stack concerns (mid control not working as expected for example), but thats for later.

Now I am working on a hum issue. I believe it is 120hz hum (approx b flat on guitar A string).

It hums without a guitar cable plugged in, and it hums the same with controls zero'd or up at normal levels. I am not sure if this is relevant, but I noticed that pulling just the 1st or both the 1st and 2nd 12AX7's didn't effect the hum at all, BUT if I pull all 3 - 12AX7's, the hum diminishes big time - almost completely gone.

For what it is worth, I didn't use the simulated center tap on the heaters - I used the real heater center tap that Moose's transformer provides. Maybe I should try the 100 ohm resistors instead???

Any and all ideas are welcome.

Thanks,
Steve

Steve

The CT on the tranny should be sufficient without adding the 100 ohm resistors. If the hum goes away when you pull the 3rd 12ax7 then the problem sounds like it is in the PI or power tube section. I would check the wiring there , maybe moving a few wires around to see if that doesn't help. It could be the long runs coming from the coupling caps on the board to the grids of the output tubes also. Your heaters are running 60hz so if in fact you are hearing 120hz then this is usually caused by something in the High voltage supply either bad filtering, leaky coupling caps, Plate wire runs too close to grid wire runs. Also have you checked your bias and made sure your EL34's aren't red plating. Just a few things to check...........

[2thick4uni]

Check out this thread :-

https://tubeamparchive.com/viewtopic.php?t=622

Paul Ruby's comments were very relevant and useful to my wreck build, I had a buzz on mine when I first fired it up but I rewired it and rearranged the grounds as per his instructions and it made a huge difference, its almost noise free after rewire - well, no, its very loud actually, but I'm sure you know what I mean

Allan

[sliberty]

Dana,

Thanks for the all of the ideas. I will check them out this evening (after I finish helping my nephew with his new notebook computer).

The only thing I have done with bias so far is to set it at -33 VDC, which is what the schematic notes. This has enabled me to run the amp without red plating the EL34's. But beyond that, I haven't made any fine adjustments yet.

I'll keep you posted.

Steve

[philmanatee]

You probably know this already, but I eliminated some noise issues by isolating the amp from the speaker cabinet. Also having the shield in the bottom of your cabinet helps too. Phil

[changeling]

You probably know this already, but I eliminated some noise issues by isolating the amp from the speaker cabinet. Also having the shield in the bottom of your cabinet helps too. Phil

hey phil,
what do you use to shield the cab?
i am waiting on my cab and i would like to shield it when it gets here.
thanks,
r

[philmanatee]

I just used a sheet of aluminum and screwed it to the bottom of the cabinet so that it shielded the bottom of the chassis. Since my cabinet was already finished to fairly tight tolerances I had to do some filing on the front edge of the aluminum, and on the inside of the front panel on the cabinet so that the chassis would slid in and out for service. There are other materials that would work such as screen like fender used. Phil