Tracking a Princeton Reverb build hum!!!

[5thumbs]

Help! I'm about ready to give up ;-)

This is a build based on the Weber 6a14 schematic/diagram, using a JJ 40/20/20/20 cap can. Triode Electronics Power Trannie for a Deluxe Reverb (that includes a bias tap).
http://www.tedweber.com/media/kits/6a14_schem.jpg

Symptom:
60 cycle hum with or without anything plugged in. The hum is strong at 0 on the volume, goes away completely from 3-5, then from 6-10 the volume of the hum increases. The amp plays fine otherwise-all effects work correctly, just the odd hum that won't go away.

Attempted remedies:
Lifted grounds at input and moved to a "star" ground at transformer bolt
Changed to shielded wires from input through the tone stack circuit
Moved tone caps off board and mounted directly on tone pots
Re-wired the heater circuit and ran wires through sockets instead of around (ala Merlin)
Insulated bias pot from the chassis
Jumped new coupling caps throughout the circuit to check for any change in hum

Diagnostics:
Only goes away with 3rd preamp tube removed
No change with reverb leads pulled
I get a fluctuating mv reading at the input and volume pot ~-.20 to +.20 mv dc
I get 1.5mv dc across the Reverb pot and at the pot side of the .033uf tone circuit cap

Anybody have an idea as to what I am doing wrong?

Thanks for your help,
Tim

[nickb]

The behavior of the volume pot suggests to me that the hum is common to V1B and V1A stages. Are you sure it's 60Hz and not 120Hz?

If it's 120Hz then I'd check the "D" supply wiring and values. You can use your meter on the AC range to measure the hum on node D. It needs to be no more than a few mV. If D turns out to be OK then I'd sort out the hum problem with the volume and reverb set to zero first to isolate the stages.

If it's 60Hz then either bad tube or a heater wiring issue in the V1A/B stages would be my first targets.

I'ts not clear to me what you mean when you say "3rd preamp stage" - what is the V#?

[5thumbs]

Hi nickb,
I said the 3rd preamp tube not stage, so you would call it v3.

It is definitely 60 hz hum not 120z. I thought it was in or before V1, but when I pull v1 the hum is still there!

[DRH1958]

I'm wondering if it has anything to do with all four of the preamp stages hooked to the same "D" PS node without any decoupling between any of the stages. This could lead to the voltage fluctuation. This is a low frequency oscillation. Did it say why node "C' isn't used? Usually, there is more PS decoupling in the preamp. Seems like there should be another filter section to isolate V1a,b from the rest maybe? V3b and V4b seems as though they should be hooked to node C. Usually, the PI had higher voltage than the preamp sections. Also, did you try to just replace V3?

[nickb]

We need to isolate the problem. Also, you may have more than one problem area. Measure the hum level after each step using your meter on AC volts across the speaker and note the audible change. It will help to write down your results.

1) With it in the hum condition measure the AC volts across the speaker - report.
2) Set the reverb and volumes to zero - report.
3) If the hum is unchanged, connect V3B pin 7 to the ground on R28 - report.
4) If it stopped, it bring up the volume (leave reverb at zero) - report
5) Set the reverb back to zero bring the vol up - report

This will isolate it to the main path, the reverb path or V3B.

[mort]

Is there a center tap on the heater loop?

[mort]

Did you ever figure this one out?