What to do about hum?

[rutledj]

What do I do if I've tried all the tricks to reduce hum and none of them work? It isn't a low level hum but a significantly loud hum.

1. Created Virtual CT with 100 ohm resistors. Quieter but not enough.
2. Tried raising the CT to 50 volts, more noise than if grounded.
3. Moved leads around all over without one iota of difference to the hum.
4. Hum level doesn't change with volume control up or down..
5. Turn reverb level up and hum increases some.
6. Unbolted reverb xformer and moved it around. No diff.
7. Changed tubes to no avail.
8. Ground out v2 output after cap. Dead quiet. Ground anywhere on tone stack and no diff.
9. Redid PS with diff caps. No diff.

At wtt's end. Please advise before I throw it out the window.

Rut

[fabiomayo]

That could be agreat thread. I have the same problem

Excuse me, but I'll hijack...

My amp is very quiet when the PI entrance is grounded.
I tried shielding the preamp heater wires and elevating them to
20v - noise is the same. BTW, noise is EXACTLY the same no matter
where I: set the multi-turn heater balance trimpot.

Noise dimishes when grounding a few tone stack spots.

My layout differs a lot from the tried and true ones - though
I tried to keep the main lead dress issues about the same.


Maybe it's the transformers?

[marshmellow]

What is really strange is that hum level doesn't change with your volume control. Hum is normally picked up by the first stages, so turning down master volume (if you have this) should make it dead quiet.

Whatever, easiest thing avoiding hum is heating with DC for the first stage(s). You could do it kinda like this.

[rutledj]

What is really strange is that hum level doesn't change with your volume control. Hum is normally picked up by the first stages, so turning down master volume (if you have this) should make it dead quiet.

Is there a definitive way to verify that the hum is from the heaters? It sounds like it by the tone.

[greiswig]

What is really strange is that hum level doesn't change with your volume control. Hum is normally picked up by the first stages, so turning down master volume (if you have this) should make it dead quiet.

Is there a definitive way to verify that the hum is from the heaters? It sounds like it by the tone.

6V motorcycle battery?

[mdroberts1243]

What is really strange is that hum level doesn't change with your volume control. Hum is normally picked up by the first stages, so turning down master volume (if you have this) should make it dead quiet.

Is there a definitive way to verify that the hum is from the heaters? It sounds like it by the tone.

Sometimes difficult to tell by ear... You could look at the frequency of the hum... 60Hz heaters, 120Hz from the HT DC supply.

[heisthl]

I know I've encountered this and I'm wracking my brains to remember all the things I've ran into in the past. I've seen hum with the same symptoms caused by 100k resistors instead of 100 ohm on the heater ground reference, I've seen the same symptoms corrected by moving the presence ground from the PI ground point to the front ground bus and vice versa. I've seen the same symptoms caused by not enough relay power supply filtering. I've caused the same symptoms by unintentionally doing a solder through that bridged to a B+ wire running under the board. I'll try to remember more....

[rutledj]

Just to clarify, This is not a Dumble build. It does use the dumble PS board (which I used in my non-hrm successfully - dead quiet). It is actually a variation on a vibraverb clone. I stuck with my dumble build as far as how I grounded things. Verified I have 100 ohm resistors on heaters.

I checked with a scope the hum freq and it appears to be about 120hz. If this is the case and I am using the dumble ps, what is left to do in that ckt? I have already changed out the caps in the PS.

Here are some pics of the build

http://s519.photobucket.com/albums/u360/jmrut/

[greiswig]

Like I said, wouldn't temporarily powering the heaters with a battery be a good way to eliminate or confirm them as the source of hum?

[heisthl]

Where's the bias supply filter cap or caps? Also I would float those PI grid wires up in the air away from the speaker leads.

[rutledj]

Like I said, wouldn't temporarily powering the heaters with a battery be a good way to eliminate or confirm them as the source of hum?

Yes if I didn't have to buy a battery

Where's the bias supply filter cap or caps?

It is on the side mounted board with the HV diodes. One diode/cap.

[heisthl]

Where's the bias supply filter cap or caps?

It is on the side mounted board with the HV diodes. One diode/cap.

I couldn't see it in the pictures - I'm assuming it is orientated correctly.

[rutledj]

Yes. The amp sounds terrific with the exception of the loud hum.

[tonelab2]

Try insulating input jack from chassis with fibre washers, sometimes works for unknown hums.

[tonelab2]

Forgot to mention, make sure ground lug of jack is still grounded of coarse.