Locating loud hum/noise

[EETStudent]

I need a little help locating a very obnoxious noise/hum in my twin reverb clone. The sound is very similar to an ungrouned volume control, a loud buzzy hum. I determined it is in the preamp by pulling the PI and removing the noise. I then disconnected the PI coupling cap and it came back. My assumption was that its in the phase inverter. However, changing the volume level increases the noise. This noise is very loud, so I feel like there is a major issue that I just cannot find.

[Prairie Dawg]

I need a little help locating a very obnoxious noise/hum in my twin reverb clone. The sound is very similar to an ungrouned volume control, a loud buzzy hum. I determined it is in the preamp by pulling the PI and removing the noise. I then disconnected the PI coupling cap and it came back. My assumption was that its in the phase inverter. However, changing the volume level increases the noise. This noise is very loud, so I feel like there is a major issue that I just cannot find.

Is this a new build?

[EETStudent]

Yes its a brand new build

[booj]

Are the filaments grounded through two 100 ohm resistors, or is the 6.3 volt winding center-tapped?

[Prairie Dawg]

Yes its a brand new build

Here's what you do, and that means to validate your build-you can't troubleshoot unless you do-sometimes you can get lucky, of course, but you can't count on it. Meaning no disrespect but 95 per cent of the time whatever problem emerges in any device can be traced back to the last person who worked in there-that's fundamental trouble shooting theory. So it's either something you did or some component that was not up to snuff (it happens sometimes).

Get yourself a nice big copy of the schematic. Tape it to the wall next to your workbench. Equip yourself with a highlighter. Then, proceed to validate each and every circuit path and all the component values along that path. As you go, mark it off on the schematic, until you have validated your build and everything in it. Look especially closely for wrong component values (easy to do late at night) and improper terminations.

Once you've done that, you can proceed to try and locate your problem but you may well have discovered what it was through this process. But what you will have is a stable platform of known good quality to work off of. That makes troubleshootign a lot easier.

[EETStudent]

Actually, under the noise, the tone is very, very good. I'm getting the correct voltages everywhere, the correct bias, correct signal and power, etc...and the result is good tone.

There must be something electrical goin on here. I mean, this buzz is LOUD. Way louder than the guitar signal. Its ridiculous how loud it is. Its not 60 cylce hum, and I have never heard 120 cylce so I dont know if its that. I have done a hundred checks for continuity and everything is fine. I have done a hundred voltage checks and they are all fine.

Let me reiterate the fact that with the phase inverter coupling cap removed, the buzz is present as ever. Someone confirm for me that the trouble is in the power amp/PI section

[EETStudent]

Should there be any AC voltage on the plates of the 6L6s?

[bkahuna]

Yep. If they're passing audio, there will be AC there.

First, you said pulling the PI couplers killed the buzz, then later you said with the couplers removed, the buzz was still present... which is it?

booj also asked a valid question which you haven't answered, yet. Do the filament winding have a center tap to ground, or do they reference ground through a pair of resistors?

[txstrat]

Are the filaments grounded through two 100 ohm resistors, or is the 6.3 volt winding center-tapped?

I agree
If you don't ground the heaters at the CT or virtual CT (with two 100 ohms resistors) you get major hum. I'd check that first.

[tubeswell]

And you swapped out the PI tube for another? (to rule out bad filaments in that tube?)

Has this LTP got a presence control? And if so, does the hum vary with the pot setting?

[EETStudent]

When I pull the PI tube, I actually do hear that buzz but at a much quieter volume. When I put the PI tube back in, its like that existing buzz is just amplified. And yes, I did try tube swaps to no avail

The filaments are artificially center tapped through two 100 ohm resistors.

Back to the PI. Sorry for the confusion, but with the PI couplers disconnected and the PI tube installed, the buzz IS there. With the PI removed altogether, that is when the buzz is toned down. Im almost positive its not in the preamp after all.

How much AC should I expect at the power tube plates? Right now Im getting about 8VAC at the 6l6 plates, and about 1.5VAC at the PI plates. Everywhere else is less that 300mv.

[txstrat]

However, changing the volume level increases the noise. This noise is very loud,

Generally spoken, when the noise changes with the volume pot the culprit lies BEFORE that pot.
IS the noise already LOUD when the volume is turned all the way down? Or does it start and get louder with volume?

In my latest build I've learned that elevating the heaters can cure even very strong hum. Have you considered that? It's just two resistors and a cap.

[Enzo]

Elevating heaters only cures hum caused by the heaters. It does that well, but won;t affect other sorts of hum.

"PI couplers removed" To me that means the caps that couple the PI signal to the power tube grids. By removing them, you break the signal path from the PI, so whether the tube is there or not shouldn;t matter. Since it still turns the hum off and on, I have to wonder if you mean some other caps. Or the PI tube is somehow loading down the B+ or something.

[txstrat]

Elevating heaters only cures hum caused by the heaters. It does that well, but won;t affect other sorts of hum.

I know, I just thought it might be similar to the problem I encountered in my latest build. The hum was not too bad when the volume was turned down but grew to the worst hum I ever heard when turning the volume pot up.
The heaters hum was somehow amplified by the preamp. At least I think so, since the hum was gone when I elevated the heaters. Could that be?

[loudthud]

Did you check the resistance to the chassis of every point that is supposed to be grounded? Especially the filter caps and in the bias supply.

Does the hum go away immediately when you put the amp in standby mode or does the hum fade out over a period of a couple of seconds?

Each side of the heater supply should measure about 3.2V AC and 50 ohms to ground. Please confirm.