Massive hum - B-15N

[txstrat]

Yeah, I though so too, but I was hoping they got lost over the resistors.
Can anybody explain this?

I changed the input jacks for isolated ones and split the grounding of the preamp and PI tube (these shared the ground before).
Put the ground for the preamp (input jacks) to the star ground at the PT bolt.
Can't say that all this helped a lot.

[loudthud]

The hum fades out in a second or so. It surely doesn't stop immediately. Sort of like when you play and switch the amp off.

When you switch the AC mains off in the play mode or simply un-plug it from the AC mains, it better stop humming or the hum has to be coming from your guitar or your amp is possesed!

[txstrat]

I haven't put it right. I mean when I throw the stand by switch off play mode.

[txstrat]

I changed the preamp tubes (and sockets) to 12AX7 and finally found that the hum was indeed lead dress related. Changed the OT from x-mounting to outside the chassis and shorended the B+ to the OT and the wires to the output tube plates. The amps is now playable even though it hums when the volume is full open (doesn't motorboat or howl anymore). The hum is more high pitched now. Might be the preamp tubes which now have an amplification factor of 100 instead of 70 (6SL7).

[Tom Phillips]

A wire routing suggestion.
I noticed in one of the photos you posted that the primary wires from the OT were routed under the parts board and they were in close proximity to the input signal leads. This is a potential cause of undesirable coupling between the input and output stages. This may have been fixed during your latest changes. If not, it is an area for potential wire dress improvement. Did the lead routing change much when you remounted the OT? With your layout there is no easy clean rout for the primary leads between the OT and the power tubes but I think there is room for improvement.
Regards,
Tom

[txstrat]

Thanks Tom for your suggestion. Right before I read your message I'd finished the remounting of the octal tube sockets.
The OT is now sitting unter the chassis and the leads don't get too close to the preamp section anymore (maybe there's still room for improvement).
I'm using different tube sockets now, to rule them out as a potential source for the hum.
The amp is still aplayable but the 6SL7's still hum a lot when I turn up the volume. Way more than the 12AX7 but also with a lot more headroom.
Think I'm gonna order a new set of Tung Sol 6SL7 tubes (these were the cheap no name ones).
And tinker around with the lead dress layout. Any suggestions?
Gonna add more pics later.

[Tom Phillips]

Looking forward to seeing the pics of the current version. Please include a shot that shows the complete layout from one side to the other.

[txstrat]

I got my two other 6SL7 (TungSol) and tried them. They hum less, but still do. Also the hum goes up with the volume pot.
When I pull the preamp and PI tubes there is just a slight hum like I would say is normal. When I put the PI tube back in it changes a little to the worse, but still nothing I would worry about. When I put the preamp tube back in (volume down), the hum keeps almost the same. If I start turning up the volume, the hum gets louder until the amp starts squealing (depending on the treble setting).
Next thing I'm gonna do is take a lead with alligator clips and ground the grids of the preamp tube to see if I can narrow down the place where the hum starts.
Also think of using shielded wires for the preamp grids. Any ideas?

Add: I left the mixing resistors (those two that mix the channels before the PI) out of my circuit, since I have only one channel and thought I wouldn't need it. Could this make the preamp signal too strong for the PI? Should I put one 270k between preamp and PI

[tubeswell]

Have you got long signal wires (to grids)? They could be picking up this.

[txstrat]

From the input jacks to the board (to the input resistors) it's about 3 inches. From there to the first grid it's about 4 inches. From the volume pots wiper to the 2nd grid and from the treble pots wiper to the 1st grid of the PI the wires are about 7 inches each. It doesn't change the hum when I move those wires, though.

One thing I haven't been thinking about before is that the OT has extra taps for the screen grids. I don't use them (just left them unconnected). Does this make any difference?

[txstrat]

OK I wired the grid taps of the OT to the grids to no avail.
Next step I'm thinking about would be an increased bias filtering.
I found that if I throw the power switch to off (not using the stand by switch), the hum stops instantly. Does that narrow it down?

[Tom Phillips]

Do you have photos of the current version?

Re: My message of 1 November. "Looking forward to seeing the pics of the current version. Please include a shot that shows the complete layout from one side to the other."

[booj]

the layout shouldn't be a problem, since I built this amp once before with almost the same layout and it doesn't hum at all (ok at least nothing to worry about).

I'd go back to that!

[txstrat]

I moved the bridge rectifier away from the bias supply. I increased the bias filtering (it's now 2 x 100uF). Still hums.
The pic is the most recent (original size).
Can the main and screen filter caps (the blue ones) be too close to the PI circuit?
Can the output (speaker) jacks be too close to the preamp?

[txstrat]

I'd go back to that!

I thought of that too, but it even hums, when I disconnect the preamp (not the PI) and use a different amps preamp.