Hafler T3 hum

[drewl]

Anyone have a schem for this?

Just found the T2, should be close enough to help.

The T3 has three channels, Ch1 is all solid state and dead quiet.
Ch2 & 3 use the two 12AX7 tubes, and I'm getting some really nasty 120Hz from those two channels, up to about 50mv of hum.

Power supplies look good with no noise, but it's picking up noise from somewhere after the gain controls and befroe the level pots.

[drewl]

Well I got a schematic for this I'll post, but I'm still not getting anywhere.
The hum is coming from the first tube , pull it and it goes away.
I think if they used dc for the filaments it would be alot quieter, but it couldn't have always hummed like this.

[g1]

How is the ripple on the B+ for the tube plates?

[Enzo]

DC heaters will ONLY improve hum caused by the heaters. it will have zero effect on hum from any other source.

[drewl]

The plate supply is clean, as are all the low voltage supplies.
The tube stages are working properly, with typical voltages and none of the coupling caps are leaking.
Don't know where it's picking the hum from.
aside from rebuilding the first stages I can't see what would cause the hum.


I'm trying to post the schem but my stupid computer keeps crashing.

[drewl]

here we go

[tubeswell]

So you swapped out the V1 tube?

[drewl]

Yes of course.

[g1]

If you leave V1 in and pull V2, the hum is still there?

[Enzo]

As far as I can see, all we know so far is that the gum is coming from the first tube or THROUGH the first tube. Pulling the tube not only silences what the tube might do, it also silences all noise from before the tube. For example is ther hum at pin 7 of the op amp? There are two selectable gain controls, soes it make any difference which is on? either of those JFETs could be injecting noise. Have you turned out the lights in the room? Aside from them making electrical noise, some transistors and ICs are reactive to light. Toss a piece of cardboard on the amp chassis, any change? For that matter, just groiund pin 2 of the first tube, then try pin 7. WHich ones kill the hum?

[drewl]

Did all that.
Gain controls have no effect.
Ground V1a input and it's quiet, so it's not the jfets, or opamps.

[g1]

Ground V1a input and it's quiet, so it's not the jfets, or opamps.

I came to the exact opposite conclusion? If grounding V1A input kills the noise, doesn't that imply the noise is coming from somewhere before that?
Or are you saying grounding anything ahead of R10 makes no difference to the hum?

[tubeswell]

Are the input jack tip-switches making contact firmly? (Or are they just lightly/intermittently making contact?)

[drewl]

Actually, let me clarify.
Ground pin 2 of the first preamp stage hum is stii there.

Ground pin 7 the next stage and the hum is gone.

So the noise is being picked up in that first tube stage, not before or after.

Started working on the filament runs to that first tube and it is already quieter.

[drewl]

Also correcting it's a 60hz hum I matched with signal generator.