Fender "the twin" hum

[timgman]

I have a 1990 fender "the twin" that has a loud hum after It's taken off standby.
I replaced all of the tubes with known good ones from my mesa nomad 100.
It did have a loose resistor (a 30k 10 watt ceramic resistor) that looked to have taken some heat and lifted the traces off the underside of the board. I re-soldered the resistor after it tested correctly.
The amp still exhibits hum in the following manner.

1. On standby the amp is quiet
2. both 25 watt and 100 watt mode exhibit the hum
3. After reading up and listening to samples it appears to be a 60hz hum not a 120.
4. the reverb / loop tubes seem to react when they are touched (they are micro phonic but known good tubes)...

Is this something the 300v 220uf caps might cause I suspect they are original. Is there a way to safely test these?

[pdf64]

See http://bmamps.com/Schematics/fender/the_twin.pdf
What are the voltages at TP281 and TP282 (power tube cathodes)? I think they may be accessible at the back panel.

[timgman]

See http://bmamps.com/Schematics/fender/the_twin.pdf
What are the voltages at TP281 and TP282 (power tube cathodes)? I think they may be accessible at the back panel.

Would those be the bios test points on the back?

[g1]

Yes, that's them. If you don't have the 0V between center and right points, you may get hum.
Center to left is bias, factory setting is .04V, later versions they said up to .08V for better tone with trade-off being reduced power tube life/reliability.
The filter caps you mentioned would give 120hz hum rather than 60hz hum. However, bad filter caps in the bias supply circuit could give 60hz hum (C305, C302).
Plug into the "power amp in" jack and see if it has any effect on the hum.

[timgman]

Plugging directly into the power section I still get the same hum.
When I get home, I'll report back what the results are on the bias readings.
Thank you for your time.

[timgman]

The bias and balance seem to be exactly on as per the chassis notes. .040 bias and balance was super close to 0.. I turned it to 0 and still have hum.

[timgman]

The bias did say "- .040" .....is the center where I put the positive or negative connector of my meter? I assumed that was a ground

[drewl]

Try removing the phase inverter preamp to to see if the noise is coming from the output tubes.

Does this amp have a hum balance pot?
I forget.

[timgman]

Try removing the phase inverter preamp to to see if the noise is coming from the output tubes.

Does this amp have a hum balance pot?
I forget.

If I remove Just that tube v4 (12at7) and power it up it is dead silent.
In fact if I remove only the one just to the left of the PI tube v3 (12 ax7) It is also dead silent.

Tube assignment As in this doc

http://music-electronics-forum.com/a...atic-parts.pdf

[drewl]

Okay, good.
It's not the output, try and narrow it down to which stage by removing preamp tubes working back from the output tubes like you are.

The first preamps are the most sensitive to noise.
Sometimes just swapping tubes around by putting the quietist one in v1 position will do the trick if there's not any other problems.

[timgman]

Okay, good.
It's not the output, try and narrow it down to which stage by removing preamp tubes working back from the output tubes like you are.

The first preamps are the most sensitive to noise.
Sometimes just swapping tubes around by putting the quietist one in v1 position will do the trick if there's not any other problems.

It seems that no matter what tube I put in the v9 position it's microphonic. does that mean anything?

[g1]

If pulling V4, or even V3, kills the hum, then I don't understand why the hum is there when you plug in to the power amp in jack. Can you double check this, and short tip to sleeve of the cord (or dummy plug) you plug in there?

[timgman]

If pulling V4, or even V3, kills the hum, then I don't understand why the hum is there when you plug in to the power amp in jack. Can you double check this, and short tip to sleeve of the cord (or dummy plug) you plug in there?

I'm positive that this I'd the behavior.
I pull the pi tube and it all goes silent.
I pull the v3 and the same
I plug into the effects return or power amp input with a shorted cable and I get the loud hum
If I tap the tip of a cord plugged in to either of these I get loud hum and signal from rapping the tip.
Thanks again

[g1]

Then I would think it must be V3 related. If it were any earlier in the circuit, plugging into the amp in jack would disconnect it. If it were from V4 or later, pulling V3 would not kill the hum.

*** to anyone reading this, the V3 and V4 designations are meaningless without the diagram from the pdf in post #9 (pg.5 of pdf).
This amp uses very non-standard tube designations.

[timgman]

Then I would think it must be V3 related. If it were any earlier in the circuit, plugging into the amp in jack would disconnect it. If it were from V4 or later, pulling V3 would not kill the hum.

*** to anyone reading this, the V3 and V4 designations are meaningless without the diagram from the pdf in post #9 (pg.5 of pdf).
This amp uses very non-standard tube designations.

To confirm
v4 = PI
v3 = effects loop?
OK, I'm going to re-evaluate the solder points on the socket ans see what I find. I'll also post a pic of that area in a few.