Fender "the twin" hum

[Mick Bailey]

It's worthwhile checking the board revision to see if the reverb recovery mod (TN96-5) has been carried out. I found this reduced the hum a fair bit even with the reverb turned down. It needs doing anyhow.

I also ended up 'auditioning' preamp tubes to get the best combination that would further reduce the hum.

[pdf64]

See p19 of http://bmamps.com/Schematics/fender/94-twin.pdf for TN96-5

Yikes - The Twin got a lot more complicated from 89 to 93 (compare the above to the one linked in post #2).

[timgman]

See p19 of http://bmamps.com/Schematics/fender/94-twin.pdf for TN96-5

I think that amp is an entirely different amp than mine?

Yikes - The Twin got a lot more complicated from 89 to 93 (compare the above to the one linked in post #2).

I'll check the board revision.
compared to the pics of the red knob images the other user posted above, mine looks super messy inside.
I had a beer last night and I didn't want to get back inside the amp after that so I'll carry on tonight.
Tonight I'll pull the rear board and look for anything out of spec or nasty.

[timgman]

V3 effects loop /send return

So It would appear that v3 has everything to do with the effects send / return.
I'll be that there's something buggered up in that area that's the root of most / all of this.
I really appreciate the guidance /confidence you guys are giving me. I think I partially understand the heater section. but is the 2.9 v on either green enough voltage? is it 2.9 x 2 = 5.9 ish or is it just 2.9v and I'm not getting enough heater voltage?

I printed out the schematics on a large sheet and I'm highlighting the signal path to try and get this ideology..

[Enzo]

The hum balance resistors are OK, as was mentioned above, they are paralleled by the transformer winding, so since they are 47 ohms, I'd expect them to read half that, or about 23 ohms. They are fine.

WHAT did you plug into the power amp in jack? Do you have a shorting plug? A plug with a wire inside grounding the tip to sleeve. Plug a guitar into the jack and turn its volume to zero. ANy difference? If that still hums, and pulling V3 kills the hum, then the triode of V3 after the loop seems to be central to it. V9 is the reverb circuit and if you plug into the power amp jack, that should eliminate any involvement of V9.

5.9vAc on the heaters may be a little low, but is within 10%, so the amp should work. It makes loud hum doesn't it? The tubes are thus working.

With my zeroed guitar or shorting plug, if the amp still hums, does the loop level switch affect the level of the hum at all? or affect it in ANY other way?

PIns 1 and 3 of V3a, schematic says 264vDC (I think), is yours remotely close? What is on pin 3? For that matter, are all the B+ voltages more or less close?

[timgman]

The hum balance resistors are OK, as was mentioned above, they are paralleled by the transformer winding, so since they are 47 ohms, I'd expect them to read half that, or about 23 ohms. They are fine.

WHAT did you plug into the power amp in jack? Do you have a shorting plug? A plug with a wire inside grounding the tip to sleeve. Plug a guitar into the jack and turn its volume to zero. ANy difference? If that still hums, and pulling V3 kills the hum, then the triode of V3 after the loop seems to be central to it. V9 is the reverb circuit and if you plug into the power amp jack, that should eliminate any involvement of V9.

5.9vAc on the heaters may be a little low, but is within 10%, so the amp should work. It makes loud hum doesn't it? The tubes are thus working.

With my zeroed guitar or shorting plug, if the amp still hums, does the loop level switch affect the level of the hum at all? or affect it in ANY other way?

PIns 1 and 3 of V3a, schematic says 264vDC (I think), is yours remotely close? What is on pin 3? For that matter, are all the B+ voltages more or less close?

I plan on having achance to test this tonight. I'll post the results (I haven't given up yet) lol

[timgman]

ok v3 has .003 volts on pin 1 and pin 3 ? hmm

[g1]

I don't think you would have got any sound if this were correct, is that DC volts?

[timgman]

I don't think you would have got any sound if this were correct, is that DC volts?

I'll retest to make certain.

[timgman]

I'll retest to make certain.

Soo.
C117 is totally missing. Does that seem sane?

[timgman]

Soo.
C117 is totally missing. Does that seem sane?

The schematic calls for a 10 pf cap there. how many volts does it need to be?

[Tom Phillips]

...C117 is totally missing. Does that seem sane?

I am not finding a "C117." Please tell us where you find the reference to C117.

[Enzo]

Look just above the reverb select switches, it is parallel a high value resistor R101. This looks to be more or less the cap across the 3.3M resistor in most older Fender reverb amps. If you find R101, it should be right next to it. And the fact it is missing is not an invitation to stick one in there. it could well be the factory decided the amp worked better without it and made the engineering change during production.

Are we still trying to fix the hum problem? if so, forget C117, it has nothing to do with hum. It might affect a bit of sparkle on the high end in reverb situations. Once you have the hum corrected, you can go back and mess with C117 if you want.


How does one even find out that little part is not there?

[timgman]

Look just above the reverb select switches, it is parallel a high value resistor R101. This looks to be more or less the cap across the 3.3M resistor in most older Fender reverb amps. If you find R101, it should be right next to it. And the fact it is missing is not an invitation to stick one in there. it could well be the factory decided the amp worked better without it and made the engineering change during production.

Are we still trying to fix the hum problem? if so, forget C117, it has nothing to do with hum. It might affect a bit of sparkle on the high end in reverb situations. Once you have the hum corrected, you can go back and mess with C117 if you want.


How does one even find out that little part is not there?

Lolita labeled on the board. I was tracing the v3 effects look tube back and it seemed to be part of it...
Is it not?

[timgman]

Lolita labeled on the board. I was tracing the v3 effects look tube back and it seemed to be part of it...
Is it not?

OK I'm certain you guys are sick of this Thread..
here's the news.
the pi tube had a loose solder connection. it fell off and i resoldered it. no ore hum.. just a static now. the amp works but the sound to static ratio is unusable. is this caps?
It happen when I'm plugged direct to the power section or the whole amp.