Troubleshooting by Tube Substitution a Fender Twin Reverb

[64deluxe]

I have some questions about a 1976 Twin Reverb that I am blackfacing but they apply to all Fender tube amps with reverb.

When troubleshooting by pulling tubes, in my case looking for a nasty hum, this is what I am seeing.

All tubes have been double-checked as "known good". With all tubes in place and volume at zero there is a hum. The hum increases with volume on both channels.

When I pull only V6 the hum stops at any volume setting.

When I pull only V1, V2, V3, or V5 the hum remains.

When I pull only V4, the hum is not present at zero volume or when I increase volume on the VIBRATO channel but the hum is present and increases when I turn up the volume on the NORMAL channel.

Does this mean the problem is in the output stage, possibly the changed values in the PI when I blackfaced it?

And because the hum was not present with V6 pulled, does this rule out the power transformer as the source of the hum.

This is my first posting and I deeply appreciate any advice.

[Alex R]

Pulling v4 will cut out the Vib channel, and the reverb. Two questions:

What happens to the hum with all tubes in if you turn the reverb knob right down? (let's just see if we can eliminate reverb hum as a contributor)

Did you change the heater wiring at all?

[Jazz P Bass]

Pulling the phase inverter to eliminate the hum tells me that the issue is before the PI.
I would test any & all coupling caps for leaky Volts dc.
The coupling caps are usually attached to the preamp plates.
They pass the the Vac signal voltage to the next stage.
Most importantly they must block the plate high voltage from getting to the next stage.
So on one side you will see the plate voltage.
The other side of the cap should read (ideally) zero volts.

[64deluxe]

Thanks Alex R. Hum still present when reverb at zero, dips a little and then increases as turned up.
No heater wiring changes. Corrected some criss-crossed on power tubes. Hum balance circuit, first time I have seen one, and if I turn pot I get a sharp hum just at centre and overall hum goes way down to either side of centre.

Jazz P Bass, I found small amounts of DC, .04 or so, on some of the caps and replaced them after reading your post. I still see trace amounts, .001 and .004 at the bottom of the .047 from pin 6 of V1 and the .022 from pin 6 of V6 where it meets the 500pf respectively. I am measuring to chassis with them in circuit. The hum remains. They are new Mallory's so I am wondering if I have to lift one end to test them properly?

One odd thing. The hum almost vanishes with vibrato channel volume at 3 and normal channel at zero. It completely vanishes when I turn the normal to 3, but only if I then turn the vib channel to 4. With vib channel at zero the hum is present and increases steadily when I turn up normal.

[MWJB]

If you turn both channels to 4 what happens to the hum? You could have a misplaced ground for the preamp filter cap. This should be grounded by the Normal input jacks, bur Fender for some reason chose to move the preamp ground to the same place as the main & screen filters. Check that preamp & PI filter caps are not jumpered to the main & screen caps under the filter cap board. If so, reground them, look at photos of the AB763 BF amp.

[JoeM]

One odd thing. The hum almost vanishes with vibrato channel volume at 3 and normal channel at zero. It completely vanishes when I turn the normal to 3, but only if I then turn the vib channel to 4. With vib channel at zero the hum is present and increases steadily when I turn up normal.

The two channels are out of phase due to the extra tube in the vibrato channel (for the reverb), so the hum in both channels pre-amps and as you turn up and match the out of phase signals they null out.

[64deluxe]

Both channels at roughly 4 the hum is gone. normal at 3 the other at 4. With both at exactly 3 the hum is back. I will check for the jumper. I did a cap job on a '72 whose cap board looked the same in terms of visible connections and sounds fine, but I will look under.

[64deluxe]

When the normal is at 4 and the vibrato at 3, or normal at 3 and vibrato at 2 etc., the hum is gone. Also at if I turn either channel to 9 it is quiet.

The preamp/PI caps have separate ground wires in the doghouse that ground on the buss, one under the normal channel middle knob and the other under the vibrato volume knob adn i can't see any hidden jumpers under the board. Both wires show continuity and seem fine. This is the same arrangement as the Twin I did last week, a '72 and it sounded great when I was done.

In working on this amp I found some bad grounds already. One of the main filter cap grounds was loose against the chassis and the original red prong AC cord had a ground pin but no continuity to the amp. I figured either of those would solve the problem but there was no discernible change in either case. It actually increased a bit with the new AC cord, I assume because it was getting more juice through the non-oxidized wire as the plate voltage jumped 20 volts!

I have replaced all the coupling caps and ceramic discs, except the ones on the pots. I have checked and resoldered every ground I can think of. The jacks all test and sound fine.

But there is a hum balance pot. ASnd when it is centred the hum gets very loud and buzzy but to either side it decreases. This seems counter-intuitive to me. I assumed the pot was just a variable voltage divider like an adjustable virtual centre-tap, so I expected hum to be the least in the centre and more on either side.

I am going to go ahead and remove the hum balance from the circuit and create a virtual centre-tap. I already corrected wiring mismatches. But can you tell me if this behaviour of the pot is normal? I know Doug Hoffman recommends removing them as he says they are prone to fail.

[64deluxe]

Sorry I attached the following to another reply but here it is again. I'm still getting the hang of this.
I found small amounts of DC, .04 or so, on some of the caps and replaced them after reading your post. I still see trace amounts, .001 and .004 at the bottom of the .047 from pin 6 of V1 and the .022 from pin 6 of V6 where it meets the 500pf respectively. I am measuring to chassis with them in circuit. The hum remains.

[MWJB]

Does the hum decrease to a useable level when the hum balance is adjusted to its quietest setting. There is no hard and fast rule that dictates that the hum should be least with the hum balance pot centred.

We know that pulling V6 kills the hum. This suggests that the hum is coming from earlier in the amp. It could still be heater related, so check that grid wires (pins 2 & 7 of the 12A#7 tubes) are well away from heater wires & also away from plate wires (pins 1 & 6). What happens with just V6 & V1 installed? With a dry wooden chopstick, or other non-condiuctive tool (not a screwdriver) try moving wires around to see how it affects the hum.

Any photos ofthe chassis, circuit board, tube socket wiring?

[überfuzz]

How does the hum sound? Ocean shore line? 50/60 Hz hum? Etc...

[64deluxe]

I believe it is 60 hz. A bit like a cicada, insect-like. I read somewhere that 120 hz is A# on the A string of a guitar and it does sound more that pitch than an octave lower, so perhaps it is 120.

[überfuzz]

Ok good, that info can be of use. 120 would suggest that it's introduced via the power supply. Check for AC in the reverb circuit, no instrument plugged in. If it's 60Hz is likely to be some issue with the grounding.

Did the amp come with a hum..? You wrote that you've nailed a couple of bad groundings prior to this. (It might be one or more left to fix, even if you thought it was 120Hz.) One way of probing for bad solder connections is tapping with a NON conducting stick. An other is to resolder.

A long shot. What kind of board is it? Some boards can be conductive and cause noise.

[64deluxe]

With V6 and V1 installed the hum is still there.

I have chopsticked every darn wire and I can't get the hum to change. I have tapped hard and can't make it intermittent, it is steady.

I saw from my photos that the '72 I just did had shielded wires from the normal channels input but the this MV/boost one didn't, so I just added those but the hum remains.

I hope these photos are useful. I can easily post more specific ones if they would help you.

[64deluxe]

Eureka!

IT'S FIXED!!!!

In the photos in my latest post you can see the new heater wiring I did last night to replace the g#$%mned hum balance pot.
I did this morning's post before I had the guts to heat up the amp again.
The hum is gone completely and it is now, after all the other things I did, the quietest, sweetest sounding amp.

I woukld never work on one of these again without ditching the hum balance. They are a useless artifact. I am willing to bet the only reason they added it was so they could stop worrying about correct heater wiring and speed up production by just sticking in the pot.

I have never heard an amp where the imbalance was caused by anything but easily correctable heater wiring mismatches.

Thanks everyone for sticking with me through my first posting.