Fender Supersonic 112, sorting out the source of the hum/hiss

[liddokun]

I have a Fender Supersonic 112 (the original with the block letter name on the plate).

This amp is known to be inherently noisy, especially with hiss. In attempt to lower the noise floor, one of the suggestions on the web is remove the send and return+effects loop. I did this by taking the signal right before it enters the pre-send part of the circuit, and tying it directly to the input of the phase inverter. This reduced a very large amount of hiss, but not all of it. I DID notice in doing so that the hiss ends up being present in only the clean channel, and further narrowed it down by pulling tubes. Hiss remains when V1 is pulled, but disappears when V2 is pulled, which explains the drive channel not having the hiss as the signal does not go through V2.

The above mod ends up removing the built in reverb, so I got to tinkering again and this time bypassed the send/return part of the circuit, but kept the signal going through the reverb circuit. Low and behold, this got rid of all this hiss mentioned above. However, now there is a 60hz hum present. It is present on all channels, does not change in volume with the controls. This hum is not present at all when I was also bypassing the reverb circuit, however, doing so brings back the very loud hiss.

I've attached a schematic (I've added how I've modded the circuit as well), wondering if someone can spot where I'm introducing this hum into the mix.

Truth be told I could probably live with the hum, and not the hiss, so given the choice I'd stick with no hiss+keep reverb+hum, but hoping to solve this too. I'm hoping more keen and knowledgeable eyes could point me in the right direction.

[Stevem]

Did you just jump out the effects loop with a short shielded cable, which is all that should have been needed?

[R.G.]

The most common sources of hiss are input stage noise and RF oscillation. Bad solder joints, intermittent pots and resistors, other things can make for hiss, but are less common.
Given that you can stop the hiss completely with some signal routing and that the input stages seem quiet when you rearrange the signal routing, it would be a good guess to suspect internal oscillation for the next step.

Do you have access to an oscilloscope?

[liddokun]

Did you just jump out the effects loop with a short shielded cable, which is all that should have been needed?

Yes, I used a shielded cable, as short as possible. The PCB is designed in a way that things kind of go all over, so I did what I could to keep it short.

The hum is present with nothing plugged in and all the volume controls at 0. However, if I ground the signal coming out of the reverb circuit, the hum disappears. I don't know if this maybe points to the hum occurring somewhere in the reverb circuit.

The most common sources of hiss are input stage noise and RF oscillation. Bad solder joints, intermittent pots and resistors, other things can make for hiss, but are less common.
Given that you can stop the hiss completely with some signal routing and that the input stages seem quiet when you rearrange the signal routing, it would be a good guess to suspect internal oscillation for the next step.

Do you have access to an oscilloscope?

Unfortunately, I do not have access to a scope at this time.

[Stevem]

If you unplug the Reverb pan fully and put a short across the output jack rca does the him go away?

Other then the pan and or something on the recovery side of the reverb circuit picking up the 60 hz hum from the power transformer I can’t see how you would be getting 60 hz hum

120 hz I can see and would sound like a B note on the E string being played.

Is that what you have?

[imjonwain]

Did you check that the reverb send/return wires inside the chassis aren't too close to the tube heater wiring? They aren't shielded if I recall. I have the exact same amp I just picked up recently.

[TUBEDUDE]

Check the code on the reverb tank, or Ohm it out, to determine the grounding of the in and the out. Then look at the circuit to make sure the grounding strategy is appropriate. The tank may be wrong for the circuit.

[Stevem]

The schematic is showing both jacks on the pan being grounded to the chassis thru c94.

Does your pan check out like this?

[liddokun]

If you unplug the Reverb pan fully and put a short across the output jack rca does the him go away?

Other then the pan and or something on the recovery side of the reverb circuit picking up the 60 hz hum from the power transformer I can’t see how you would be getting 60 hz hum

120 hz I can see and would sound like a B note on the E string being played.

Is that what you have?

A short across the output RCA does not make the hum go away. I double checked against a few youtube vids and I think it sounds like 120hz.

The schematic is showing both jacks on the pan being grounded to the chassis thru c94.

Does your pan check out like this?

There is indeed continuity between the chassis and jacks at the correct places in the schematic.

Check the code on the reverb tank, or Ohm it out, to determine the grounding of the in and the out. Then look at the circuit to make sure the grounding strategy is appropriate. The tank may be wrong for the circuit.

It is the stock reverb pan, and the grounding scheme is correct according to the schematic and measured with my meter.

Did you check that the reverb send/return wires inside the chassis aren't too close to the tube heater wiring? They aren't shielded if I recall. I have the exact same amp I just picked up recently.

Yes, I've ensured they're far away enough. I actually redid the heater wiring in this amp and really tightened up everything in this aspect.

[liddokun]

A few extra pieces of info:

As I mentioned in a previous reply, upon closer listening my hum sounds like 120hz.

This hum is still present with all preamp tubes, the reverb tubes, AND the phase inverter tube pulled. This would possibly imply that the hum is happening after the PI?

What confuses me is when I bypassed both the effects loop AND the reverb, this hum was not present (although there was an obnoxious amount of hiss), but with only the effects loop bypassed, there is the hum. If the noise is occurring post PI, it would be present in both scenarios, no?

[R.G.]

This hum is still present with all preamp tubes, the reverb tubes, AND the phase inverter tube pulled. This would possibly imply that the hum is happening after the PI

Yes, that's what it implies. It's possible that the two output tubes are picking up hum through their grid wiring; or from ripple on the bias supply; or from magnetic-loop pickup by wiring being near the PT or choke.

Try this (carefully!) - unplug the power cord, let the filter caps bleed down. Pull all the tubes up to and including the PI. Temporarily install a shorting wire across R136 and R137, effectively shorting the inputs to the output tubes. Plug in, turn on, listen. Hum better or worse?

What confuses me is when I bypassed both the effects loop AND the reverb, this hum was not present (although there was an obnoxious amount of hiss), but with only the effects loop bypassed, there is the hum. If the noise is occurring post PI, it would be present in both scenarios, no?

Your amp has more than one problem and they interact. Throwing a "fix" at it may make one problem better, but either not affect or make the other problem worse. That is, there is no "the" noise, the amp has multiple issues. I really suspect, given that you say that this model is known to have hiss and hum issues that the original layout and wiring design has problems and instead of redoing it from scratch to fix this, the manufacturer used circuit "bandaids" to get it shipped on time.

It is possible for there to be two different sources of hum, injected at two different places, and they partially phase-cancel. Completely fixing one source can leave the other source un-cancelled, so the apparent hum sounds worse. Hiss is not related to hum, but high levels of self-oscillation can mask the underlying hum by driving a stage so hard that no hum can get through.

[liddokun]

I hooked it up like this, which I hope is what you meant by shorting wire across R136 and R137.

It did not change the level of hum. I did rotate the OT 90 degrees so it wouldn't be the same orientation as the PT. Would this make any difference? The output tubes are right next to the OT. Though I've never heard of this being an issue, but thought I'd ask.

[martin manning]

I did rotate the OT 90 degrees so it wouldn't be the same orientation as the PT. Would this make any difference?

Definitely could be an issue. The axes of the coils should be oriented at 90 degrees to one another to minimize hum pickup in the OT from the PT. Hum produced in this way will be present without the tubes installed, and it will not be affected by the controls. I would like to believe that Fender had them in the correct orientation, but anything is possible.

The output tubes are right next to the OT. Though I've never heard of this being an issue, but thought I'd ask.

Not generally a problem in my experience.

I believe what was meant was meant by shorting across R136 and R137 was to short both resistors so that any signal coming from the PI coupling caps (C71 and C72) will be grounded.

[liddokun]

I did rotate the OT 90 degrees so it wouldn't be the same orientation as the PT. Would this make any difference?

Definitely could be an issue. The axes of the coils should be oriented at 90 degrees to one another to minimize hum pickup in the OT from the PT. Hum produced in this way will be present without the tubes installed, and it will not be affected by the controls. I would like to believe that Fender had them in the correct orientation, but anything is possible.

The output tubes are right next to the OT. Though I've never heard of this being an issue, but thought I'd ask.

Not generally a problem in my experience.

I believe what was meant was meant by shorting across R136 and R137 was to short both resistors so that any signal coming from the PI coupling caps (C71 and C72) will be grounded.

Right, I misunderstood so I'll try it again.

FYI You would think that Fender would have it in the correct orientation, but alas, stock, this amp comes with both transformers in the same orientation. The transformers are on opposite sides of the chassis, though. I rotated the OT to be at 90 degrees to the PT. The hum goes away with the output tubes pulled.