76 Twin Reverb SF 100W: 100Hz hum

[Lambertini]

Hello!
Working on this Twin for a while. It is now rather quiet, with fine tuned coupling caps values, selectable NFB and 100K-log pots for MID and Reverb, thanks to the advices from the forum, with a special thank to Sluckey who helped a lot.
I'm now working on what I hope will be the last fix: a 100 cycles hum coming from, I assume, the filter caps.
Not that loud (~-50dB on the cellphone app I'm using to measure it) but clearly present. I would like to get rid of it.

I tried to:
- changed the rectifier diodes with Vishay's.
- planted a forrest of 0.02 1kV caps in parallel of each diodes of the rectifier as suggested by RobRob.
- changed some JJ 100µF 350V filter caps with F&T same specs, as nobody seems to be using the JJs.
- removed the filter caps dropping resistors I've previously changed (1k and 4.7k, AB763 specs) and put back the stock values (2.2k and 10k, SF 100W specs).
- fully unmount, unwax and resolder the filter caps board.
- swapped the 6L6.
- swapped each preamp tube individually.

None of these work. Even worse, increasing the resistors values seems to have slightly increased the hum (+2dB measured with cellphone).

The hum can be heard:
- very lightly with V1 to V6 removed.
- more present with the PI tube.
- even more present with all preamp tubes.
- it increases with channel volumes (the <60dB 50hz hum also)

Any suggestion for further investigations or mods?

[punkykatt]

The two 100r resistors for the tube heaters virtual ground usually mounted on the pilot light. Did you test them? In your picture they are not visible

[pdf64]

Good point.
Have you checked the heater circuit balancing pot on the back panel?
They often get damaged when an output valve shorts.

[Lambertini]

The two 100r resistors for the tube heaters virtual ground usually mounted on the pilot light. Did you test them? In your picture they are not visible

There's a hum digger instead, very efficient against 50hz, no effects on 100hz.

[Lambertini]

Good point.
Have you checked the heater circuit balancing pot on the back panel?
They often get damaged when an output valve shorts.

The pot is brand new and this was really killer against 50hz. My current hum is 100hz and moving the pot has no effect at all on it.

[pdf64]

Does removing all the 9 pin valves affect the hum?
If not, what cathode or anode current is each 6L6 drawing?
If yes, refit the valve in V6 and assess hum, then V5 and so on.

[Lambertini]

The 100hz hum can be heard:
- very lightly with V1 to V6 removed.
- more present with the PI tube.
- even more present with all preamp tubes.
- it increases with channel volumes (the <-60dB 50hz hum also, hard to distinguish possible harmonics)

[WimWalther]

I assume you have 50Hz mains?

If so, the 100Hz noise is originating in the rectifier, just as does 120Hz noise on 60Hz mains.

There are a number of ways for that noise to enter the audio circuit other than directly from the B+. One is via electrostatic coupling into the PT and then into the filament & bias circuits. It can also be inductively or capacitively coupled into adjacent wiring.

[Lambertini]

I assume you have 50Hz mains?

Yes, 220/230V 50Hz.

If so, the 100Hz noise is originating in the rectifier, just as does 120Hz noise on 60Hz mains.

 
Yes, however adding the 0.02 1kV cap in parallel of each diode didn't change anything.

There are a number of ways for that noise to enter the audio circuit other than directly from the B+. One is via electrostatic coupling into the PT and then into the filament & bias circuits.

I've swapped every single tube, with no success. One 12AX7 was more sensitive than the others, I replaced it.

It can also be inductively or capacitively coupled into adjacent wiring.

I've tried to chopstick any wire without noticing an effect. However I might have overlooked this hypothesis, will double check.

The fact that increasing the filter cap dropping resistors values slightly increased that hum too made me focused on the filter caps track.

[BrownIsound]

Perhaps adding parallel film caps in the circuit to the filter caps might be worth a try?

From Aiken’s site: 

I also tend to put the preamp filter caps in the circuit where they are used ("local" bypassing). For example, the first preamp tube's filter cap will be physically placed in the area of the first preamp tube, with the ground and B+ connections made right to the bottom of the cathode resistor/cap and to the top of the preamp tube resistor. If I decide to instead group all the filter caps together, I'll be sure to also locally decouple each filter cap node with a smaller capacitor, typically a 0.1uF/400V) directly from the top B+ node of that stage to the ground node at the bottom of the cathode resistor. You'd be surprised how many brand new filter caps have very high reactances at frequencies within the range of a distorted guitar. Sometimes you can take a "bad" filter cap and bypass it with a 0.1uF cap and it will sound fine. It never hurts to have good high-frequency decoupling at all nodes.

https://www.aikenamps.com/index.php/grounding

[tdvt]

A shot in the dark, but maybe try adding another filter stage to the bias supply?

[PRR]

...however adding the 0.02 1kV cap in parallel of each diode didn't change anything.....

How would it?? 20-100uFd caps for hum. 0.01uFd caps could only be for really high thin buzz, above 10kHz.

And all those yellow blobs make it hard to see the real problem: the routing from rectifier minus to first filter cap. (The experimental larger filter cap making problem worse suggests filter return mis-route.)

[Lambertini]

...however adding the 0.02 1kV cap in parallel of each diode didn't change anything.....

How would it?? 20-100uFd caps for hum. 0.01uFd caps could only be for really high thin buzz, above 10kHz.

That's a suggestion from Rob Robinette website:
100 or 120Hz buzz can be caused by a noisy rectifier. This can usually be eliminated by placing .02uF 1KV caps in parallel with each rectifier diode.
https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Hum_and_Buzz

And all those yellow blobs make it hard to see the real problem: the routing from rectifier minus to first filter cap. (The experimental larger filter cap making problem worse suggests filter return mis-route.)

I don't understand what you mean by "routing from rectifier minus to first filter cap".
I didn't experiment larger filter caps. I previously changed the dropping resistors to AB763 values to increase voltage on preamp tubes plates. I reset them to stock SF 100W larger values, which caused a slight 100Hz increase. Caps are identical in both configs.