Soldano Astroverb 16 Reverb Hum

[telecaster]

Am attaching a schematic
Basically as you turn the Reverb control up, the amp starts to hum which increases with the Reverb level. A 60Hz AC signal appears at the output of V4.b, a perfect sine wave of around 0.8V peak-to-peak. This happens even if the reverb tank and V4 are unplugged. I measure 0 AC voltage at V4.b grid or cathode.
One other note vs the schematic - it shows the plate resistor of 10K, and cathode resistor of 100K. But the circuit board is marked, underneath the components with values, and for V4.b the plate resistor is marked 220K and the cathode 1K. I changed the values to match the circuit board markings since a 10K plate/100K cathode 12AX7 stage would be low gain for a Recovery circuit.
Appreciate any theories or advice on what to try (amp has new tubes and Filter caps).

[Helmholtz]

Schematic values are wrong.

Can you post a scope pic of the V4b hum?

What does V4 unplugged mean? V4 pulled?

[g1]

Did you follow the resistors on the board to verify they were going to plate and cathode?
If it was ever working right with those values, you should not have to change them. What values did you find installed in the circuit?

[telecaster]

Helmholtz - V4 unplugged means pulled. Scope picture attached - 5mS/cm horizontal, 0.2V/cm vertical. Probe connected to junction of V4.b 22nF coupling cap and 47K resistor.

g1 - yes, I verified the correct location of the plate and cathode resistors and their connections. I double-checked by flipping the circuit board. I think 220K plate and 1K cathode were installed when I started working on this amp.

[Helmholtz]

If the hum is there with V4 pulled, it's not entering via the tank.

I think 220K plate and 1K cathode were installed when I started working on this amp.

These values make much more sense than the schematic values but still don't fit the schematic voltages.

[g1]

These values make much more sense than the schematic values but still don't fit the schematic voltages.

Supply node 'B' is shown as 272V. So I think those voltages fit?

[Helmholtz]

Supply node 'B' is shown as 272V. So I think those voltages fit?

Oh, you're right. I mixed up B and B+ .

BTW; what is actual B+?

[g1]

I mixed up B and B+ .

Me too. I only caught it later.

[telecaster]

DC Voltages measured with all tubes installed. B+ = 380V, A=366V, B=285V, V4(1)=341V, V4(6)=105V, V4(7)=0.7mV, V4(8)=0.8V

Is it possible that somehow the heater voltage is coupling to one of V4.b's pins (plate pin 6 for example) ?

[tedmich]

I might start replacing the caps in V4b, starting with the 22nF and 2200pF (why is this one even there?)

[Jon Snell]

It will not be a smoothing issue, your hum is the wrong frequency.
If V4 is removed and there is still hum but the hum reduces to zero with the reverb vol at zero, then the hum is coming from the feed to the reverb pot.
That makes no sense at all as there is no signal path present with no V4.
Please check the valve ID and maybe something is weird with the pot grounding or V3 is the issue.

[telecaster]

tedmich - I did replace the 22nF cap, no change. I'll try the 2200 next, probably won't be until Monday.

Jon Snell - I agree it's not smoothing, that's why I asked if it might be some weird coupling to heater voltage. I'm certain which tube is V4.b, and that the problem appears mysteriously in the plate circuit whether the tube is plugged or not. FWIW, grounding V3.a's grid stops the hum regardless of Reverb pot level. I checked the Reverb pot ground and validated the sweep is correct.

[Helmholtz]

FWIW, grounding V3.a's grid stops the hum regardless of Reverb pot level.

Grounding V3.a grid is essentially the same as dialing the reverb pot to zero.

Does the hum stop when you disconnect the 22nF cap from the V4.b plate pin?

[Jon Snell]

tedmich - I did replace the 22nF cap, no change. I'll try the 2200 next, probably won't be until Monday.

Jon Snell - I agree it's not smoothing, that's why I asked if it might be some weird coupling to heater voltage. I'm certain which tube is V4.b, and that the problem appears mysteriously in the plate circuit whether the tube is plugged or not. FWIW, grounding V3.a's grid stops the hum regardless of Reverb pot level. I checked the Reverb pot ground and validated the sweep is correct.

If you take the valve out, that stops any heater cathode leak that may or may not be present.
The hum cannot be coming from the B supply as that would provide 120HZ if the smoothing was insuffucient. If there was 60HZ on B then all valves would be affected.
Think logically, replacing components without knowing why, is going to add to your problems.

[telecaster]

I lifted the 22nF cap from V4.b pin 6 and measured 0.6VAC still present on the other end of the cap (none on pin 6 of V4.b). Reconnected the cap and pulled one leg of the 47K resistor connected to 22nF - measured 0.58VAC on the other end. Last, I pulled the top end of the 220K in parallel with the top (non-ground) lug of the Reverb pot - measures 0.498VAC there. So I looked at the land patterns on the circuit board - first, V4 is on the opposite side of the board from the Reverb pot. Second, the long connection between the 47K resistor and the 220K in parallel with the Reverb pot crosses over both AC heater lines - most, if not all, other signal lines cross just 1 heater trace maximum. Crossings are at right angles and the traces are on opposite sides of the board.

To test the theory that the AC is coming from the 60Hz heater voltage, I'm thinking of bypassing the land pattern that crosses both heater lines and inserting a coax wire grounded at the Reverb pot end (only). Let me know if you disagree with the theory or approach.