67 Bassman Hum

[Valvehead]

Swap the tubes around and see if the low value stays with the socket, or with the tube.

That was with the OT un wired from the amp

[The Dude]

Not necessary as we have the individual voltage drops across the primary halves.

I suggested measuring cathode current because if the OT is intermittent or fails under load, measuring voltage drop & resistance at the OT wouldn't tell the whole story.

[pdf64]

Ok, found something ....voltages checked again at 6l6 sockets and bias current is 27ma/18ma . I swapped many 6l6 in and out trying to get that closer .

Everything looks ok except pin 3 on one of the 6l6 sockets . When i touch the probe there my meter starts beeping like crazy and the screen goes wild ! Over voltage ?? Also , the hum gets Way louder when i touch that pin. This is with or without a tube installed .



EDIT------- This might be a clue -- it only does that with the PI tube in . If i remove the PI the voltage is normal at 463vdc . Tried different 12 at7's

Probing an output valve anode often induces oscillation, which if ultrasonic might not be apparent per se; the probe, lead and meter become a transmission antenna whose field gets received by the grid of preceding stages.
The only time it’s worth probing an output valve anode is when the valve in the preceding stage is pulled, as that (generally) breaks most potential positive feedback loops.

Fitting 1R cathode current sensing resistors is incredibly useful in getting an insight into what’s happening in a circuit. Measurements made there are electrically much safer (eg than at the anode), have pretty much zero likelihood of causing measurement errors or of disturbing normal circuit behaviour, and provide information not otherwise easily obtainable.

[Helmholtz]

Everything looks ok except pin 3 on one of the 6l6 sockets . When i touch the probe there my meter starts beeping like crazy and the screen goes wild ! Over voltage ?? Also , the hum gets Way louder when i touch that pin. This is with or without a tube installed .
EDIT------- This might be a clue -- it only does that with the PI tube in . If i remove the PI the voltage is normal at 463vdc . Tried different 12 at7's

These are all typical signs of ultrasonic oscillation.
It is possible that the amp only oscillates when a probe is touching the plate pin.
Do you have a scope?

[Valvehead]

These are all typical signs of ultrasonic oscillation.
It is possible that the amp only oscillates when a probe is touching the plate pin.
Do you have a scope?

no

[Chuck H]

If the OT is back in place I call for fresh voltage readings on all power tube pins.

[Valvehead]

If the OT is back in place I call for fresh voltage readings on all power tube pins.

I can tonight. Should i put the PI tube in and the 6L6 ??

[Chuck H]

I can tonight. Should i put the PI tube in and the 6L6 ??

Power tubes, yes. not the PI tube. Thanks.

[Valvehead]

Power tube sockets read the same . PI tube removed . 6L6 tubes in .

filaments are 6.73 vac pin 2 to 7

pin 1 -048vdc
pin 3 459vdc
pin 4 460vdc
pin 5 -048vdc
pin 6 459vdc
pin 8 0v

[pdf64]

What’s the V DC at the bias supply outputs, the other leg of the 220k grid leaks?
What’s the idle anode or cathode 6L6 current?

[Helmholtz]

I think he means - 48V at pin 5.

[pdf64]

I think he means - 48V at pin 5.

Yes, I assumed that.
Rather I recalled a thread on another forum, where the protagonist was using a DMM with an input resistance of what must have been about 1M, as opposed to the normal 10M.
That resulted in the lug 5 voltages being read with a lower magnitude that was actually correct (due to voltage being lost across the grid leaks as the meter drew current).
Measuring at the bias supply output avoids measurement errors of that nature.

[Helmholtz]

Oh, I see.
I thought you had interpreted the number as 48mV like I first did.

With a 1M meter actual grid bias would be around -60V explaining the low idle currents.

[Valvehead]

What’s the V DC at the bias supply outputs, the other leg of the 220k grid leaks?
What’s the idle anode or cathode 6L6 current?

idle current at plates is 27ma / 18ma

[g1]

idle current at plates is 27ma / 18ma

If you swap the tubes around, does the lower value (18mA) stay with the socket, or follow the tube?