Champ 12 red plating

[drewl]

Well I fixed some bad connections on this Fender Champ 12, but I'm having trouble with the output tube drawing too much current.

It starts out with 27v across the cathode resistors, at about 44ma, and the cathode voltage continues to increase until the tube breaks down.

I tried a few different 6L6's, new and used. The one that hangs the best is an old Ampreg branded 6L6.

I tried increasing the cathode resistance by another 100 ohms, but then the amp starts to lose power, only putting out about 8w instead of 10, but that may have to be the only way to keep the output stable.

I'm also getting around 9vdc on the input grid from R30, and that voltage increases as the bias goes up, naturally.

I don't think coupling caps C11 or C15 are bad/leaky as there is no dc voltage on the 6L6 input grid when that tube is removed. I'm going to swap them out just to make sure.

Any other ideas on taming the current draw of the output tube?
Thanks. champ_12_schem.pdf

[Helmholtz]

It starts out with 27v across the cathode resistors, at about 44ma, and the cathode voltage continues to increase until the tube breaks down.

I'm also getting around 9vdc on the input grid from R30, and that voltage increases as the bias goes up, naturally.

How do you measure cathode current? Does cathode current increase when the cathode voltage rises?
There's a possibility that one of the resistors R31/32/33 is unstable and changes in value with temperature.

It is normal with this circuit to have a positive voltage at the power tube grid. R30 provides a grid bias of around +13VDC. Your meter reads lower because it loads the circuit.

[drewl]

Well I was just measuring the cathode voltage to ground across the cathode resistors.
I guess the resistance could be rising, I will have to see.

Thanks for the suggestion.

[Helmholtz]

Well I was just measuring the cathode voltage to ground across the cathode resistors.

Don't see how 27V at the cathode would give you 44mA. Did you ignore R33?
Cathode current should be close to 65mA.

[drewl]

Yes.

The actual DC resistance to ground is 570 ohms, for 47ma at 27v.

But, that cathode voltage keeps rising, the original tube in it had it rise to 48v when I was testing the amp, and that is how I discovered the problem.

[Helmholtz]

Yes.

The actual DC resistance to ground is 570 ohms, for 47ma at 27v.

From the schematic resistance from cathode to ground should be around 430R. You higher value indicates some drifted resistor or a bad solder joint.
A CC resistor that has significantly drifted might be instable with temperature because of internal cracks.

[J M Fahey]

For testing ground the grid with a piece of wire.
Straight from tube socket.
Make certain grid pin has good contact with socket "sleeve" , give it more tension by slightly bending it inwards with a sewing needle.
I suspect a poorly ground referenced grid.

Post result.

[drewl]

Ground the input grid, pin 5?

[Helmholtz]

For testing ground the grid with a piece of wire.
.

The grid is biased to around 13V via R30.
It should not be grounded.

[drewl]

Then what do you want me to ground?

If I ground pin 5 the cathode voltage is solid at 20v, of course the amp won't produce sound, but does this mean there's a leaky coupling cap?

[Helmholtz]

Then what do you want me to ground?

I don't see a sense in grounding the grid.
Check/replace the resistors in the cathode circuit as mentioned before.

If you suspect C16, lift one end. But as the node before C16 essentially sits at ground potential, a leaky C16 will not increase grid voltage.

[drewl]

Yeah, c16 and either c11 or c15 would have to be bad.

I think Helmholtz nailed it the first time.

I disconnected the cathode circuit and just attached a 470 ohm from pin 8 to ground for testing and have a stable cathode voltage.