5F6A Bassman bias help...

[boroman]

I have a 58 Bassman on my workbench. Changed some dried caps amps is sounding great but one 6L6WGB tube was redplating. I measured and whoah... 88mA on one

So
455V on plates
88ma / 66mA - tube measurement

I put 120k resistor instead of 56k but change is very small
Now: approx #1 is 65mA / #2 is 33mA now---- still too much!
Now: Plate voltage has risen up to 500V

Both 4,7k and 15k resistors are in specs.
Both 8uF caps replaced.

Quite confused.
I know that new matched set must be bought soon, but at the moment I want to make amp just playable within safe limits.
Is there anything I'm missing here? Usually, changing bias to a 10-20k more (or a pot, in series) resulted with quite hardcore range - and here, over double resistance and not much is changed.

https://robrobinette.com/images/Guit..._Schematic.pdf
https://robrobinette.com/images/Guit...der_Layout.gif

[Jon Snell]

I would suspect the 100n or 0.1uF coupling caps feeding the 6L6.

[Chuck H]

I would suspect the 100n or 0.1uF coupling caps feeding the 6L6.

This could be tested by swapping the tubes to see if the high current follows the tube or stays with the socket.

[Helmholtz]

To tell if the coupling caps are leaky, lift one end of each and measure bias again
Those old Astron caps are notorious for developing leakage..

[boroman]

Thanks!

I lifted two 0,1 caps and still have same issue.
Swapped tubes and it just switched sides. So, not the coupling caps...
(Currently 68k resistor = 79mA one one tube and 49mA on the other - and they all should be in 30mA range)

Any other ideas?

[Helmholtz]

What voltage do you measure at the junction of the grid leaks?
Too little bias voltage here would explain the high currents.
Reason might be a bad selenium bias rectifier.

As B+ is high, screen voltage will be high as well. High screen voltage increases idle currents.
Is the PT original?
What is the heater voltage?

[boroman]

What voltage do you measure at the junction of the grid leaks?
Too little bias voltage here would explain the high currents.
Reason might be a bad selenium bias rectifier.

As B+ is high, screen voltage will be high as well. High screen voltage increases idle currents.
Is the PT original?
What is the heater voltage?

PT original
Heater: 6.7V measured on the power tubes between pin 2&7.
Screen +/- same as PV (475V)
Juction of grid leaks same as PV
Between each coupling cap: 310V
Bias resistor: 54V

[Helmholtz]

PT original
Heater: 6.7V measured on the power tubes between pin 2&7.
Screen +/- same as PV (475V)
Juction of grid leaks same as PV
Between each coupling cap: 310V
Bias resistor: 54V

At the junction between the 220k grid leaks you should have a negative voltage to ground.
What is "PV"?

Surprized that Fender offered a 220V export PT in the 50s.

[boroman]

At the junction between the 220k grid leaks you should have a negative voltage to ground.
What is "PV"?

Surprized that Fender offered a 220V export PT in the 50s.

Sorry - my mistake=, so at the junction (220k) to ground is 54V
PT is original 120V, I am using step down transformer at 122VAC measured now.

[Helmholtz]

Sorry - my mistake=, so at the junction (220k) to ground is 54V

I guess that means minus 54V. Should be large enough.
Still don't understand what you meant with PV.

I'm beginning to doubt your idle current measurements.
How exactly did you do it?
Also make sure your meter battery is fresh.

[Jon Snell]

I would suspect a faulty or as we call it 'soft' valve. Replace them with a new matched pair. That will at least save you time deliberating.
Have a look at the glass for signs of condensed vapour from the cathode.

[Dave H]

I guess that means minus 54V. Should be large enough.
Still don't understand what you meant with PV.

I think it stands for Plate Voltage

[The Dude]

Regardless of the problem, I'd replace that selenium rectifier. Even if good, it's not worth keeping. If it goes bad, the smell alone will make you wish you'd replaced it.

[Chuck H]

Regardless of the problem, I'd replace that selenium rectifier. Even if good, it's not worth keeping. If it goes bad, the smell alone will make you wish you'd replaced it.

I've read this many times. Never experienced it. I hope I never do.

[The Dude]

It's quite possibly the worst thing I've ever smelled and it lingers.