22VDC at Pin 8 across Bias Measurement Resister

[Contech]

Hello Friends,

I have an interesting bias problem that I can’t figure out. I have a late 60’s Twin Reverb on my bench. It’s just a simple tune. I added “bias measurement” resisters (1 ohm, 1w) between Pin 8 (Cathode) and ground on all four tubes. As you know, this is an east mod and helps quickly measure bias voltage for biasing the output tubes.

I adjusted and measure the voltage across each resister starting from the far outside 6L6. I’ve set the bias voltage to 40mV on the first tube. Here is what I get on all four tubes.

Tube 1 (Pair 1a): 40mV
Tube 2 (Pair 1b): 40mV
Tube 3 (Pair 2a): 34mV
Tube 4 (Pair 2b): 22V

Yes, I am measuring 22V (not millivolts) on Tube 4 across the bias measurement resister. I’ve checked the following.

• Solid ground connection on pin 8 (via the 1 ohm resister)
• Swapped Tubes to see if the problem followed the tube. It did not.
• +460V at Plate
• +470V at Screen Grid
• -52V at Control Grid

Any idea what this might be? Any ideas for me to check?

I’ve never seen this before. Thanks in advance for your help.

[Helmholtz]

Hello Friends,

I have an interesting bias problem that I can’t figure out. I have a late 60’s Twin Reverb on my bench. It’s just a simple tune. I added “bias measurement” resisters (1 ohm, 1w) between Pin 8 (Cathode) and ground on all four tubes. As you know, this is an east mod and helps quickly measure bias voltage for biasing the output tubes.

I adjusted and measure the voltage across each resister starting from the far outside 6L6. I’ve set the bias voltage to 40mV on the first tube. Here is what I get on all four tubes.

Tube 1 (Pair 1a): 40mV
Tube 2 (Pair 1b): 40mV
Tube 3 (Pair 2a): 34mV
Tube 4 (Pair 2b): 22V

Yes, I am measuring 22V (not millivolts) on Tube 4 across the bias measurement resister. I’ve checked the following.

• Solid ground connection on pin 8 (via the 1 ohm resister)
• Swapped Tubes to see if the problem followed the tube. It did not.
• +460V at Plate
• +470V at Screen Grid
• -52V at Control Grid

Any idea what this might be? Any ideas for me to check?

I’ve never seen this before. Thanks in advance for your help.

22V across the 1 Ohm resistor would mean 22A bias current. This is absolutely impossible. Please measure the resistor value, might be 1k instead.

[Contech]

I know it is impossible...but that's what I'm reading. See attached pics. Tube 3 = 40mv. Tube 4 = 24.34V!

[Helmholtz]

I know it is impossible...but that's what I'm reading. See attached pics. Tube 3 = 40mv. Tube 4 = 24.34V!

[ATTACH=CONFIG]50039[/ATTACH]

What is the measured Ohm value of the resistor? Don't trust colour codings.
Otherwise you could measure resistance from cathode to ground at power off.

[mac dillard]

swap tube 4 with another tube and remeasure....See if the problem goes with the tube.

[Contech]

What is the measured Ohm value of the resistor? Don't trust colour codings.
Otherwise you could measure resistance from cathode to ground at power off.

Yep, I think that is it. The resister is not 1 ohm. I removed it and it reads 40M ohm. I'm gonna replace and see if it measures correctly.

[Contech]

swap tube 4 with another tube and remeasure....See if the problem goes with the tube.

Yes, I did that and the problem did not follow the tube.

[Contech]

Confirmed! Problem fixed. It was a bad 1 ohm resister. I replaced it and I now can read 40mA across it.

Thanks everyone for your troubleshooting ideas.

Also, I'm not sure why I get a different bias reading from pair to pair. There is about a 5mV different

Tube 1 (Pair 1a): 40mV
Tube 2 (Pair 1b): 40mV
Tube 3 (Pair 2a): 34mV
Tube 4 (Pair 2b): 35mV

The voltages are in the pocket and the amp sounds great but each pair of tubes read a different bias voltage. Any thoughts on that would be helpful.

[Helmholtz]

Well, 22Vdc across a 1 Ohm resistor can only mean that it's not 1Ohm. Otherwise it would burn trying to dissipate around 500W. And no tube power supply can deliver 22A.

Your different bias currents should follow the tubes. Tubes are individuals, i.e. the same grid bias may produce different cathode currents. Your values are O.K.

[J M Fahey]

... already answered, thanks ....

[pdf64]

Whatever its merits, the amp may be better balanced with a 40 and a 34 on each push pull ‘side’.

[Helmholtz]

Whatever its merits, the amp may be better balanced with a 40 and a 34 on each push pull ‘side’.

Isn't it?

[pdf64]

It depends exactly what is meant by

Tube 1 (Pair 1a): 40mV
Tube 2 (Pair 1b): 40mV
Tube 3 (Pair 2a): 34mV
Tube 4 (Pair 2b): 35mV

[Enzo]

Assuming he numbered the tubes in order, we have 40mv tubes on push and 35mv tubes on pull.

As to "balanced", we only have a 5mv difference. Pretty close in my book.

[pdf64]

Yes, probably not an issue in this case, but from the way the tube designators were arranged and how things were worded, it may be that contech hasn’t grasped how the parallel push pull pairs are laid out in the amp. So pointing that out may be helpful down the line.