AB763 Twin Reverb bias current gone crazy (resolved)

[MoparWade]

So, I set my bias current on my new build and initially all 4 tubes were at 31mA. I don't remember for sure if I had it on the current limiter at that point or not. At any rate I decided to see what kind of sound I could get out of this thing. Turned it on and no sound accompanied by blue plating. Shut it down put the bias checker back in and it was at 370mA! Not good. I unfortunately am out of town so I don't have the amp in front of me. Hoping for some direction on what I should check first when I get back to it. Did notice that the intensity pot was affecting my bias current. Double checked all wiring and component values and it is all correct, that being said it's certainly not beyond possible that I am missing something. Thanks in advance.

[MoparWade]

I guess I'll pull the power tubes and see what voltages I'm getting at the pins. Hopefully that will show an out of whack value and I can trace it from there. I am certain that I have built my bias circuit correctly but I may have a bad component in there.

[labb]

If your bias circuit checks out, check the coupling caps for leakage.

[MoparWade]

Can they be checked for leakage without removing them? And what's the best way to check for leakage?

[PRR]

What voltage on the G1 pins?

[MoparWade]

What voltage on the G1 pins?

I'll check that first thing when I get back on the 5th. It's just driving me nuts not having it in front of me until then.

[tubeswell]

Was the speaker hooked up?

[MoparWade]

Was the speaker hooked up?

Do you mean when the current skyrocketed? Yes, it was. Not when I biased it though.

[MoparWade]

What voltage on the G1 pins?

Here are my numbers (all in VDC):

Intensity pot set to minimum with bias pot at minimum:
V7 = -13.93
V8 = -13.91
V9 = -73.9
V10 = -73.9


Intensity pot set to minimum with bias pot at max:
V7 = -4.88
V8 = -4.88
V9 = -26.17
V10 = -26.15


Intensity pot set to max with bias pot at minimum:
V7 = -6.75
V8 = -7.62
V9 = -36.5
V10 = -39.3


Intensity pot set to max with bias pot at max:
V7 = -2.1
V8 = -2.2
V9 = -13.6
V10 = -14.5

[sluckey]

INT pot should not change bias, even in the Hoffman modified AB763 circuit. Something is not wired correctly. Pull all output tubes and leave them out until this is resolved. You should have approx. -50VDC on pin 5 of every one of those 6L6s. They should all be the same.

Which schematic and layout did you use? Did you deviate any? Show us some hi-rez gut pics.

[p2pAmps]

INT pot should not change bias, even in the Hoffman modified AB763 circuit. Something is not wired correctly. Pull all output tubes and leave them out until this is resolved. You should have approx. -50VDC on pin 5 of every one of those 6L6s. They should all be the same.

Which schematic and layout did you use? Did you deviate any? Show us some hi-rez gut pics.

I agree with Sluckey, pull the tubes and start tracing your wiring...  INT pot should have no effect on BIAS. 

[MoparWade]

I used the Hoffman AB763 2-channel layout with appropriate modifications for a 100W Twin Reverb. It is clear to me at this point that I must have something other than a 220K resistor in position R47 or R48. That being said I do not see how the intensity pot could have zero effect on the G1 values if the diagrams are correct. In both the layout and the circuit diagram it shows the bias supply running through the wiper of the Intensity pot, won't this guarantee a variable bias as Intensity is adjusted? I will get some good gut shots in a few. Thank you for your help so far.

[sluckey]

There is no dc current flowing through the INT pot, or R47, or R48. Therefore, there can be no voltage dropped across any of those three resistors.The voltage that appears on pin 5 of the output tubes will be the same as the voltage on the wiper of the BIAS pot. (Practically, since your meter has a high resistance, you will see a slightly lower voltage at pin 5, but this is strictly due to the resistance of the meter.)

The fact that your voltage changes with the INT pot and is so different between pairs of tubes indicates a wiring error or bad tubes/sockets if your tubes are still plugged in. Did you remove the power tubes and recheck voltages on pin 5 of each output tube? If not, please do so and post the new voltages.

[HotBluePlates]

What voltage on the G1 pins?

...
Intensity pot set to minimum with bias pot at minimum:
V7 = -13.93
V8 = -13.91
V9 = -73.9
V10 = -73.9
...

Agree with the others.

Also note you have two different voltages for the group of tubes. That has to be an error of some kind, because all should be getting the same bias voltage (unless you had a bias balance pot, but that's not what the Hoffman board uses).

Something appears very wrong with the things which connect to the grids of V7 and V8.

[labb]

Ok, guys, help me here. I'm like MoparWade. I don't understand what the intensity pot in that schematic is suppose to do. It is not like the original Fender schematic. Way it is shown it has to affect the bias.

[sluckey]

The INT pot varies the amplitude of the low frequency sine wave (tremolo) signal that rides on top of the negative bias voltage. That's all it does.

Read reply #12 for the explanation for why the negative bias voltage will not change.

[labb]

Slaps head! Got ya. TKS. A question, The voltage divider and cap shown on the single channel schematic in the shaded area that can be removed for more gain. You like it in or out of the build...Sorry to hijack thread.

[MoparWade]

Yeah, something is not right I agree, and obviously it is on the one side.

 

Did you remove the power tubes and recheck voltages on pin 5 of each output tube? If not, please do so and post the new voltages.

Those numbers were with the power tubes out. I will do some further investigation and see what I come up with. Although I am sure it makes no difference it might be worth noting that when measuring the voltages I noticed that if the tremolo was turned on I could definitely see the voltage on pin 5 fluctuating as you would expect the tremolo circuit to, with speed clearly changing the bias. I have a feeling I have a short somewhere that is allowing current to flow within my bias circuit.

[sluckey]

Although I am sure it makes no difference it might be worth noting that when measuring the voltages I noticed that if the tremolo was turned on I could definitely see the voltage on pin 5 fluctuating as you would expect the tremolo circuit to, with speed clearly changing the bias.

That's exactly what you should see with a voltmeter. The trem signal is properly modulating the bias voltage. That's why it's called bias vary tremolo. If you look with a scope you would see a low frequency sine wave riding on a negative dc level.

Just turn the trem off while troubleshooting this. It's a very simple circuit that should be easy to fix unless you put some mod in the bias path (such as master volume). Just start at the bias pot wiper and trace the circuit through the INT pot, two 220K resistors to the 1.5K resistors on each tube socket at pin 5.

[MoparWade]

Although I am sure it makes no difference it might be worth noting that when measuring the voltages I noticed that if the tremolo was turned on I could definitely see the voltage on pin 5 fluctuating as you would expect the tremolo circuit to, with speed clearly changing the bias.

That's exactly what you should see with a voltmeter. The trem signal is properly modulating the bias voltage. That's why it's called bias vary tremolo. If you look with a scope you would see a low frequency sine wave riding on a negative dc level.

Just turn the trem off while troubleshooting this. It's a very simple circuit that should be easy to fix unless you put some mod in the bias path (such as master volume). Just start at the bias pot wiper and trace the circuit through the INT pot, two 220K resistors to the 1.5K resistors on each tube socket at pin 5.

Guess I could have been more clear. With the tremolo on the bias fluctuation was quite large, on the order of 30 VDC. I have a couple theories about what might be wrong but it will be a bit before I can check. Hopefully I'll have some results show soon. Thank you again for your help.

[sluckey]

Guess I could have been more clear. With the tremolo on the bias fluctuation was quite large, on the order of 30 VDC.

You were very clear. Your meter doesn't know there's a big ole slow sine wave on that dc bias line. For all it knows, you could be turning the BIAS pot up and down. 

[MoparWade]

Found the problem. Pin 6 post was resting on heater wires, when I soldered my connection it melted through the jacket and shorted. Lesson learned. Thanks for everyone's help.

[HotBluePlates]

Congrats on finding it!

[sluckey]

Good deal! You were very close to finding the problem all along.

[HotBluePlates]

... when I soldered my connection it melted through the jacket ...

And that's why I don't use PVC-insulated wire. But I'll use Doug's cloth- or teflon-insulated wire any day of the week!

[MoparWade]

And that's why I don't use PVC-insulated wire. But I'll use Doug's cloth- or teflon-insulated wire any day of the week!

Agreed, that could have been easily avoided. I've noticed the PVC is quite worthless if you give it any heat and I'm no expert at soldering.

[p2pAmps]

... when I soldered my connection it melted through the jacket ...

And that's why I don't use PVC-insulated wire. But I'll use Doug's cloth- or teflon-insulated wire any day of the week!

100% agree HBP