5f6a balanced biasing

[pbiagi]

On a new tweed bassman I have NOS Tong-sol 5881 tubes which I currently feel sound great, better than other 6L6s - but at this early stage that could be vintage thinking, need more time.
Problem is the tubes are not well matched, I biased one at 35mA but the other is 21.5mA - pretty big difference. Amp sounds decent, brand new, but I'm thinking it could sound better.
I disconnected what I believe the correct voltage divider resistor in the negative voltage grid supply, and placed a 300K pot in place using jumper cables. The pot is not in the photo but I've drawn a red box where I did this. This side of the divider appears to go to the left power tube, per ohm measurement.
But no matter the setting on the pot, the lower current tube stays around 21.5mA.
Maybe a clue - working at night outside, by chance I turned off the lights and noticed the 21.mA tube had a blue glow on just one side of it. One side without, one blue. Could this be a bad tube, and there's no way to bias more current?
Or am I doing something wrong?

More details -
B+ 445, 441, 398V
Pin 5 of both power tubes is -52V.

BTW, I switched the output transformer wires - didn't get a howl but a loud whistle. Switching the wires resolved it.

[Jazz P Bass]

First off, a tube that biases 'colder' than normal will have a lower overall voltage output.
Second, unless you totally split the bias arraignment, you cannot bias the tubes separately.
Even if you do reroute the bias, the lower tube will have a lower peak voltage.
How does the amp sound?
If you like it, play it.
If you truly believe that the amp will sound better with matched tubes, then that is what you will have to do.
Install matched tubes.
As to the blue glow, that is normal for an output tube.

[pbiagi]

Hi Jazz Bass, thanks for the response.
I realize blue glow can be normal but it does not appear on the good tube..
I must be reading it wrong but was following the book 'A Desktop Reference of Hip Vintage Guitar Amps'.
The idea being temporarily substitute a pot for the voltage divider resistor to balance bias the tubes, then solder in the resistor value found on the pot.
Mostly this is learning for me and I'd be curious to hear the difference on balanced tubes.
... For now I'm going to put the resistor back and put in a pair of better balanced 6L6s.
So that I understand - this balance resistor mod could be done on certain amps but not the tweed bassman (without changing more than one resistor)? Which amps would this work on? blackface?
Thanks

[Jazz P Bass]

I cannot follow the 'mod'.
I would not mess with the bias circuit.
As stated, the lower biasing tube will not be the same, output voltage wise, as the hotter biasing tube.
In itself, a push pull output section tends to balance the tubes out.
That & the output transformer.
Truth be told, a slight imbalance in the tubes can sound good.
And I honestly do not believe that Leo Fender even considered it an issue. (imbalanced tubes)

[Diablo]

You could build yourself another bias supply so you could bias each output tube.

[Jazz P Bass]

You could build yourself another bias supply so you could bias each output tube.

I really thing we are missing the point of bias.

[g1]

I'm in agreement with Jazz, bias/balancing isn't going to make a huge difference to the sound (unless it's real far out).
You said the 21mA tube wouldn't change with the pot adjustment, how about the other tube? Using the pot to replace one of the bias feeds (220K) is only going to allow you to adjust one.
Swap the output tubes around and see what happens.

[pbiagi]

Thanks for the responses - really appreciate the learning.

Current-wise they are pretty far out I think, 35mA vs. 21mA - that's 14mA difference.
Good point g-one, should have thought of changing tubes. The idea would be to adjust one side to approximately equal the other, I believe that is the idle current, (but Jazz Bass points out the operating performance would be different anyway).
Now the original resistor is soldered back in, with a different 6L6 tube set, and they have a 10mA difference. Would like to do some more experimenting but have other projects to get to. Look forward to coming back to this at some point.

Jazz Bass, I get the operating difference. My understanding is that on Silver Face amps the bias pot is actually a balance between the power tubes.
I've also read somewhere on this forum, some members (or a member) sets up a bias pot and balance pot on all his amps.
Since bias makes such a tone difference, I'm no expert but would be curious to the tonal difference of balancing bias.

[Jazz P Bass]

[QUOTE=pbiagi;291239] My understanding is that on Silver Face amps the bias pot is actually a balance between the power tubes.

Since bias makes such a tone differenceQUOTE]
Most tech;s would opt to remove the goofy bias balance pot.
As to bias giving the amp a tonal quality, I don't buy into that one.(unless it is set within reason)
Under bias the tubes & you get crossover distortion (blech!)
Over bias them & you are literally beating the piss out of the tubes.
Set it correctly & the amp sounds as it should.

[loudthud]

I think the OP is trying to balance the bias on the tubes by changing the grid leak resistor with a pot. This won't do anything because virtually no current flows in the grid leak resistor. See the attached schematic. The left pot sets the overall bias and the right one allows you to adjust one side to match the other. You may need to change the 15K resistor if there isn't enough range on the left pot to get both tubes to proper bias.

[pbiagi]

Thanks loudthud!
This is what I was looking for. It's a little more involved than replacing one resistor.. and I have other projects to attend to now, but look forward to coming back to this.. to try out.

[g1]

loudthud: thanks, I thought the grid circuit impedance would be too high for the grid leak resistor to make a difference, but I was trying to guess what the Weber mod was about. Turns out it was not about balance but about adjusting the fixed bias overall, without installing permanent pot (replace top resistor in bias voltage divider with pot, set bias to taste, measure pot, replace with proper value resistor).