Need a pot size to adjust bias current

[pbman1953]

.412 on a 2k setting

What does this mean? 2K setting? .412, is that a voltage drop?

No, someone asked the value of the balance pot

[pbman1953]

.412 on a 2k setting

What does this mean? 2K setting? .412, is that a voltage drop?

[pbman1953]

No, someone asked for the pot value

[Willabe]

Full Counter Clockwise  -  Full Clockwise   -  all in negative
1-58.3  4-57.6               1-57.6  4-58.2
2-58.4  5-57.8               2-57.6  5-58.2
3-58.3  6-57.6               3-57.6  6-58.2

10k pot swing    -48.4  to -76.1

Ok, that's pin 5 on the power tube socket? Yeah, that's not moving them, just barely. 

That's a good -dcv swing.

[pbman1953]

Full Counter Clockwise  -  Full Clockwise   -  all in negative
1-58.3  4-57.6               1-57.6  4-58.2
2-58.4  5-57.8               2-57.6  5-58.2
3-58.3  6-57.6               3-57.6  6-58.2



10k pot swing    -48.4  to -76.1

Ok, that's pin 5 on the power tube socket? Yeah, that's not moving them, just barely. 

That's a good -dcv swing.

Yes, all 6 pin 5's

[Willabe]

No, someone asked for the pot value

So the balance pot measures .412K with your meter set to read resistance 2K setting?

[pbman1953]

No, someone asked for the pot value

So the balance pot measures .412K with your meter set to read resistance 2K setting?

Yes

[Willabe]

Yeah, it's interesting.
I wonder if the 2 pots are in parallel, maybe 800R each out of circuit.
This isn't a suggestion, but say instead of changing the voltage dividers between the 2 sides, you go back to the original circuit but instead of the 2.7k resistor on the ground side, replace it with, say a 1k safety resistor to ground and a 5k pot in series.
Again, this isn't a suggestion, just an idea.

See reply 29.

[AlNewman]

So which 2 pots are you referring to?

My bad, I see the hum balance pot now.   
So the output balance is a tapped pot?

[Willabe]

I wonder since the tubes measure so closely that I should bypass the balance pot and just use the bias pot

Well, as tubes age they age differently and go out of balance. That slowly kills bottom end and the amp gets more and more noisy. So a balance pot can be a good thing.

Dual balance/bias adjustment pots can be a good thing too. You already have the 10K bias adjust pot installed.

I would try this; I'd take out the 15K R that's on the balance pot to ground and put the 2K7 back in. Then put the 15K on the open lug on the 10K bias pot to ground. And swap the 2 wires with each other that are on the bias pot now. The wire going from the bias pot to the balance pot should be hooked up to the bias pots wiper. The raw bias feed should go to the pots X end lug, no resistance between the wiper and end lug when turned full clockwise, of the bias pot. The 15K R should go to the 0 end of the bias pot to ground.

I'd try that, then take all those -dcv measurements at pin 5 of each power tube socket. If both the bias adjust pot and the bias balance pot now both work, then set the -bias for the most negative bias, largest number, -76dcv? And set the balance pot so the -bias is very close side to side. Then put all 6 tubes back in, set the bias, then set the balance, check bias again and adjust as needed, then check balance again.

You can also switch tubes from 1 side to the other. It takes a little while, but doing this, you can get them even closer. But your readings look really close now, if they stay that way, your fine.

[Willabe]

My bad, I see the hum balance pot now.   
So the output balance is a tapped pot?

Yes, I said that in reply #40. It's a tapped pot. It has 4 lugs.

Or that balance pot has 4 lugs, it's a tapped pot. So, looking at the schematic, 1 wire goes out of the pot to the left of the pot, wiper, the right side of the pot is the tap.

 

[pbman1953]

I wonder since the tubes measure so closely that I should bypass the balance pot and just use the bias pot

Well, as tubes age they age differently and go out of balance. That slowly kills bottom end and the amp gets more and more noisy. So a balance pot can be a good thing.

Dual balance/bias adjustment pots can be a good thing too. You already have the 10K bias adjust pot installed.

I would try this; I'd take out the 15K R that's on the balance pot to ground and put the 2K7 back in. Then put the 15K on the open lug on the 10K bias pot to ground. And swap the 2 wires with each other that are on the bias pot now. The wire going from the bias pot to the balance pot should be hooked up to the bias pots wiper. The raw bias feed should go to the pots X end lug, no resistance between the wiper and end lug when turned full clockwise, of the bias pot. The 15K R should go to the 0 end of the bias pot to ground.

I'd try that, then take all those -dcv measurements at pin 5 of each power tube socket. If both the bias adjust pot and the bias balance pot now both work, then set the -bias for the most negative bias, largest number, -76dcv? And set the balance pot so the -bias is very close side to side. Then put all 6 tubes back in, set the bias, then set the balance, check bias again and adjust as needed, then check balance again.

You can also switch tubes from 1 side to the other. It takes a little while, but doing this, you can get them even closer. But your readings look really close now, if they stay that way, your fine.

-54.3 is negative voltage at the input of the balance pot

Full Counter Clockwise  -  Full Clockwise   -  all in negative
1-54.1  4-50.9               1-50.9  4-54.2
2-54.2  5-50.9               2-50.8  5-54.2
3-54.2  6-50.9               3-50.9  6-54.2

Much nicer swing , right?

The higher bias number now is -61.4

[Willabe]

-54.3 is negative voltage at the input of the balance pot

Full Counter Clockwise  -  Full Clockwise   -  all in negative
1-54.1  4-50.9               1-50.9  4-54.2
2-54.2  5-50.9               2-50.8  5-54.2
3-54.2  6-50.9               3-50.9  6-54.2

Much nicer swing , right?

The higher bias number now is -61.4

Yes, that's a little better.

What did you do?

[pbman1953]

I did exactly what you said-

Put the 2.7k back in
Move the 15k to unused leg of the bias pot then  grounded it
Reversed the the wires on the bias pot

Measured , reported the findings

Installed the tubes and per side and now I have an average 21 watts per side

[Willabe]

Ok, and it's working. 

Do you know what dissipation % your running the power tubes? Since it's a bass amp, you don't need ditortion and those tubes are $$, I'd run them no more than at 60%.

If you ever need more  -dcv, increase the 15K R on the -bias pot, say, like 22K, or maybe even a little more. You want the try and have the -bias swing centered on a -dcv around what you'll be using so you can run the tubes a little hotter or a little cooler. And then be able to balance them.

 

[Willabe]

What are the new bias numbers when biased and balanced? How close are they?

[Willabe]

You had a -dcv swing of  -48.4  to -76.1 on the 10k pot.

What do you have now?

[pbman1953]

21.8
23.3
20.6

21
20.7
21.8

I too have a Fender Super Twin set up as a bass head. I used newer Tungsol 7581a's in that and it sems louder. On a side note, I installed a Mercury Magnetics transformer in order to take advantage of the 4 & 8 ohm capability. MM say that the transformer is a factory replacement but has both the 4 & 8 ohm taps. The footprint is the same too, which made it easy.

The Studio Bass is 8 ohm off the shelf but I installed the 4 ohm tranny from the Super twin because most of my cabs are 4 ohm.

Thanks again for your help!

[pbman1953]

You had a -dcv swing of  -48.4  to -76.1 on the 10k pot.

What do you have now?

I had -76.1 before the changes , then -61.4 after

Current is -60.5

[Willabe]

21.8    21
23.3    20.7
20.6    21.8

That's pretty good, it's just that 23.3, but you can try this;

Put the 3 highest and 3 lowest together;  23.3/21.8/21.8, and 21.0/20.7/20.6.

Then re balance them. Should come out a little closer now. 

 

[Willabe]

You had a -dcv swing of  -48.4  to -76.1 on the 10k pot.

What do you have now?

I had -76.1 before the changes , then -61.4 after

Current is -60.5

No, the -dcv swing, lowest to highest.

Any time you make changes to a -bias adjust pot or a bias balance pot you must measure the new swing, FC to FCC, lowest to highest.

[Willabe]

Put the 3 highest and 3 lowest together;  23.3/21.8/21.8, and 21.0/20.7/20.6.

Then re balance them. Should come out a little closer now. 

And often, just swapping the tubes on 1 side, from socket to socket, without changing the -bias or the balance, the current readings will be different. Rearrange 1 side, then do the other side.

Then re balance.

So you can get even a little closer. Takes a little time, but you can often get them very close doing this. 

I did this on a Fender 6 power tube amp, came out very closely matched.

[pbman1953]

19.4
20.9
21.2


19.2
19.9
20.3

[Willabe]

19.4
20.9
21.2


19.2
19.9
20.3

See, better.

Now swap the 19.4 with the 20.3, all 19.x and 20.x together, re balance, see what happens. You have to keep playing around with it. 

[Willabe]

You have to get them in the % range you want before doing this because those current draw numbers will go up/down on each tube when you bias them hotter or cooler.

The tubes react differently running them hotter or cooler.

[pbman1953]

22.4
22.9
22.3
22
22.1
21


That should enough right?

[Willabe]

Yes, that's real good. 

Probably anything within 2 or 3 mA's is very close.

Now you know how to get them closer in a multi tube power amp.   

[Willabe]

You could try and tweak the balance pot just a hair, but it depends how sensitive it is. That's something you'll get used to.

You might go past where you want with just the slightest touch. 

[pbman1953]

I did before you wrote back and they're even better. I think it's going back in its cab


Thanks!

[Willabe]

The side with the 22/22.1/21, you could try swapping those 3 around in those 3 sockets, then re balance. 

It depends how much you want to spend time on doing it. 

I've messed/played with this to see how close I could get them. Took a good 1/2 hour until I couldn't get them any closer. 

[Willabe]

Now that you have a bias adjust and a bias balance that work;

Write ALL the numbers down for each tube, -dcv bias AND current and keep it where you can find it.

Now depending on how much you play through that amp, maybe in 6 months, pull the chassis and see how much the tubes have drifted from each other.

Then re adjust both -bias and balance as necessary, that's why you have those 2 adjustments.