A better bias pot?

[Dr Tony Balls]

What are people here using for bias pots? I've been using the CTS screwdriver-adjustable ones but they only come in 10K and I find that range a bit limiting. Does anyone have a favorite bias pot option they're using? I like to have things externally adjustable so panel-mount is key.

[Ten Over]

What are people here using for bias pots? I've been using the CTS screwdriver-adjustable ones but they only come in 10K and I find that range a bit limiting. Does anyone have a favorite bias pot option they're using? I like to have things externally adjustable so panel-mount is key.

The range is usually determined by the circuit rather than by the value of the pot. That is, a sufficient range can almost always be attained with a 10k pot if the circuit is designed properly.

[WhopperPlate]

What are people here using for bias pots? I've been using the CTS screwdriver-adjustable ones but they only come in 10K and I find that range a bit limiting. Does anyone have a favorite bias pot option they're using? I like to have things externally adjustable so panel-mount is key.

The range is usually determined by the circuit rather than by the value of the pot. That is, a sufficient range can almost always be attained with a 10k pot if the circuit is designed properly.

Or if you don’t want to change a thing about the rest of the circuit you can always pop a higher value track into the pot casing . Simple enough

[jabguit]

What are people here using for bias pots? I've been using the CTS screwdriver-adjustable ones but they only come in 10K and I find that range a bit limiting. Does anyone have a favorite bias pot option they're using? I like to have things externally adjustable so panel-mount is key.

The range is usually determined by the circuit rather than by the value of the pot. That is, a sufficient range can almost always be attained with a 10k pot if the circuit is designed properly.

Or if you don’t want to change a thing about the rest of the circuit you can always pop a higher value track into the pot casing . Simple enough

Yep - done it many times. The 450 series CTS pots are easy to open up and change resistance tracks in.

cheers,

[Stevem]

You do not want to use a pot in the first place you want to take advantage of using a 20 turn Ciramic trimmer in the resistance value that you need.

There far more accurate and they do not drift.

[Dr Tony Balls]

What are people here using for bias pots? I've been using the CTS screwdriver-adjustable ones but they only come in 10K and I find that range a bit limiting. Does anyone have a favorite bias pot option they're using? I like to have things externally adjustable so panel-mount is key.

The range is usually determined by the circuit rather than by the value of the pot. That is, a sufficient range can almost always be attained with a 10k pot if the circuit is designed properly.

Well, do you care to share how to extend range with a 10K pot? I tend to use something like this Dynaco example, but in about every example I can think of there's a simple voltage divider being used to dial in bias voltage. Thus the pot is only able to adjust R1 or R2 +/- 5K

[Dr Tony Balls]

You do not want to use a pot in the first place you want to take advantage of using a 20 turn Ciramic trimmer in the resistance value that you need.

There far more accurate and they do not drift.

Right I know about this option bu what i'm really looking for is something that is panel mount so it could be adjusted from the exterior of a chassis.

[sluckey]

The 10K pot that Fender used is capable of providing proper bias range for Deluxe Reverb up to Twin Reverb simply by slightly changing the ACV input and the tail resistor on the pot. And the Fender pot can be adjusted internally or externally. Sounds like you already have the ultimate pot.

The Dynaco circuit (Sunn uses the same) you posted uses a 10K pot. Assuming -60vdc input allows the bias pot to swing between -37v and -58v. That's a respectable voltage range.

[Dr Tony Balls]

The 10K pot that Fender used is capable of providing proper bias range for Deluxe Reverb up to Twin Reverb simply by slightly changing the ACV input and the tail resistor on the pot. And the Fender pot can be adjusted internally or externally. Sounds like you already have the ultimate pot.

The Dynaco circuit (Sunn uses the same) you posted uses a 10K pot. Assuming -60vdc input allows the bias pot to swing between -37v and -58v. That's a respectable voltage range.

Perhaps my problem has more to do with tube variability than anything else. I recently built an amp for a guy that was supplying his own tubes. I built it at my shop, tested and configured with a set of JJ 6L6GCs for an optimal bias setting at 40mA (60% dissipation) +/- 7mA (50% - 70%). I shipped it to him without tubes and on install he had a vintage set of JAN 6LCGC where the lowest he could set it was 52mA, and another vintage set got him a high of 32mA. A set of JJs was right on the money, though. So while Fender, Dynaco, Sunn, etc's use of the 10K pot was fine at the time, tube tolerances were also much tighter. A slightly larger (25K, say) bias pot would help adjust for a wider range of tube tolerances in the current market.

[Ten Over]

Well, do you care to share how to extend range with a 10K pot?

Make the other resistors smaller.

[Ten Over]

Perhaps my problem has more to do with tube variability than anything else. I recently built an amp for a guy that was supplying his own tubes. I built it at my shop, tested and configured with a set of JJ 6L6GCs for an optimal bias setting at 40mA (60% dissipation) +/- 7mA (50% - 70%). I shipped it to him without tubes and on install he had a vintage set of JAN 6LCGC where the lowest he could set it was 52mA, and another vintage set got him a high of 32mA. A set of JJs was right on the money, though. So while Fender, Dynaco, Sunn, etc's use of the 10K pot was fine at the time, tube tolerances were also much tighter. A slightly larger (25K, say) bias pot would help adjust for a wider range of tube tolerances in the current market.

There is only a 5V difference in the bias voltage between 50% and 70% dissipation for a bogey 6L6GC with 400-some-odd volts on the plate and screen grid. Did you build a bias circuit that only had a 5V swing?

What is the RMS AC voltage powering the bias circuit and where is it coming from?

[Yoda]

To answer OPs question I like to use the Bourns 50K precision trimpots. I just completed a dual bias circuit for a 6V6 amp and it gives me as low as 10-12 up to 40+ mA of bias adjustment, that’s a huge range that guarantees I can bias any 6V6 on the planet. Can’t give exact numbers because the particular trimpots I used on this build don’t have a firm end to the screw travel and I don’t want to strip them.

[sluckey]

Can’t give exact numbers because the particular trimpots I used on this build don’t have a firm end to the screw travel and I don’t want to strip them.

They do have an end of travel. You won't strip them. You should be able to feel a click when you reach either end. If you listen closely, you can probably even hear a slight click.

[TUBEDUDE]

Designing with the 10k pot makes it easy to Keep the total resistance low, as seen by the power tube grids, preventing slightly gassy tubes from thermal runaway.

[Ten Over]

Designing with the 10k pot makes it easy to Keep the total resistance low, as seen by the power tube grids, preventing slightly gassy tubes from thermal runaway.

And to make the higher resistance bias circuit even worse, all of the current from all of the power tube grids flows through the bias circuit making its effective resistance significantly higher.