bias balance pot

[txstrat]

I recently bought two matched 6L6 tubes of which one had drifted.
When I bias one tube to 40ma the other goes up to 48ma.
I now think of adding a balance pot to the bias circuit (see picture) to balance the tubes.
The span of the bias voltage is pretty broad.
Any suggestions appreciated.

Matt

[tubeswell]

Hi txstrat

Looks as if it might work, but the way it is shown on the schematic, if any of the wiper contact(s) burn(s) out, your bias will stop working and the tubes will go ballistic

Therefore perhaps it might be more fail-safe to have two separate bias circuits (or at the least, two separate adjustable voltage dividers), with the wipers in each trimpot tied to one end of the pot as a backstop.

[Robert M. Martinelli]

Hi Matt, hi TW,
I would create two separate bias circuits ( adjustable voltage dividers ) as per TW's advice, but with one end of the pot(s) to -V, the wiper(s) to the grid(s) and resistor(s) in series between the other end of the pot(s) and GND to keep the voltage to go too "positive" ( actually less negative ) for the tubes' sake; this way, even if the pot fails the resistor(s) will protect the tubes.
Best regards
Bob

[txstrat]

How about this.
In case either wiper contact burns out, the bias voltage should be more negative and protect the tube. Right?

[tubeswell]

How about this.
In case either wiper contact burns out, the bias voltage should be more negative and protect the tube. Right?

Right - but you still need a DC connection to ground to do that

[txstrat]

Thanks TW,

think I'll do it that way.

[telemoxy]

Thanks TW,

think I'll do it that way.

I believe the Traynor 50Blue has just such a balancing scheme.

The schematic is on their website.

[txstrat]

Thanks telemoxy,

gonna have alook at it.