Modify Bias Circuit for More Range

[Blindog]

I've been working on a Hughes & Kettner Duotone that I recently aquired, and I have a situation where I need some more range on the bias trimmer to bias the pair of EL34's I have. I've included pic of the bias circuit, and from what I remember, I would need to lower the value of R78 (22K) to accomplish this, no? Rght now, the trimmer is at it's full clockwise range and I'm still drawing about 26mA or so, which is a bit cold but not terrible. New matched set JJ EL34's.

Many thanks!

Mark

[Leo_Gnardo]

Your'e correct and I would leave the 22K in place & parallel another resistor with it, say in the 56K-100K range. Beats pulling out the circuit board. Then if you get a juicier set of EL34 at a later time & can't dial bias current down enough you can unsolder or clip out the parallel resistor.

[Stevem]

You could also parallel a 50k resistor across the outer legs of the pot, but whatever you do fire up the amp without the 34s in and check the bias and confirm that the highest voltage is what you had before and then you should have enough range to idle them up into the 60 to 70% range.

[roberto]

Any resistor across the pot is useless, because at its minimum value the bias is still too cold. The right solution is to reduce R78 by paralleling another resistor.

I'd choose the value of that resistor by setting the trimmer at its minimum value, then adding a 50k trim across R78 (CHECK to have it at its maximum value before turning on the amp!), then lower the value until you reach the right bias current. Then turn off and add in parallel to R78 a resistor with the closest lower value. This way you are sure you can adjust the bias to the right value.

[Blindog]

Thanks for confirming gentlemen, appreciate the assistance!

Mark