Vox V100/AC100 MKII Bias Mod

[parentheticalfact]

Hello there!
On my bench is a Vox V100 head. The bias supply is much like the AC100 MKII in that it uses 2 zener diodes to create a 35v regulated bias supply.
Trouble is the amp runs the EL34 output tubes quite hot. One neat thing about the circuit is that it's a regulated bias supply. One bad thing is that it maintains approximately -35 volts no matter what the mains input is, which can fluctuate.

My question is can I run a potentiometer in series with the zener network to create a larger negative voltage supply to the output tubes? I considered running a potentiometer and resistor in series to replace the zeners (similar to what I often do in a V4 bias circuit to add adjustment) but considered keeping a zener network in there.

[The Dude]

It's a good idea to include a schematic when you start a new thread so we can see the circuit.

[parentheticalfact]

It's a good idea to include a schematic when you start a new thread so we can see the circuit.

I heard you in my head before I saw it and updated the post

[The Dude]

I think either of your ideas would work. If you keep the zeners, and add a pot in series, the supply will no longer be regulated (which is what you want), so the end result will be similar either way. I believe this is a PC board amp? If so, it might be less clunky looking to just remove the zeners and use the holes where they were to install a resistor and pot.

[parentheticalfact]

I think either of your ideas would work. If you keep the zeners, and add a pot in series, the supply will no longer be regulated (which is what you want), so the end result will be similar either way. I believe this is a PC board amp? If so, it might be less clunky looking to just remove the zeners and use the holes where they were to install a resistor and pot.

It is a PC board amp and a pot/resistor do make more sense from that standpoint. 27k resistor and 25k pot give a range of -35v to -50v which should get the tubes in a more comfortable bias range. They are in place of the diode network so theory is that if they burn up or go open a greater negative value will hit the grids and be a safer failure for the tubes.

[parentheticalfact]

It is a PC board amp and a pot/resistor do make more sense from that standpoint. 27k resistor and 25k pot give a range of -35v to -50v which should get the tubes in a more comfortable bias range. They are in place of the diode network so theory is that if they burn up or go open a greater negative value will hit the grids and be a safer failure for the tubes.

25k resistor and 25k pot had the tubes running cold. 75k to ground across this network yielded a good adjustable range for this amplifier.
All is stock except zener diodes were replaced with 25k pot with one side/wiper feeding R22 and R33, remaining leg grounding through a 27k resistor and a 75k resistor in parallel with the pot and 27k resistor. Hopefully this arrangement could work for someone else as well. I would imagine the fixed bias AC100 could use the same arrangement.

I actually think the zener regulated setup is clever but when mains voltage can fluctuate so much I think it's better to have bias voltage sway with the mains to help keep things in the ballpark of correct.