Vox V100 bias circuit

[Marcomel79]

So im gonna do a recap on a friends Vox V100. Looking a the schematic, it has a non adjustable fix bias circuit. I was wondering if it was possible to convert it more like jcm900 type bias circuit since the bias voltage is being tapped from a FWBR, with the bias input cap. It would be nice to fit a trimpot so it can be adjusted... here is the schematic bit

What say you? @Pete Farrington?

 

[2L man]

Yes, after -56V junction you can implement "typical" EL34 bias circuit. When you remove Zeners -56V come more negative so there possibly come a need to change "tail resistor" which go to 0V so that bias current come high enough. Install also four 1 ohm resistors, one to each cathode circuit, and bias balance checks and bias adjusts come easier.

 

[Pete Farrington]

Yes, remove the zeners, fit a 22k variable resistor in seeies with 22k fixed (those values should be ok but might benefit from tweaking).

In the absence of the screen grid supply also being regulated, regulating bias supply is a pretty bad idea.

 

[Marcomel79]

In the absence of the screen grid supply also being regulated, regulating bias supply is a pretty bad idea.

Thank you for input Pete, not sure i understand what you mean by the screen grid supply being regulated.....

 

[Pete Farrington]

Thank you for input Pete, not sure i understand what you mean by the screen grid supply being regulated.....

It's not, my point was that it's a bad idea to regulate one and not the other

 

[Marcomel79]

It's not, my point was that it's a bad idea to regulate one and not the other

Isnt that like any other Marshall bias circuit is?

 

[Pete Farrington]

Isnt that like any other Marshall bias circuit is?

The zener diodes are regulating the bias voltage in the V100, I can't think of any Marshalls (or anything else, offhand) with that?

 

[Marcomel79]

The zener diodes are regulating the bias voltage in the V100, I can't think of any Marshalls (or anything else, offhand) with that?

sorry, i totally misunderstood your previous comment and replied with something that clearly didnt make sense

 

[Marcomel79]

Right, i removed the zeners and fit a bias pot. Good range as it is. Thank you @2L man and @Pete Farrington for your help!