fixed bias vvr

[passfan]

Is anyone hand building a fixed bias vvr with a dual pot and would you be willing to share the schematic?

[katopan]

It hasn't been released in the public domain.

[passfan]

I found one.

[JazzGuitarGimp]

There's a bias-tracking VVR discussion in Merlin's power supply book.

[gingertube]

Look at the "Standard" design in TUT5.
I built a "Standard" with a quad of 6V6 and it was glorious.
Switchable Fixed/cathode bias with full powerscaling, tracking bias reg for the fixed bias mode.
Cheers,
Ian

[Invertiguy]

There's a bias-tracking VVR discussion in Merlin's power supply book.

Which is unfortunately now out of print, just as I was getting around to order it! :cry

[Tillydog]

You can just 'mirror' the positive regulator with a 'P' channel MOSFET (reverse the direction of the zener, if you include it).

The MOSFET doesn't need to be a very high power device - I used a ZVP2106A - there are better devices out there with lower voltage drop, but I happened to have some, and it gets the job done.

This is the basic design, but the values were for a 4-4-0 amp with switchable 6SN7 or EL84 power stage.

[IMG834]http://i255.photobucket.com/albums/hh14 ... pdated.gif[/img]

I zener regulated the raw bias zoltage as it was derived from a capacitor coupled to the bridge rectifier (I didn't have a separate bias winding) and wasn't very stable - YMMV.

I haven't got an updated schematic, but for an Express style circuit (different PT and B+ dropping resistors), IIRC I used a 39V zener to end up with ~-35V max bias, a 100k resistor between the pot and ground on the B+ side, and a 51k resistor between the pot and ground on the bias side.

This means that the bias gets (relatively) hotter as the VVR is reduced which I thought did a better job.

This will likely need some tweaking to meet your needs, but hopefully it will give you some ideas

[passfan]

Many thanks to all. I'm collecting the parts for a couple.