Adding VVR to fixed bias amps

[TIMBO]

Hi guys, Can anyone confirm that using a VVR for a Cathode Bias Amp is not suitable for a Fixed Bias Amp. Thanks

[mcrracer]

Not suitable. Two different units,

[LooseChange]

You need to add another circuit that tracks the Bias voltage with the B+ voltage.

[Raybob]

SkipZ's Vari-Watt FX works real good for fixed bias amps, tracking the bias with the power.  Can scale just power tubes or wired to scale pre-amp as well.  I've installed a few of those.

Raybob

[TIMBO]

Has anyone got a schem or a layout for this?

[daven]

I saved this a while back from a now-dead geocities link (www.geocities.com/ge0ne0).  I successfully used the bias control portion with a different main power supply scaler in a single-channel deluxe reverb build.  I used a different scaler because the ST STW9NK90Z MOSFETs I've been using don't seem to like the circuit here or dana hall's similar VVR, but they're a lot cheaper than the NTE parts.  (it could just mean that some of the component values need tweaking, but I had a working circuit and was too lazy to mess with this one.)

[jojokeo]

Hi guys, Can anyone confirm that using a VVR for a Cathode Bias Amp is not suitable for a Fixed Bias Amp. Thanks

Timbo, why two EZ81s? Where's the preamp? The pi values are for a 6V6/6L6 and not EL84s. Where'd that schem come from?

[TIMBO]

Hi jojokeo, its all very much an experiment. A combo of different amps, the preamp and PI Mesa boogie, power amp 18w marshallish so the values at the moment are just a starting point. Feel free to add anything. Thanks

[jojokeo]

That's okay, just curious. It would be a good exercise to follow along w/ Aiken or Merlin on how it's done say for the pi stage. You get the info free and just follow along with it...you should end up close to the 18 watt values and/or some of geezer & tubenit's values on their amps that you see. (or others using two el84s)