Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on January 22, 2011, 03:54:07 am

Title: Adding VVR to fixed bias amps
Post by: TIMBO on January 22, 2011, 03:54:07 am

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

Title: Re: Adding VVR to fixed bias amps
Post by: mcrracer on January 22, 2011, 05:59:29 am

Not suitable. Two different units,

Title: Re: Adding VVR to fixed bias amps
Post by: LooseChange on January 22, 2011, 06:16:34 am

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

Title: Re: Adding VVR to fixed bias amps
Post by: Raybob on January 22, 2011, 12:59:55 pm

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

Title: Re: Adding VVR to fixed bias amps
Post by: TIMBO on January 22, 2011, 02:23:46 pm

Has anyone got a schem or a layout for this?

Title: Re: Adding VVR to fixed bias amps
Post by: daven on January 22, 2011, 04:44:18 pm

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.)

Title: Re: Adding VVR to fixed bias amps
Post by: jojokeo on January 22, 2011, 05:36:23 pm

Quote from: TIMBO on January 22, 2011, 03:54:07 am

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?

Title: Re: Adding VVR to fixed bias amps
Post by: TIMBO on January 22, 2011, 05:43:54 pm

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

Title: Re: Adding VVR to fixed bias amps
Post by: jojokeo on January 23, 2011, 12:12:27 am

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)