Ampeg V4 Twin Bias Mod

[Diablo]

I want to put two adjustable bias trim pots in my V4. My plan is two build two bias circuits just like the standard V4 circuit, but sub a 60K resistor + 20K trim pot for the 75K resistor. My intention is to be able to bias both sides of the push-pull output tubes independently. The schematic shows that the phase inverter (12AU7) outputs from pin 1 and pin 6 are bridged by two 100K resistors with the bias output voltage connected between the two 100K resistors. If I disconnect this bridge to supply my two bias output voltages for each side of the push-pull, will this create a problem? I plan to keep the 100K resistors in each circuit from the output bias.

[Rob Mercure]

Diablo,

Are you going to duplicate the entire bias set up - voltage divider cap, diode, filter, etc. - for both sides or just use the original set up and install two variable bias outputs? The second is easier but you've got to remember that if you install two 80K (75K) sections in parallel you're dropping the resistance in half. So rather than use the 60/20K setup use two 120K fixed resistors and two 20-25K pots - the resistances aren't critical as long as you don't use too low a resistance and pull down the bias supply - could use two 100K fixed resistors and two 50K pots (make sure your pots are linear). And do feed the bias to each side using the stock 100K resistors.

While some folks are uncomfortable with measuring current on the high potential side the V-4's come with 10 ohm 5W anode resistors and I like to replace these with 1% tolerance resistors and use this to monitor plate current - and this way you're not getting the screen grid current also.

Rob

[Diablo]

Diablo,

Are you going to duplicate the entire bias set up - voltage divider cap, diode, filter, etc. - for both sides or just use the original set up and install two variable bias outputs? And do feed the bias to each side using the stock 100K resistors.
Rob

Thanks Rob for the hints! I was going to duplicate the entire bias set up for both sides and just finished ordering the parts before posting the question. Actually, I bought enough parts to build 4 duplicate bias supplies, . If I want to get crazier in the future and put in a bias control for each tube. I ordered up some 1 ohm 1% resistors for the cathodes to measure current the chicken's way. The only thing I wasn't sure about was disconnecting the junction between the two 100K resistors. Doesn't part of the signal travel from the PI to the opposite power tube?

[Rob Mercure]

No! Each half of the PI feeds one bank of tubes - perhaps what's getting you confused is that this is an AB stage so each bank conducts for more than 180 degrees. But each half of the PI puts out all 360 degrees just opposite in phase.

While I personally like being able to adjust the bias on each output tube individually I don't bother to duplicate the basic bias source of cap, diode, filter, and resistor only the individual pots (I even share the "limit" resistor that sets the lowest bias voltage). And I don't know why but somehow it just seems silly to add a second current sense resistor when the design came with a perfectly good one - easy to use a DMM with leads with mini clips stay away from the B+ when adjusting current.

And you preferences may vary but I really don't like using the reversed biased diodes from each OT primary end to ground to limit flyback - instead I permanently solder a 180 ohm 20W mil spec vitreous resistor (got these surplus and they're tiny for the dissipation) across the two ohm secondary and then remove the diode. I've seen too many of the diodes go down during performance (and done a couple of "on stage rescues" during a performance).

Rob

[Diablo]

Thanks again Rob for the clarification.