Bias Tracking VVR

[craigmillard]

Hi Guys,

I have been reading Merlins book for power supplies:) and want to implement a high wattage VVR for an amp.

I have drawn up the circuit and think all is correct

Am i missing anything?


Can i adjust the R1, R2, R5, R10 and R11 to more standard values? i.e 220k and 330k. All other resistors will be 1/2w.

Cheers all!

[PRR]

It appears approximately correct at a glance.

Be very very careful!

> Can i adjust the R1, R2, R5, R10 and R11 to...

Did you design this yourself? Or just copying an existing plan? If you developed it, you should be able to figure what may change if 300K is changed to 330K, etc. If not, maybe you should work it out before you throw $99 of power bottles on it.

[craigmillard]

Hi Guys,

I have finally got round to testing this out and the bias tracking works great!

I am having an issue with the MPSA92 Q4 transistor, it keeps blowing, not straight away but will eventually break down. No smoke or sparks just turns into either a diode or straight short across all legs.

The B+ in the amp is 470v and its a 50w so should be with range of these transistors as they can handle upto +/- 300v and at that point should only be getting half the total voltage, max 250v..

Anyone got any ideas?

[PRR]

[Phoenix]

Errata

[craigmillard]

Thanks for the responses guys,

Phoenix - I had seen this but its related to the bias part of the circuit and mine is stable.

PRR - I know these are in a complementary differential pair, base of q5 will try to match q4 with the q5 collector output feeding the regulator but I don't understand what the diodes are trying to accomplish? Could you explain a bit?

[PRR]

You said "blowing, not straight away but will eventually break down." That makes me wonder about reverse emitter breakdown in transient conditions. The diodes clamp that.

Note also a resistor in the collector so current can't go "to infinity". It maybe can not be as big as shown.

[craigmillard]

Ah ok i see, so the diodes are clamping the base of both transistors from being to far apart (2v), therefore limiting the the Emitter−Base Voltage to below the -5v limit.

I didnt notice the resistor at first glance either!

I will give both a go see if it helps!

Thanks a lot PRR!!