Zener regulation for fixed-bias supply

[EL34Dave]

I am considering adding zener regulation to a simple diode-C-R-C bias supply. My first question is how beneficial is a mod like this? Next, do I simply add the zener in series with the above components, and if so where in the chain is it best placed? And how should its polarity be set? Also, assuming the input is approx 50 volts, and the desired output voltage is about 40 volts, what value zener should I use?

Thanks in advance,
Dave

[bigwill]

Not really recommended unless you also regulate the screen / HT voltages. With typical mains voltage variation and no regulation, the bias voltage gets more negative in the same proportion as the HT rises, resulting in steady quiescent current. If you give the bias supply a steady reference then increasing HT will result in an increase in quiescent current, which may cause drift you haven't considered...

[kevinkr]

As bigwill indicated if you regulate the grid bias at minimum you would also need to regulate the screen supply (assuming an SEP amp) otherwise variations in line voltage will result in significant shifts in operating point, changes in the unregulated bias voltage to some degree help compensate for the varying B+. (Not perfectly it is admitted) In the event that this is a SET you would need to regulate the plate voltage and this is altogether a more complicated undertaking.

As an aside most of my DHT SET amps have fully regulated supplies..

[SY]

You also have to regulate the heater/filament supplies if you want the outcome to be stable with changes in mains voltage.

[bigwill]

Originally Posted by SY

You also have to regulate the heater/filament supplies if you want the outcome to be stable with changes in mains voltage.

An easily overlooked point!

[richwalters]

I use both B+ regulation and neg bias regulation in my series of power amps; but not the 6.3V power stage heaters. On this I cannot see much point as the specs for heater operation covers the mains voltage variation with very little cathode current performance change.
Some power tubes are emission sensitive to heater volts despite the spec quotes; the 6550B is a lovely tube but by golly does it vary with heater volts. In comparison, the EH KT90 & some KT88 versions remain rock solid.

richy

[kevinkr]

I almost always use regulated DC on the filaments, never occurred to me to mention that..

Have found most beam power types don't care much at all as rich points out - these I usually heat with AC because of the currents involved.