P-Series Valves, what to do with them?

[Terry]

I'll think twice before using a diode in the future.

Not a problem, surely, if you also include a thermistor?

[Murphyv310]

Yes I don't like diode droppers. There were more heater failures in Thorn sets with diode dropper than any other set we came across. Initially in the TV22 I'd dispensed with the HT & Boost diode valves and was trying silicon, so the chain was even lower resistance. Wasn't happy with silicon so added a PY33 & PY81.

[pwdrive]

Figures Figures Figures, my head is nipping.
Ok so can we explain why we don't get the switch on surge?

With a resistor dropper the resistance will always be less than the reactance of an equivalent capacitive dropper, that's because the voltage across C is not in phase with the current flowing via C, this means that if the resistor dropper was connected across an AC supply more current would flow compared to the current that would flow if the capacitor in question was connected across the same AC supply.

Lawrence.

[Murphyv310]

That's correct. My microwave oven has two 2.2uf caps in parallel across the mains.
God knows where I was yesterday, an Avo 7 with a sticky pointer then discovering I'd bought a batch of wrongly marked caps and my brain doing a wobbly. Oh dear.

[ppppenguin]

Presumably the caps in your microwave are for power factor correction.

[Refugee]

I capacitor dropper is likely to have a high switch on surge if the contacts make at peak AC with a fully discharged capacitor.
A bridge rectifier with a shunt regulator across the DC terminals controlled by an R/C timer would be my component of choice with modern silicon prices.

[ppppenguin]

The surge in a heater chain is limited by the resistance of the chain. So for Trevor's eaxmple it would be 235/90 = 2.6A. Hardly damaging, as it will be going on for less than 10ms as the cap charges. This is also much less than the thermal time constant of the heaters. This is a different surge to the one caused by the low cold resistance of the heaters. That will go on for signifcant fraction of a second.

The capacitor is a good KISS solution. Easy, simple, cheap.

You can't use a cap dropper for HT, for example to replace resistive line cord. You might just about get away with it on a "cut and try basis" but the peak pulse currents taken by a halfwave rectifier make it very hard to predict what will actually happen. I suppose that if the heater chain current is a lot greater than the HT then you'd get away with a single dropper cap for everything but that's unlikely to happen in practice.

[Murphyv310]

You can use a cap for HT so long as on the input side to the rectifier there an other diode strapped to ground.    

[Kalee20]

Oooh be careful!! Because then you've got a voltage doubler and (depending on capacitor value) HT voltage could actually INCREASE!

Would be OK under normal conditions, but if the output valve stopped passing current (dodgy cathode valveholder pin, for example), the load would reduce and it could be bye-bye HT electrolytics!

[Murphyv310]

Indeed it does but using the original rectifier and careful selection of the cap your HT won't rise, I'd not contemplate using two silicon diodes TBH as the HT will surge up well above the working volts of the caps before the set warms up and HT stabilizes.