Yes, a good quality film capacitor like the 716P series works well for this purpose,
if there is no better circuit alternative.
if there is no better circuit alternative.
3rd reason: the B+ voltage change (and ripple) has opposite influence to the anode current than the negative grid voltage (and ripple). Their change cancel each other, more or less.
I don't see the purpose of using fancy film capacitors for just some bias voltage?
There is no audio passing through.
So using any cap that can sustain the voltage and is somewhat stable is fine.
Because of how the impedance of a capacitor works.
One example in the context of tube amplifier;
http://www.valvewizard.co.uk/bias.html
Bottom of that page.
But basically, the impedance Z of an capacitor is;
Z = 1 / (2*pi*f*C)
So you can reach a much higher resistance for dropping the voltage, without getting to hot (or at all in practice)
@b_force:
So you actually mean more efficient rather than more effective.
Both methods work, so are equally effective.
I agree that using a cap to drop the AC voltage is more efficient, because it avoids the dissipation of the resistor.
But considering the total power a tube amp wastes at idle the resistor dissipation (typically being below 0.5W) is insignificant.
I have never seen a resistor bias dropper fail.
But I have seen capacitor droppers fail shorted causing the bias filter cap to explode.
I think the reason is short but high mains voltage spikes.
A resistor tpically can stand low energy high voltage spikes, while a normal film/foil cap might develop a short.
Apart from sufficient voltage rating based on actual peak voltage in the circuit, a cap should be certified for class X2 in this application.
Using a capacitor dropper will not eliminate the slow ramp-up when using excessive filter capacitance.
So you actually mean more efficient rather than more effective.
Both methods work, so are equally effective.
I agree that using a cap to drop the AC voltage is more efficient, because it avoids the dissipation of the resistor.
But considering the total power a tube amp wastes at idle the resistor dissipation (typically being below 0.5W) is insignificant.
I have never seen a resistor bias dropper fail.
But I have seen capacitor droppers fail shorted causing the bias filter cap to explode.
I think the reason is short but high mains voltage spikes.
A resistor tpically can stand low energy high voltage spikes, while a normal film/foil cap might develop a short.
Apart from sufficient voltage rating based on actual peak voltage in the circuit, a cap should be certified for class X2 in this application.
Using a capacitor dropper will not eliminate the slow ramp-up when using excessive filter capacitance.
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That is the same thing?
We are working on the secondary voltage here, please don't do this directly on mains 🙁
So spikes are not nearly as bad.
These capacitors droppers are widely being used in all kinds of appliances.
So I don't share the experience of them being less reliable.
@b_force :
Effective : providing a desired effect.
Efficient: having good efficiency.
Mains voltage spikes will appear on the secondary when the winding is unloaded.
This situation happens e.g. with a Marshall JCM 900 when the standby switch is open.
Marshall seem to know as they specify a class X dropper capacitor.
Nevertheless failures have been reported (on the MEF) with the cap failing open, taking out the HT fuse.
Effective : providing a desired effect.
Efficient: having good efficiency.
Mains voltage spikes will appear on the secondary when the winding is unloaded.
This situation happens e.g. with a Marshall JCM 900 when the standby switch is open.
Marshall seem to know as they specify a class X dropper capacitor.
Nevertheless failures have been reported (on the MEF) with the cap failing open, taking out the HT fuse.
The capacitor voltage drop depends on the stability of its impedance at 60Hz.
And it must be bipolar as well. I would never use an electrolytic for this purpose.
You certainly don't want this part to fail and give 0V bias on the output tubes.
And of course film types are much more reliable. When I've done this with a 716P,
it has worked fine for decades.