Estimate negative bias voltage

[Mick Bailey]

This is slightly off topic, but now when I started to talk about the size of capacitors in the power supply section. How does the size of capacitors affect the amplifier. My point being, why now just used some BIG capacitor after the rectifier so that the supply voltage is super smooth?

Big filter caps means more current to charge up - especially from cold when the cap looks more like a momentary short. Then you have to look at fusing and how to balance that initial current surge with 'normal' running. Tube rectifiers have a limit on the cap size immediately after the rectifier. Not so bad with SS rectifiers. Big caps also stress the HT winding at startup.

With PP amps only minimum smoothing after the rectifier is required due to hum cancellation in the OT primary. The rest of the filtering nodes and stage decoupling take care of the rest, with the result that V1a will have less than 3mV ripple. Increasing the first filter cap size doesn't make much difference with PP, though an SE amp can benefit to some degree. Either way it's diminishing returns. With most amps heater hum contributes more to noise than PSU ripple.

Back in the day amps were low-gain and tube rectified, and high-value high-voltage caps expensive and bulky, so 22uf smoothing caps were adequate. SMPS developments nowadays means that there are plenty of larger 500v clip-in caps to be had upwards of 220uf. In a well designed amp increasing filter capacitance causes more problems than it solves, though. The modest current draw of a tube amp means that relatively low-value caps can be used without suffering undue PSU ripple.

[überfuzz]

Thanks! Clears thing up, for me! (I opened an other thread, guess it could be killed now)