voltage doubler question

[smashguitars]

Hi and happy holidays!

i got a question about using a voltage doubler. in the arrangement where the current draw is rather minimal (the doubler will be used to supply plate voltages for 2 tubes)

what kind of filtering will be required in the doubler circuit? this will be a high gain tube pedal, so space is minimal. do i need to have two 220uf caps in the double arrangement? or can i get away with using much smaller size caps like a 47uf? or even a 22uf?

thanks

[Steve Conner]

It depends how much ripple you can tolerate and what current you're drawing. But you should get away with far less than 220uF. That's the kind of capacitance you would find on the main B+ rail of a 50w amp, not a pedal.

Try 22uF, and if it hums too much, add a series resistor of a few kOhms, and another 22uF to ground after that, to make it a pi filter.

[smashguitars]

Steve, thanks for your reply. Basically, this will be a tube preamp in a stomp box. so i got 2 12ax7, where i'm using all 4 stages. I want to supply approx 350V B+ to the plate resistors just like i would in a 50watt amp. So 4 stages with typical plate current of say 1.5mA i get a total of 6mA of current draw for everything.

i guess the ammount of ripple i can tolerate is what i'm trying to figure out. Normally i would just use a 220uf and be done with it, but space is really tight and i need to use only as much as i need to filter. Also i'm doing a pcb for the pedal, so adding another filtering stage is not as easy.

[Steve Conner]

I think the hum might be a little too much if you just hooked all your tube stages straight to a doubler with 22uF caps. However, if you add a 10k dropper resistor and another 22uF filter cap, it should be fine. I don't know of any amps that don't use some pi filter arrangement like this.

If you're cascading all four tube stages to get huge gain, you might need to isolate them by breaking it up into two dropper resistors and two filter caps. One for the first two stages and another for the second two. With all four off a single filter capacitor, you might get problems like motorboating.

It's always a good idea to make a rough prototype and check that the circuit works, before designing the PCB for it. Then resist the temptation to change anything! :P

[soundmasterg]

With a doubler, your actual cap value as far as the cicuit is concerned is half of what is printed on the side of the caps. So two 22uf caps for the doubler end up being one 11uf cap. I'd suggest to go a bit higher...say a couple 40uf caps so you end up with a 20uf that the circuit is seeing.

Greg