Bias cap value - Sluckeys Plexi

[pjd3]

Hi there,

Just curious if for Sluckey's 6V6 plexi I could use two 33uF bias caps as opposed to the 22uF that are called out on the schematic without running into any difficulties. It appears that in certain situations there is an acceptable range but, at my level I'm not qualified to make a call on this.

Thanks!

Phil D

[sluckey]

That'll be fine. I used 25µF/150V because I have a bag full. If I had to buy them I'd get 10µF/100V.

[pompeiisneaks]

The most I can find on this as a good answer is the valve wizard's discussion of the bias caps, his statement found here;

http://www.valvewizard.co.uk/bias.html

is that anything from 10uF to 47uF is usually fine for a bias supply. Maybe someone else can confirm that's kosher.

~Phil

[pompeiisneaks]

Heh sluckey beat me to it... looks good

~Phil

[pjd3]

Thanks Sluckey and Pomp.

That's the value available at Amplified Parts for a 160volt F&T cap, figured I'd make it part of the order since I'm just about ready to go with a list over there.

There was also a big thread (maybe at Gear Page) with alot of tech talk and math to justify when bias cap values would be too low or too high and why. I think the basic theory was that larger values would usually improve higher frequency noise but you would eventually run into a rise time that would put the amp in danger, maybe the power tube dissipation running away before the needed negative bias was present on the grid, I think it was. Also, some explaination about the impedance of the bias effecting different things in the power section, frequency response I think being one of them. The latter theory was just a bit beyond me but the other one made a little sense. Someone said that if your bias is wired pre-Standby switched and you actually used it you would protect your amp.

As long as I know I'm picking a safe one that works I'm good! Thank you! Its really nice to be able to bounce these things off experienced folks that have been down these roads.

Phil D