Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fazeka on June 04, 2025, 12:40:55 am

Title: Grid leak decoupling/bias pot cap and bias reservoir cap in bias supply
Post by: fazeka on June 04, 2025, 12:40:55 am

Hi all,

Have a question on failsafe bias for a Fender.

I am referencing https://www.valvewizard.co.uk/bias.html (https://www.valvewizard.co.uk/bias.html) and find the following:

"C2 is added to decouple the grid leak circuits from one another (when more than one valve share the same circuit) and simultaneously acts as an RC smoothing filter with P1. Since the bias supply draws very little current we only need one stage of smoothing in addition to the reservoir capacitor. Both capacitors are typically 10u to 100u –they’re not critical."

(https://i.imgur.com/yvcjYs0.jpeg)

My question is: do C1 and C2 have to be equal in capacitance? i.e., Would there be a detriment to use a 22uF and a 47uF for each of these? I'm trying to avoid placing an order for a matching cap that's going to cost less than the shipping...

Thank you!

Title: Re: Grid leak decoupling/bias pot cap and bias reservoir cap in bias supply
Post by: Merlin on June 04, 2025, 03:53:16 am

No they don't have to be equal. C2 isn't even essential, it just improves smoothinig and decoupling if that's something you want.