Grid leak decoupling/bias pot cap and bias reservoir cap in bias supply

[seveneves]

Hi all,

Have a question on failsafe bias for a Fender.

I am referencing 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."



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

Thank you!

[tristanc]

I used the same cap value and rating in my builds following Merlin’s pages. But only as I did the design and BoM before ordering.

I’d fire ahead and use what you have. Maybe the bigger value in C1.

edit: Merlin himself may reply on el34.

[Roe]

different values should work. but smaller caps are faster to charge than the bigger ones. Thus smaller caps are safer to use

[tristanc]

edit: Merlin himself may reply on el34.

And just like that: "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."

[martin manning]

I prefer this circuit: https://ampgarage.com/forum/viewtopic.p ... 77#p476377
It is fail safe, and preserves its ripple filtering ability even if the bias is set to the most negative value. In that case the circuit above will parallel C1 and C2.

[hayun]

There are no reason to match two cap values. Since the current is very small, 10uF is sufficient.

Smaller cap has advantage for faster charging.

Slow discharging also important for stablity, it could be achieved by the diode//resistor combo as shown on that page of Merlin’s.

[martin manning]

If the ripple gets too high, it will show up in the audio. Common mode cancellation is imperfect.