Fender Bias Electrolytic Value

[bbmade]

I've been searching the forum for this answer but am wondering what opinions are on the electrolytic capacitor value on the bias board.  I see most people up the voltage and many use a 47uf/100V capacitor there.

I have a bunch of 100uf/100V electrolytics but no 47uf.  Is too much capacitance here an issue?  Just seems like more ability to smooth and not a tonal issue.

I have no idea why I don't have 47uf/100V considering I usually only work on Fender amps.

ALSO - the 470ohm resistor often gets replaced with a 2 watt metal oxide.  Any issue with using a 3 watt?

Any insights are appreciated (as always).

[PRR]

That's all OK.

You don't want to go 1,000uFd because (depending on details) it could take too long for the bias to come-up on cold-start.

Resistor wattage is space and cost.

[Latole]

Don't go up to 100 mfd, less is better. 

A too high value will make the bias voltage arrive more slowly on the tubes grid, while the capacitor is charging.
And that's not what the tubes want.

[HotBluePlates]

... the electrolytic capacitor value on the bias board.  ...

I have a bunch of 100uf/100V electrolytics but no 47uf.  ...

I put 100µF 100v in the 1966 Pro Reverb and 1967 Super Reverb I used to own.  They performed flawlessly, and I never used the Standby switch on either amp.

[rake]

why not put 2 caps in series? If that's all you have.
Correct me if I am in error but wouldn't 2 100uf 100v caps in series
end up a 50uf 200v equivalent?

[HotBluePlates]

why not put 2 caps in series? ...

Typical Fender amp has 470-1000Ω between the bias tap & the bias filter cap.  For example, my old Pro Reverb had 470Ω and 25µF/50v but the bias voltage was over 50v.  So I really needed ~100v rating for safety and a 100µF ca was convenient.

Let's use worst-case 1kΩ and a 100µF cap.  1 Time Constant = R x C, where R is in Ohms and C is in Farads.

     100µF = 0.0001 Farads

     R x C = 1000Ω x 0.0001 Farads = 0.1 second

     The link explains a cap is substantially charged in 5 time constants:  0.1 Second x 5 = 0.5 second

Even at worst-case, my Pro Reverb's bias has built up in about half a second.  However, its indirectly-heated GZ34 rectifier will take another 10-11 seconds to heat up and start passing high voltage.

The above is why I wasn't concerned about switching to the 100µF 100v bias cap, and why I never used the Standby switch (both of which are perfectly safe).