Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bbmade on December 26, 2022, 12:54:38 pm
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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).
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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.
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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.
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... 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.
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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?
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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 (https://el34world.com/charts/Schematics/files/Fender/Fender_pro_reverb_aa165.pdf) 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 (https://www.allaboutcircuits.com/tools/capacitor-Charge-and-time-constant-calculator/) = 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).