question about bias cap values?

[texwest]

I want to learn more about this cap in blackface fender amps. One amp tech I went to replaced it with a 100uf/100v cap on my Super reverb. Hoffman recommends a 47uf/100v. Hoffman said that you need at least 100 volt there, not the 50 volt in the schematic. Being less experienced I just replaced some of these with the ratings in the schematic on my other amps.

1--Does it affect the tone or feel of the amp to use bigger values for this cap i.e., the 100uf?

2--Is it important to use the 100 volt rating? Will 50 volt caps that I already installed work ok?

3--Do you use a different value for different amps? do you use a bigger value in a twin vs. a princeton?

Thanks for all the help

Wes

[Enzo]

All the bias supply does is provide a steady low current negative voltage. it is not in the signal path, it doesn't experience sag like the B+. (Though high power demands CAN draw the whole power transformer down a litle on peaks)

In the old days, caps were more expensive, so they used smaller ones than we would today. Plus there was the whole "good enough" manufacturing philosphy rather than "as good as we can."

IN some cases, the bias supply voltage would be 55 volts with 50 volt caps or worse not good, but they got away with it. Caps don't automatically fail the instant you exceed their rating. SO it makes sense to use higher voltage caps for reliability and safety. 100v caps provide plenty of room to breath for the cap.

Looking at a Deluxe Reverb AB763 I see a 25uf 50v cap. But by the A1270 version, I see they raised it to 50uf 70v.

The main affect on tone would be less hum, in my opinion.

Will 50v caps work? Maybe. Measure the voltage right across the cap. What's there? If it is more than 50v, you tell me how comfortable you are with it.

SOmeone recommends 47uf, someone else 100uf. The point there is not using a 20uf or an 8uf. I use 100uf, but if I ran out, I would be perfectly comfortable using 47uf. I wouldn't hold up the repair just to order that one cap. But I keep a pretty large drawer of 100uf caps.

It is a simple supply with minimal draw on it, just trying to make a steady clean voltage. I see no reason to use different values for different amps.

[Dave Curtis, dB AudioTech]

All the bias supply does is provide a steady low current negative voltage. it is not in the signal path, it doesn't experience sag like the B+. (Though high power demands CAN draw the whole power transformer down a litle on peaks)

In the old days, caps were more expensive, so they used smaller ones than we would today. Plus there was the whole "good enough" manufacturing philosphy rather than "as good as we can."

IN some cases, the bias supply voltage would be 55 volts with 50 volt caps or worse not good, but they got away with it. Caps don't automatically fail the instant you exceed their rating. SO it makes sense to use higher voltage caps for reliability and safety. 100v caps provide plenty of room to breath for the cap.

Looking at a Deluxe Reverb AB763 I see a 25uf 50v cap. But by the A1270 version, I see they raised it to 50uf 70v.

The main affect on tone would be less hum, in my opinion.

Will 50v caps work? Maybe. Measure the voltage right across the cap. What's there? If it is more than 50v, you tell me how comfortable you are with it.

SOmeone recommends 47uf, someone else 100uf. The point there is not using a 20uf or an 8uf. I use 100uf, but if I ran out, I would be perfectly comfortable using 47uf. I wouldn't hold up the repair just to order that one cap. But I keep a pretty large drawer of 100uf caps.

It is a simple supply with minimal draw on it, just trying to make a steady clean voltage. I see no reason to use different values for different amps.

I'd add that in some amps (generally not Fenders), 100uF bias caps are way too much; namely older Ampegs (like my VT22) or newer Marshalls (JCM2000). In my experience, 100uf caps in these amps cause the bias V to take a very long time to get where it needs to be. There are 10uF (stock value) caps in my Ampeg now. In general, don't go much more than 2X the stock uF rating.

[Bruce / Mission Amps]

I'd add that in some amps (generally not Fenders), 100uF bias caps are way too much; namely older Ampegs (like my VT22) or newer Marshalls (JCM2000). In my experience, 100uf caps in these amps cause the bias V to take a very long time to get where it needs to be. There are 10uF (stock value) caps in my Ampeg now. In general, don't go much more than 2X the stock uF rating.

OK I think that is fair statement.

I would think you would only have to worry about a 100uF cap (over a smaller one) if the Bias supply diode is being fed with a 100K to 220K resistor from the high voltage secondary tap, such as you'd find in quite a few amps using smaller value caps or as a JCM800, a small cap, a resistor and then another cap... like a CRC pi filter.
The time constant there could make a difference, but probably not in an amp like a Fender Reverb, with a 40v-55v bias tap and a 470 ohm resistor to the diode...

[Enzo]

And I guess it makes a difference what a "very long time" might be. I am not too concerned over "cathode stripping" being an issue in a little Fender sitting under-biased for 4-5 seconds.

[Bruce / Mission Amps]

And I guess it makes a difference what a "very long time" might be. I am not too concerned over "cathode stripping" being an issue in a little Fender sitting under-biased for 4-5 seconds.

Probably not but you are highly experienced.
I'm sure you've seen what happens with SS rectifiers in a B+ rail and a bias fed from one of the high voltages legs before the standby and through a 220K resistor... the charge time for full negative bias voltage is rather long compared to how long it takes to hit the plates with full B+... read that as the novice tube guy that doesn't understand how to use a standby switch.

Cathode stripping... what an internet driven rash of crap with respect to indirectly heated audio tubes at around 500vdc or less.
As you must, I always have to laugh when I read some Net guru spewing that near worthless bit of nonsense.

[Enzo]

Oh that, and skin effect, and a host of other phenomena that are real but not pertinent to our stuff.

I just count my blessings I am not in the hifi game. "My new speaker wires sound very good, excellent soundstaging, and once they are broken in thoroughly, they'll sound even better." Actual speaker wire customer...