blackface bandmaster bias cap

[spicoli]

Hi - just picked up a 66 bandmaster will all original electrolytics, which I intend to replace. The cardboard mallory bias cap in the amp is 50uf 50v, while the fender schematic (AB763) lists 25uf 50v.

Which should I replace it with?

Thanks

[MWJB]

100/100v, don't use a 50v cap because the cap will be run over voltage.

[deci belle]

A 100µF 100v electrolytic with the (+) end to GND, spicoli. Or you could have a 47µF 100v electrolytic (+) to GND in parallel with a 4.7µF 100v film cap. I've had it both ways on my '66 Showman. Maybe the experts can confirm why the caps are in parallel. I assume it is to lower the ESR value of the electrolytic cap…?

[spicoli]

Thanks - Is this because of modern wall voltages being higher - or was the voltage always under spec'd ?

[Fee]

Use the sprague atom 100/100 and then adjust your neg voltage (if not in range you need ) by changing the resistor to ground on the bias pot. Fender later upped that cap to 70mfd but you won't find them anymore. Best wishes.....

[Gtr_tech]

You don't need to use Sprague....just use a *decent* quality cap and yer fine. The reason for going to the 100v rated part is due to bias requirements of modern output tubes being all over the map. So if you have a pair of tubes that don't bias into the safe range until you have -61v on the control grids, the bias filter won't be stressed and possibly short....taking your bias with it.

[spicoli]

Thanks - what is the effect of the increased capacitance? The schematic list 25uf, mine has a 50uf and I'm being advised to go with 100uf. Is it just that you can't get a 50uf 100v?

[Fee]

it's true that some of the old values are not around anymore and also true that you need a little more range on the negative voltage to deal with some of the new tubes and that's what you can get with the bigger cap. Another thing is that they were a little under rated to begin with. Fender changed those to 70mfd which is no longer around. These days 100/100 does the job just fine. (and I didn't even say "Sprague")

[Enzo]

I didn't look at this one, but on some fender schematics it calls for a 50v cap, while the schematic calls for 56v across it. Clearly underspecified. 100v is common, cheap, and won;t be bothered by the circuit voltage.

This is a half wave power supply, so ripplke is potentially a little higher. But the current through it is minimal. But the larger value cap will potentially allow less ripple, ie hum can be reduced.

25, 50, 100uf are all commonly available, it is not a matter of supply. Caps are a lot cheaper now than they were 50 years ago, so when they spec'd a smaller cap to save a few bucks, it made more sense to them than it would now.

This cap is not in the signal path, it won;t change the tonal response. And it is not even in the power supply to the tube, in the sense of stiffening B+ or something. The ONLY way a larger cap will affect the amp is it might reduce hum some. That and it will be more reliable in the long run.

[Gtr_tech]

it's true that some of the old values are not around anymore and also true that you need a little more range on the negative voltage to deal with some of the new tubes and that's what you can get with the bigger cap. Another thing is that they were a little under rated to begin with. Fender changed those to 70mfd which is no longer around. These days 100/100 does the job just fine. (and I didn't even say "Sprague")

Nice. I was starting to think you were an outside sales rep for Vishay

[Fee]

Nice. I was starting to think you were an outside sales rep for Vishay

I'm not "not saying" Sprague

[spicoli]

I'll say F&T (although I did use sprague's in the aforementioned bandmaster).

[Fee]

good man

[jrfrond]

100/100 is a pretty universal bias cap value nowadays. The 50V cap Fender used was under spec, but they still lasted for decades.

[pdf64]

Remember - positive end to ground!