Films for bias caps?

[rp]

Any reason not to use these as bias filter caps? They are about the size of a .1/630V 715 Orange Drop which isn't too huge. Bullet proof bias supply?

http://uk.farnell.com/vishay-bc-compone ... dp/1166121

BTW apparently there are metalized film Orange Drops out there, besides the above there's also these. Probably like the Panasonic or Xicon brown drops but orange dipped. When describing caps orange is just a color at this point.

http://www.hificollective.co.uk/capacit ... s-dme.html

[Stevem]

No reason not to if there on hand already, but a fast recovery diode if it's not tube based will be a better change to make, or atleast add a .01 400 volt Ciramic across the current diode in use.

[rp]

Steve, do you mean in the bias or rectifier? And the disc cap across the bias diode? Does a fast recovery diode help in the bias? Never occurred to me to try, I have both but my UF ones are big high amp, would look like silly overkill in the bias - I worry about looks

I'm already a fan of fast recovery diodes in the rectifier, in the mid 90s I put hexfreds in my C-J hifi amp and was amazed at the difference. Definitely got rid of some silicon-ness to the sound, blacker background, more liquid. Been sold on them ever since.

[Leo_Gnardo]

Any reason not to use these as bias filter caps? They are about the size of a .1/630V 715 Orange Drop which isn't too huge. Bullet proof bias supply?

Kinda big, kinda 'spensive. Bulletproof, yeh that too. Sort of way overkill for a bias supply especially in a guitar amp.

FWIW I started using UF4007 in most hi voltage and bias supply repairs & builds a couple years ago, not awfully expensive, in fact just a bit more than regular old 1N4007. Recommended by audiophiles, those nutty hi fi types, but truthfully I don't hear any improvement. Guess I'll have to set up an A/B test sometime. You can read acres of print about it on Audio Asylum.

For a bias supply, all you need is simple and well-filtered, to the point you don't hear it injecting hum or buzz into your signal. One 100 uF 100V or a couple of smaller caps in a 2-stage filter has proven to be plenty enough for most any amp. Some claim large bias filters make the sound slow or sludgy - I don't hear any problem at all. OTOH insufficient bias filtering leaves you with a buzz that won't go away and makes clean playing sound crummy especially when playing at low volume and during note fades.

[Stevem]

Really! Your concerned about the looks of a diode that even in a 8 amp version is only twice the size of a 1N1007, so adding a Ciramic cap across a regular diode looks better?!

[David Root]

I used three Panasonic 10uF 250V films in the bias for (of all things) my '57 Twin build. Worked fine.

[Stevem]

When your plate voltage starts to sag by 30 volts what does your bias voltage dip to with those 10 uf values in there?

[JoeCon]

Guys

I see a couple of 8uF caps in Marshall bias supplies? What gives?
I also used a couple of 10uF caps in my Plexi 6V6 build recently, do I need to change those out??

Thanks...

[sluckey]

Been working fine for Marshall for a looong time.

[John_P_WI]

The capacitor charging time constant, tau=c x r.

Since we are dealing in micro farads, the difference is 0.002 t per 1k of resistance. About 2/3 of the charging or discharging of the cap is within the first t period, therfore a couple of micro farad should not make much difference unless the resistance is absurdly high.

[Leo_Gnardo]

I see a couple of 8uF caps in Marshall bias supplies? What gives?

And in the larger tweed Fenders too. That's one way to do it, a two stage filter with smaller caps. Or you can single-stage it with a big cap. Or - if you want 70's Fender overkill, two stage filter with a couple of 70 uF caps. These days you'd have to select 100 or 47uF, or special order 68 where you can find 'em.

Solder n listen. If one way sounds better to you, stick with it. Plenty of perfectly acceptable amps have been made either way. If you did a double blind play/listen test I bet anyone would be hard pressed to tell the difference by ear.

[rp]

Been working fine for Marshall for a looong time.

And those old Marshall and Fenders sounded glorious, so who knows if the underspecd bias caps are part of the mojo. Hardcore violin makers still use ground up insects for the lacquer. I know chopped up, boiled, dried and re-constituted old horses make a better acoustic guitar and amp cab. Why mess with a good thing?

Honestly though, I think Marshall & Fender used such small values in the bias 'cause caps were big back then and they had a hard time cramming anything bigger into the bias section.

Still haven't figured out if stevem recommends placing a snubber disc cap across the bias diode. I've never seen it done there, just across the power supply rectifier diodes, but sounds like a good idea. And my defense of not using a 3A diode in the bias is simply because I don't want my amps to look like the were built from random parts, rather that I made the extra effort to source what's appropriate.