Filter Cap Values?

[michaelj]

I am repairing a Bedrock 1200 head. The first filter is a pair of 150uf /350volt in series [with swamping resistors. Then to choke..]

I need to replace the first pair and am having trouble finding 150uf /350volt.

Help me if I'm wrong here but that pair in series works out to 75uf / 700volt if I'm not mistaken.

100uf / 350volt are readily available so would that work ? would end up 50uf /700volt? What could I expect having made that change?

Help and suggestions would be appreciated and any other changes I would have to make... new choke of a different value?

Enzo where are you!

Mike

[Anthony]

Filter caps bedrock

Hey Mike. I'm under the assumption that you are looking at Sprague TVA series axial capacitors. I would check the tolerance on the caps that went bad. My guess is that they'll at least have a + or - 20% tolerance. So you're going to at least gain or drop 15uf from that value on each cap. Point being, for a tube amp 15uf is not going to make or break the amp. In fact if there are cheaper caps in there now it might actually improve the longevity and low end in the power amp with the spragues in there. Go with the 100ufr's.

Anthony

[michaelj]

Thanks Anthony

Exactly what got me headed in that direction in the first place. The caps in there are Spragues and I think most of these axials were listed as having quite a wide "tolerance range" so I think I'll go ahead with the 100s.

Of course getting an component out of this amp is an adventure.

When these guys built these they must have had a deal with Dow or whoever makes clear silicone caulking. Everything is gooped down with what must be a half a dozen full tubes of the stuff.

A used high E string with a couple of wooden handles seems to do a pretty good job of getting in the tight spots with out screwing any thing else up.

AND NO I DIDN'T HAVE ANY PREVIOUS EXPERIENCE WITH A GARROTE.

Thanks again
Mike

[tubeswell]

I reckon that 2 x 40uF 350V - 500V caps in parallel would work as well. So long as you have roughly the same capacitance (within a 20% ballpark) and roughly the same (or slightly more) voltage handling capability you will be fine.

As for the way the caps are mounted, that caulking is so slap-dash and PITA! A couple of well-placed permanently mounted nylon mounting clamps which you can attach cable ties to is much neater and more recyclable :-)

[michaelj]

Voltage

I should have mentioned that these amps ran pretty high voltage. After the SS rectifier 600vdc + or - a bit hence the 350v series caps for a 700v rating for some margin.

So I need to keep the series thing going which means that I'm going to have to stick with values that will basically be half the capacitance of one of the pair.

From what I've been able to find that will also fit physically as far as axials go 100uf seems to be the most common at 350v or above. I have seen 220uf/ 350 or 475 that I might be able to squeeze in there but that seem excessive.

Mike