Metropoulos Forum

I have some .22uf, 630v Mallory 150 series caps. Would these be any good as snubber caps?

I also have some Xicon polypropylene .22uf, would these be any better?

Thanks

Snubbers across the HT diodes? Yes, they are suitable. Though I'd prefer caps with higher voltage ratings there.

george

VelvetGeorge wrote: ↑

Tue Aug 22, 2017 1:40 pm

Snubbers across the HT diodes? Yes, they are suitable. Though I'd prefer caps with higher voltage ratings there.

george

Aren't they usually 1000v rated?

Ive seen Dubliers makerd with 600v on some stock Metal panel amps

Thanks guys, better play it safe and get some higher voltage ones.

Right then fellas I need some schooling. I've been looking at caps and have got myself completely confused.....

So I've been looking up 1000v caps and I'm turning up 1000vdc which turns out to be 500 to 600/630vac. I can appreciate the difference between a.c. and dc but for the hv diode snubbers is it 1000v a.c. or dc I'm after?

Am I right to think a.c., pre rectification?

Also best cap type for this application. Metalised polyester or metalised polypropylene? And the reason behind it?

I have used the Xicon metalized polypropylene before but am thinking astheticaly that a tubuar cap would look the part. Obviously performance for the task is most important but i dont know enough to make an educated decision.

Any recommended type and or brand?

Cheers

as I understand it modern spec diodes do not create much hash so you dont really need them

but yes 1000vdc is what I would expect.

If the cap is in parallel with the diode you really have pulsating DC, with high frequency switching noise. If it's from the output of the bridge to ground, then it's DC, still with switching noise.

Correctly rated would be 1000V DC, over twice the expected DC voltage.

If you use faster diodes a lot of the noise is eliminated and the snubbers are not necessary. Try UF4007's.

george

Thanks guys, I've already staked out the circuit boards I made from scratch. I don't really want to try cut off the ends

One of the bords I made from paxolin and it chiped so easily it took quite a few attempts to make one without any chips or blowouts.

I've got the big fat fast recovery diodes so It sounds like I don't really need the snubbers. I'm going to get some anyway to be vintage correct and because I don't want to cut the end of the bords off.

http://www.ebay.co.uk/itm/1-Pkt-RS-Comp ... Swe4ZZlsY9

These RS branded Dubiliers will look the part!

Neikeel you are truly a gentlemen.

I bought some Cornell Dubilier 940c 1000vdc metalised polypropylene this morning. Those RS are really the business so they are going in instead, perfect! Thank you.

So when George said about the snubbers being in parallel with the diode is this like what I have seen on the newer re-issues? I was planning on doing it like in most of the layouts I've seen here with the two caps on the end of the board.

Didn't think of this but is there a reason not to use snubbers?

revolver1 wrote: ↑

Thu Aug 24, 2017 1:47 pm

Didn't think of this but is there a reason not to use snubbers?

Only if they fail!
You see many people remove them on 50w amps when the PT fails (for the bias winding problem usually) so they rip out the snubbers and the diode block, typically use two 1N4007s and leave out the snubbers in a shotgun approach.
I doubt they fail but are whipped out randomly - someone else may be able to disabuse me of this theory.

Looking at building a 50w, dare I ask what's the bias winding problem?

In 1969 with advent of the stand up PT and brown fibre board, block rectifier and snubbers Marshall committed a howler where the white bias feed wire came off the HT after the standby switch so the output tubes saw HT a the same time as the bias voltage, the theory goes that big inrush currents ensued, output tubes suffered and PTs died. Earlier laydown PTs take bias feed before the standby switch so the tubes get bias before HT. Later amps (72 on IIRC) had this changed. later amps also get simultaneously switched live and neutral via a DP mains switch - much better as well.
You need to ensure the bias wire comes directly off the ac feed from the PT (before the HT).
If you use a GZ34 rectifier you could avoid a standby switch. Other amps with less marginal PTs (like 70s Oranges) cope without a standby but they have dedicated bias windings (not just a spur off one leg of the HT ac).