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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Feb 2019, 9:57 pm   #1
G6Tanuki
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Default Leaky as a sieve: Waxies in metal cans.

Just excised a couple of these Dubilier Type 4703C capacitors from a late-1940s PA amplifier [EF37A, 6SN7GT, two 6L6, 5U4G]

Doing my traditional test - wiring the capacitor in series with a NE-2 neon and hooking up a 350V supply - the neon glowed impressively brightly even aftert the initial charging 'flash'.

They're being replaced with nice modern reliable yellow capacitors.

Does the "Z115507" designation imply they were perhaps originally made for the Army? Or is that only "ZA"-marked stuff??
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Old 27th Feb 2019, 11:20 pm   #2
ex seismic
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Default Re: Leaky as a sieve: Waxies in metal cans.

My WS31 has quite a lot of these, although 0.01 not 0.1. The all have a Z number.
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Old 28th Feb 2019, 10:01 am   #3
M0FYA Andy
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Default Re: Leaky as a sieve: Waxies in metal cans.

I'm always amused when folk use phrases like 'nice modern reliable yellow capacitors'. When they are as old as the ones you are replacing (70 years old) they may be just as leaky - we simply don't know!
So many threads on here discuss the problems caused by various plastics starting to degrade after maybe 20 years.
It may well be that in 30 years people will be cursing those 'nasty leaky old yellow plastic capacitors'!

Andy
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Old 28th Feb 2019, 10:46 am   #4
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Default Re: Leaky as a sieve: Waxies in metal cans.

Good point Andy I find that I am replacing those nice blue/grey Radio spares ones now . Mick.
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Old 28th Feb 2019, 2:26 pm   #5
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Default Re: Leaky as a sieve: Waxies in metal cans.

What about the class X capacitors we are recommending which half in value over five years?
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Old 28th Feb 2019, 2:40 pm   #6
Maarten
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Default Re: Leaky as a sieve: Waxies in metal cans.

Use the slightly more upmarket ones. Those that are also rated for series supplies should be fine by definition.

Of course, X-rated capacitors are only needed when you use them across the mains (even series supplies capacitors or snubbers need not necessarily be X-rated). On the secundary of the transformer or elsewhere in the circuit, a good quality MKP capacitor with a suitable WVDC rating is always the best replacement.
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Old 28th Feb 2019, 3:06 pm   #7
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Default Re: Leaky as a sieve: Waxies in metal cans.

Dunno, I think you've got it wrong Tanuki

Just look at those capacitors, shiny! almost like new and with military type numbers printed on them... and that number begins with a 'Z' one of the most promisingly mysterious letters in the alphabet. They look just like NOS and MIL-SPEC. They must be worth a fortune. They sure are going to sound very authoritative with more soundstaging than you can fit in a domestic room without folding it. The problem is that you've measured them and that is known to ruin the sound.

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Old 28th Feb 2019, 3:29 pm   #8
Hartley118
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Default Re: Leaky as a sieve: Waxies in metal cans.

It's that extra leaking grid current in the output bottles which lends so much authority to the sound.

I remember buying brand new wax paper capacitors in the 1950s. They were ex-WD surplus stock, beautifully packaged, and probably no more than a few years old. But those brand new capacitors were already leaky: it seemed that wax wasn't a sufficiently waterproof encapsulation.

By comparison, Hunts Moldseal capacitors were a leak-free joy to use back in the day. By now of course, the Mold has usually lost its Seal and they leak. I guess that the problem was that paper dielectric which has to be completely dry and stay that way for over half a century. Some 'breathing', drawing in damp air, would seem almost inevitable in the long term.

By comparison, our modern capacitors usually use a polyester dielectric which is inherently resistant to damp and may well be longer lived. We shall see........

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Old 28th Feb 2019, 4:14 pm   #9
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Default Re: Leaky as a sieve: Waxies in metal cans.

The wax capacitors relied on the set being used daily to keep them dry.
You would have to put the set on a warm surface for days to dry them out.
For occasional use in a collection the capacitors would be best changed.
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Old 28th Feb 2019, 5:37 pm   #10
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Default Re: Leaky as a sieve: Waxies in metal cans.

Quote:
Originally Posted by Hartley
By comparison, our modern capacitors usually use a polyester dielectric which is inherently resistant to damp and may well be longer lived. We shall see........

Martin
Well, judging from the longevity of those nice Philips/Mullard C296 series “mustard” axial Polyester capacitors, available from around 1962, I think we know.
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Old 28th Feb 2019, 6:27 pm   #11
G6Tanuki
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Default Re: Leaky as a sieve: Waxies in metal cans.

Quote:
Originally Posted by Hartley118 View Post
It's that extra leaking grid current in the output bottles which lends so much authority to the sound.

Martin
In this case the problem seems to have been that at switch-on, because of the directly-heated rectifier, the HT rail came-up quickly (to something close to 500V) and sat there for 10 seconds or so before the 6L6 pair started to draw enough current to pull the HT down.

The offending capacitors, taking a dislike to this (despite their voltage-rating) initially allowed something around 6V positive on the 6L6 grids, dropping back to about 4V once the HT rail voltage dropped - which it did somewhat more-than-expected because the 6L6s were passing more current than they should have.

With the capacitors replaced, the 6L6s are now saved undue stress and are each passing around 15mA less anode-current, which can only be good for their future life [they're 'Haltron' branded, from memory Haltron were one of the 1950s/60s rebranders of Eastern European-manufactured valves].

The sound - such as it is - doesn't seem to have any detectable change in 'authority' or 'presence' or 'fullness' - but it _is_ a PA amplifier - so loudness has more of a premium than quality!
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