Capacitor question

[paddy ryan]

Hi, I have just stripped out an old wax capacitor from a projector: a 0.05uF 750 volt, trouble is I can't find an equivalent value cap anywhere, either on e bay, amazon, or other electronics websites. The nearest I have are a couple of TCC duomold caps in my odds and sods box valued at 1000 volts and 0.05uF. But... one reads 0.06 and the other 0.07, which I presume are out of tolerance. There are new ones on e bay, but could I use the ones I have got or the new ones on e bay, bearing in mind the old wax one was for 750 volts? Advice appreciated please.

 

[Les Jones]

Without seeing what the capacitor is being used for we can't say if the difference in capacitance would matter. Post a schematic of the area where the capacitor is being used. I suspect it is only being used as a coupling capacitor between stages of an amplifier. If that guess is corrrect then either of the capacitors that you have in stock should be OK.

Les.

 

[paddy ryan]

Hi Les, Yes it is a coupling capacitor, so thanks for your advice, I can now go ahead and replace it. Thanks again, great website!
Paddy Ryan

 

[Tony Stewart]

The Duomolds 0.06 or 0.07 will be fine as this type is generally high tolerance error and 25% error is insensitive to this application.

 

[paddy ryan]

Thanks Tony thats great, learning a lot on here! This old vintage equipment is a nightmare to equate values, apart from the old caps, some of the resistors are just one colour, bad enough with the 2 colour ones, but have found some good sites to help me out. Thanks again.
P

 

[rjenkinsgb]

some of the resistors are just one colour,

One common early standard was "Body, Tip, Spot" for the three colour coding - the main body colour, a different colour on one end plus a colour dot somewhere on it..

The other end may have a tolerance colour as well, otherwise they were 20% if I remember correctly.

 

[Lax Luthier]

Whether or not your .06uF and .07uf caps are out of tolerance depends on what the
specified tolerance of the original .05uf was. The .06uF would be +10%, and the .07 +20%.
From your description of the original, there was no discernable marking of tolerance. You were able to measure the duomolds, was the value in uF of the original .05uf measurable, or had it tested bad?
That said, as was previously mentioned, these (duomold) values would be acceptable in the real world. I would also surmise that a wax type in a device as old as a film projector would stand a likely chance that its value would have drifted over its lifetime.

 

[Nigel Goodwin]

Whether or not your .06uF and .07uf caps are out of tolerance depends on what the
specified tolerance of the original .05uf was. The .06uF would be +10%, and the .07 +20%.
From your description of the original, there was no discernable marking of tolerance. You were able to measure the duomolds, was the value in uF of the original .05uf measurable, or had it tested bad?
That said, as was previously mentioned, these (duomold) values would be acceptable in the real world. I would also surmise that a wax type in a device as old as a film projector would stand a likely chance that its value would have drifted over its lifetime.

Jesus! - it's a coupling capacitor in a valve amplifier - it's value is in no way critical, and pretty well anything 'sensible' will work perfectly well. There's certainly no concern about the tolerance on the original ones

Assuming the following stage has a grid leak resistor of 1Meg (a typical value) then even a 0.01uF will give a 3dB point of about 16 Hz. Anything from 0.01uF to 1uF (or even wider) would be perfectly fine, and not affect the performance.

Coupling capacitors back in those far off days VERY commonly went leaky (and certainly the wax coated ones did), causing the grid of the following valve to become positive, causing it to pass excessive current. Luckily 'modern' capacitors rarely exhibit the same leaking issues, and replacing those old caps means they never need doing again - you would really have thought they would have been replaced back in the 70's or 80's?.

A typical valve refurb would consist of replacing the coupling capacitors, the anode loads of any triodes - and any capacitor that said Wima on it, as it WOULD be faulty

If the desire is to leave the amplifier looking as original as possible, then carefully disassemble the old capacitors, and fit the new ones inside the old casings - they should fit easily, as they are much smaller. The same applies if you're replacing electrolytics.

 

[paddy ryan]

Cheers gentlemen, all clear now, this projector is nearly 80 years old! (Ampro Premier 20), and very fiddly to get at anything, but I soldier on, only way to learn! Thanks again.

 

[Nigel Goodwin]

Cheers gentlemen, all clear now, this projector is nearly 80 years old! (Ampro Premier 20), and very fiddly to get at anything, but I soldier on, only way to learn! Thanks again.

As time goes by, 80 years old isn't that distant any more

WW2 era electronics were getting relatively 'modern', they had learnt how things were best done - going back to the 'early' valve days circuits were often quite obscure.