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Old 19th Apr 2019, 10:40 pm   #1
ColinK!
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Default Grid coupling capacitors

Just looking at changing a leaky grid coupling capacitor in a Murphy TA160, C72 and whilst I'm at it, I'm going to change C73. In the parts list its telling me that C72 is a Sealed paper tubular, and C73 an Insulated sealed paper tubular, capacitor.

I'm wondering what the difference is between these and a plain old Paper tubular cap, is it just the fact that they are Insulated and sealed? I'm guessing that they can be replaced with a normal axial polyester cap or should i be fitting something else in their place?
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Old 19th Apr 2019, 11:16 pm   #2
Sideband
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Default Re: Grid coupling capacitors

An ordinary polyester will be fine. Don't get too bogged down with terminology. It's important sometimes especially in RF stages and tuned circuits but with AF coupling caps, polyesters of the same or higher voltage will be fine.
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Old 19th Apr 2019, 11:31 pm   #3
ColinK!
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Default Re: Grid coupling capacitors

Cheers for that. I've already changed them, with much improvement to audio output, but I did wonder afterwards if I done the right thing !
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Old 20th Apr 2019, 6:58 am   #4
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Default Re: Grid coupling capacitors

A modern polyester will be 100s times better than an old paper type whatever construction the original claims to be!
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Old 20th Apr 2019, 10:11 pm   #5
Mike. Watterson
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Default Re: Grid coupling capacitors

Quote:
Originally Posted by ColinK! View Post
In the parts list its telling me that C72 is a Sealed paper tubular, and C73 an Insulated sealed paper tubular, capacitor.

I'm wondering what the difference is between these and a plain old Paper tubular cap, is it just the fact that they are Insulated and sealed?
The differences in the paper caps (ALL "oil"*/grease impregnated) is the lifetime. The cheapest waxed card was maybe 8 years. The domestic metal can with good end seals maybe 15 to 20 years. The metal tubes with rubber bungs maybe 10 to 12 years.

I'd not trust any NOS expensive Audiophile PIO (Paper in Oil), some are leaky.

* Cheap "oil" is some grade of paraffin or petroleum jelly.
The problem is that paraffin and the thicker than Vaseline/petroleum jelly coating will gradually allow moisture into the paper. They will often still test fine at even 20V. I use a modified flash gun charger from a single use camera and 2 x 1M in series with + (300uF replaced by plastic 1uF). The - has a neon with 100nF plastic 250V to 630V type. A slow flash is Giga ohms. An almost continuous flicker is more than 1uA. Oscillating so fast it looks like solid on is typical for ANY type paper cap.

Any modern non-electrolytic is fine. Metallised Polyester is best value. I do use XY rated ceramic disks at 1KV or 2KV for "tone correction" cap on final anode (may be to HT across transformer, or to 0V or via a tone control) and any sockets on live chassis sets. Good for aerial sockets anyway.

Polystyrene melts too easily. Polycarbonate, Teflon etc too expensive and no advantage. Older plastic caps may be separate plastic film and foil rather than metallised. Doen't matter. Sometimes on battery valve sets I use all ceramic, most only need 150V.
The caps should have a higher voltage rating than HT. The screen grid and caps on anode to next stage etc get full HT if a heater fails or valve unplugged. On output stages rating at least HT x2
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