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Old 30th Jul 2009, 9:35 am   #1
'LIVEWIRE?'
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Question Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

These are the small-about 0.5" long red coated tubular types-(from memory Hi-K)used in VHF Tuners. I have some disc ceramic 220pf 1kV caps(CPC CA05045) and wondered if other similar ones in appropriate values, whilst not looking 'authentic', would be suitable. Obviously they need to be at least 250v DC working, high stability and close tolerance(+/-5% or better in some cases) I can see no mention of tolerance, etc. on the above, which are marked RR 221K 1KV.
Alternatively can the tubular types still be obtained?

Last edited by Darren-UK; 30th Jul 2009 at 9:54 am.
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Old 30th Jul 2009, 9:57 am   #2
ronbryan
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

The first 'K' in the RR 221K 1KV description specifies a +/-10% tolerance.

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Old 31st Jul 2009, 8:35 pm   #3
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

If they are "Hi-K" they're not particularly good on tempco, so presumably just used for coupling / decoupling. In which case pretty much any ceramic with a suitable voltage rating should do. Beware of excess lead length, though, or the self resonant frequency may be too low.
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Old 31st Jul 2009, 9:49 pm   #4
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

If they will fit, silver mica types should work here.

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Old 31st Jul 2009, 9:52 pm   #5
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

It may be that my memory is selective regarding the 'Hi-K' part, but i'ts something I associate with those, usually red, tubular ceramic caps of yesteryear. Didn't come accross them much in Car radios, which I repaired for over 23 years for a firm in Oxford, but they were used extensively in Domestic Valve sets of the 50s/early 60s, often as decouplers/couplers, etc., in VHF Tuners. Anyway I guess I will use modern ceramics similar to those I mentioned in post # 1. BTW What did the 'K' in 'Hi-K' stand for?
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Old 2nd Aug 2009, 10:14 am   #6
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

It's related to the dielectric constant of the ceramic used. The higher the K, the higher the volumetric efficiency but the lower the general goodness and stability of the capacitor.

A general description is here: http://www.capacitors.hk/Ceramic-Capacitors/
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Old 2nd Aug 2009, 1:31 pm   #7
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

It's related to the dielectric constant of the ceramic used. The higher the K, the higher the volumetric efficiency but the lower the general goodness and stability of the capacitor.
Yep, spot on !
HI K means high relative permittivity, (large values/small phyical size)
Mostly they were temperature sensitive, wide tolerance, suitable for RF decoupling at HF VHF as self inductance low, (values say from 1000pfd to 0.47ufd.)

Other type of ceramics (Low K) were more stable, with lower capacitance values per unit volume. Values probably from 0.5 pfd to a few hundred pfd, (can used for tuning and frequency drift correction (n type) but a cheaper capacitor than a silvered mica.
Available in both negative and positive temperature coefficient.
Would need to activate the grey cells for more info
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Old 2nd Aug 2009, 2:01 pm   #8
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I still think of the capacitance as K*A/(4*Pi*D). K is the dialectric constant in cgs units, A is the 'plate' area and D is the distance between the plates.
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Old 2nd Aug 2009, 5:03 pm   #9
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I agree Alan, K is used in this sense as a multiplier,
Permittivity of a vacuum being 1, ie no increase over that due to the dimensions of plates and spacing.
For air, just a tiddle over 1, and then increasing for various material used as the dielectric.
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Old 2nd Aug 2009, 5:12 pm   #10
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I've just read this Thread again and can I ask a dumb question? Why? I don't remember having any problems with these unless you subject them to too much mechanical strain and break them physically, or get them too hot and melt the connections to the silvered rings.
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Old 2nd Aug 2009, 7:43 pm   #11
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

Personally I found them to be quite reliable, apart from being broken by accident, or physical strain, as you suggest, they can go microphonic/ intermittent o/c, but rarely go leaky.
Being a disc they have low inductance which is ideal for RF/HF/VHF
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Old 3rd Aug 2009, 10:45 am   #12
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I posted the question because I have aproblem in a Bush VHF90A(Poor or no reception, and a lot of crackly noise), which may be due to decoupling silver ceramic capacitors having changed value. Although +/- 20% types were used, presumably they could have gone higher in value after over 50 years, which would surely have the effect of bypassing VHF signals to earth ( suspect cap is nom. 560pf) BTW What would be the Impedance & Reactance of a 560pf cap. @ 88Mhz. I know that the formula for reactance is 1 divided by 2PIx FxC, but i've forgotten the units involved. Is it uF & Mhz or uF & kHz? Neither gives a sensible figure!

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Old 3rd Aug 2009, 11:05 am   #13
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

Quote:
Originally Posted by 'LIVEWIRE?' View Post
BTW What would be the Impedance & Reactance of a 560pf cap. @ 88Mhz. I know that the formula for reactance is 1 divided by 2PIx FxC, but i've forgotten the units involved. Is it uF & Mhz or uF & kHz? Neither gives a sensible figure!
Farads and Hertz. Always the fundamental units. That's the beauty of SI units. We had a long discussion on units recently in the forum. Clearly you can uF and MHz because the 10E6 and 10E-6 will cancel.

X= 1/(2*PI*88E6 * 560E-12)

Gives a reactance of about 3.3R which will be the same as impedance for a capacitor.
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Old 3rd Aug 2009, 11:11 am   #14
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I (actually my calculator) make it 3.23 Ohms.
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Old 3rd Aug 2009, 12:43 pm   #15
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

presumably they could have gone higher in value after over 50 years, which would surely have the effect of bypassing VHF signals to earth
That would be fine, the job of the decoupling capacitor is of course to bypass unwanted signals to ground,(or rather prevent them from going where they should NOT go !) so if the value increased, it would (all other things being equal) be great !

More likely to reduce in value actually, rather than increase, perhaps you meant to say that?
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Old 3rd Aug 2009, 7:24 pm   #16
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Default Re: Suitable replacements for Tubular Silver Ceramic capacitors in vintage radios.

I had forgotten the 'Farads & Hertz' rule, Jeffrey, so thanks for reminding me(well it was about 40 years ago when I learned the theory!)I got 3.23 ohms, too, but it didn't register that the decoupler NEEDS to be a virtual short circuit to the frequencies it's intented to bypass. On a thread in the Vintage Radio section someone has said that, in a Murphy Radio, the corresponding cap. had increased in value, which reduced the sensitivity of the tuner, whilst in another, the cap. had decreased causing instability
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