Silver Ceramic Capacitor Replacement

[Mark R]

Ah right, on the HT rail, polyester cap of the same value or there abouts will fix it. make sure it is it is 240v working voltage or more.
I know I tought yon better Robert!

Cheers Mark

ps ta for the circuit young Joe

[Joe]

The VHF90 and VHF70 are certainly the nastiest Bush sets I've done.

Here's what I'd bung in:-

   

The big one is 630V, the small one 400V. Yell if you want either.

- Joe

[Bushbaby]

Thanks chaps.
I can replace easily if its not absolutely necessary for a ceramic cap. However, it has prompted me to buy some.

Thanks for your kind offer your Joe!

R

[Skywave]

You'll find a use for them, Rob. Ceramic and mica caps. can be used for almost any task where a capacitor is required. The limiting factor with them is predominately their low capacitance-to-volume ratio.

Al.

[Yorkie]

One thing that we need to remember is that the working voltage on a cap that the maker has used bears little relationship to the actual working voltage that the component has to cope with in the radio. The fact that a cap can safely be used at say 350V simply means that it can be used in all circumstance likely to be found in a radio. It would make no sense at all for makers to make caps in a range of voltages to suit the actual operating conditions and the economies of scale would be lost.

There's nothing wrong with using higher Voltage caps where lower voltages would be fine, but not of course, the other way around.

Most caps in a radio (except suppressor caps) only have to cope with DC voltages while allowing RF/AF signals to pass. In Rob's case, the anode voltage applied to C50 is only 60 Volts, so any working voltage over say 100V DC would be fine. I guess most of us tend to use 630V polyester caps (the yellow axial lead ones) but the case style or dielectric material in just about every case is uncritical. The only exception is in tuned circuits, where - for frequency stability - silver mica or polystyrene caps are usually used. If space is an issue, small high voltage ceramic disc caps (rather than say yellow polyester caps) are both cheap and compact, and have working voltages 20 times what Rob needs in this instance. EG a 2,200 pF 2000V disc ceramic from Rapid Electronics is just six pence:

http://www.rapidonline.com/Electronic-Co...or-08-1541


10nf 2000v High Voltage Ceramic Capacitors will set you back double that price - 12.4 pence each:


http://www.rapidonline.com/Electronic-Co...or-08-1543

In Rob's radio, C50 isn't a very critical component, but the one cap that should be changed is C51 'that cap' .01uF.

The term 'Voltage' is often bandied about without a full appreciation of what voltage is being referred to. In radios, mostly it will be DC, and usually caps will have 350V, 600V or whatever on them, inferring that to use anything less would be risky. A glance at the circuit will show that many such caps only have to handle from 10V - 100V at most. Also, we might wonder why a Class X or Y suppressor cap only has a working voltage of 275V. The reason is of course that it is AC rated. The peak to peak voltage of AC is 1.414 times DC, and RMS is .707 x the P-P Voltage.

There’s a very informative and easy to understand paper on Power and Voltage ratings at this link:

https://www.samtec.com/documents/webfile...102007.pdf

I’ve rambled right off Rob’s initial query as to a suitable replacement cap for C50 in his set, (the answer to which is just about any cap with a DC WV of 100V or more) but I hope it’s of relevance, so that rather than slavishly adhering to what the makers originally used for simplicity and economies of scale – standard voltage throughout a set – say six 0.1uF 350V DC paper caps, we consider what WV is actually needed. Often, a much smaller cap such as the disc ceramic ones referred to above would make life far more easier for us in tight spaces, and the WV is so high that it's not an issue.

Hope that’s of interest.

Rambled and dribbled Yorkie.

[pwdrive]

If a capacitor is connected to the anode or screen or HT I always make sure that any replacement has a working voltage that equals the main HT voltage + at least a 100v margin as a minimum, my own preference is the usual yellow jobies at either 400 or 650 vwkg or higher if across the output transformer etc. Capacitorst might only be subjected to low HT in the circuit under certain normall operating conditions but this voltage can rise substantialy under certain fault conditions ie: valve cut off etc etc.
Having now seen the circuit I would go with a bog standard polyester type if a ceramic one isn't to hand.

Lawrence.