When is it a bad idea to use ceramic capacitors?
 

Hey Guys,

Just wondering, when is it a bad idea to use a ceramic capacitor in a set. I'm nearing the end of my first TV recapping and have only a 1000 volt tubular paper vertical discharge capacitor left to replace. Would it be ok to use a ceramic disc capacitor here? I have one on hand...

Thanks!

Vertical is probably the worst place to use Ceramics, you will probably have linearity problems if you use it there.

I avoid using ceramics in vertical and horizontal sweep circuits in general, as Eric noted.

If you don't have the needed value in 1000-volt caps, you can combine two 630-volt caps in series to make one cap with half the capacitance and twice the voltage rating.

For instance, say that you need a .1mfd/1000v cap. Wiring two .22mfd/630v caps in series gives you a .11mfd/1260v cap. That should work for the time being and allow you to progress in your restoration, while you order the 1000v cap you need.

Phil Nelson

Glad I've been using polyester caps then. So would it be wise to just avoid ceramics all together?

Generally speaking, yes.
I have used them in the Horizontal circuit of some 7" Electrostatic sets with no problem but I know they won't function properly in the Vertical circuit of the same sets, something to do with the frequency capabilities or so I was told.

Ceramic caps are fine for RF bypass duty, you see them in tuners, IF strips and sound IFs. They rarely go bad, so you shouldn't need to replace those. You usually avoid using ceramic caps in any tuned circuit (though some specially made ceramic caps, made to not drift much over temperature, are found in RF circuits, they are usually 5 to 100pF range). And avoid using ceramic caps in RC oscillators, like vertical osc circuits. Or horizontal oscilators, where the cap is part of the timing circuit.

Audiophiles avoid ceramic caps in audio amplifiers for hifi work, but if the amp in question is the usual 1 watt tube driving a 4 inch speaker in the side of the TV cabinet, ceramics will work well enough

Hear hear! Well spoken and much too the point.

You need to be careful when referring to ceramic caps as there are different types. Class 1 ceramic caps can be used where film capacitors are used but are usually expensive. Class 1 ceramics (typically NPO/COG) are extremely stable, non-microphonic/piezoelectric with very low ESR and dissipation factors. In modern electronics I routinely design in Class 1 ceramics in sensitive areas such as coupling, timing/oscillators and PLL loop filters as surface mount film capacitors are expensive. The down side to Class 1 ceramics is they have very low volumetric efficiency so they are limited as to value/voltage rating, however values up to and beyond 0.01uF and 1KV are not unheard of. This still doesn't get us to the vertical coupling caps on an electrostatic set without multiple parts which is why films make sense in this application and why these parts work fine in the horizontal section.

Class 2 and 3 ceramics contain barium titanate which has a much higher volumetric efficiency which is why it is used, but is also what gives what people think of as ceramic capacitors a bad name.
Class 2 and 3 ceramics have very poor to extremely poor temperature stability, poor to very poor dissipation factors, poor aging and because barium titanate is piezoelectric, microphonics. You can take a typical Class 2 or 3 ceramic capacitor connected to a scope and by just tapping it, observe the voltage it generates.
Class 2 caps can be used in some coupling and filtering applications, but Class 3 are strictly designed for power supply filtering applications where they can replace tantalum and aluminum electrolytic caps.

There was at one time Class 4 ceramics with even worse parameters, but these are pretty much gone.

Darryl

You can also hear the capacitor 'click' from a voltage spike applied across it (as in the breakdown test on an old style cap tester).

True, that's why there are ceramic phonograph cartridges and ceramic speakers. They can be used both ways. Not such a great thing in a capacitor though :(

Darryl

So what was the characteristic of the capacitor elements used in those ceramic-plate vertical integrators that were once so popular in Zenith TV's?

Good question. My '62 Motorola BW has one called a "vertical integrator", an integrated RC network intended to reduce assembly costs.:thumbsdn:

Are these are also called "gimmicks" in the audio stages of many tube radios?:scratch2:

****

Those were generally called 'couplates'.
Um.. a 'gimmick' usually referred to a very small value capacitor (2 - 3 pf or so) consisting of pair of insulated wires twisted together for a few turns. Sometimes used for neutralizing an RF or IF stage prone to oscillation.

Gimmicks, ah yes.
I saw those in Zeniths IIRC, except they were just 1/2" long parallel pieces of enameled copper wire, not twisted.