18-10-2012, 09:30 PM
No, a ceramic cap. is not your only option here. In actual point of fact, since this cap. is only passing audio frequencies (if my understanding of what you have written is correct), I am surprised that a ceramic cap. has been fitted at all. Usually ceramic or mica caps. only appear in circuits where there is substantial r.f. currents flowing, such as the mixer, local oscillator and I.F. amplifier anode circuits.
Now if this cap. sits between the anode of one valve and the control grid of another, then the most critical parameter is its leakage current under the d.c. and signal conditions. As is well-known, in that position a capacitor with zero leakage is required, and although a 'good' ceramic or mica capacitor will be 99.9% guaranteed to meet that requirement, there are other types that will also me suitable - especially new, unused, modern ones. But whatever type (or vintage) of cap. you choose for such a coupling capacitor, I would subject it to a high-voltage test first and measure the leakage current at that voltage. Anything over about 20 µA I would reject as unsuitable. But if that cap. is doing duty as a screen grid or HT decoupler, then the leakage current requirements are far less stringent.
That is why I asked "what job does it do in the overall scheme of things ?"
Al.
Now if this cap. sits between the anode of one valve and the control grid of another, then the most critical parameter is its leakage current under the d.c. and signal conditions. As is well-known, in that position a capacitor with zero leakage is required, and although a 'good' ceramic or mica capacitor will be 99.9% guaranteed to meet that requirement, there are other types that will also me suitable - especially new, unused, modern ones. But whatever type (or vintage) of cap. you choose for such a coupling capacitor, I would subject it to a high-voltage test first and measure the leakage current at that voltage. Anything over about 20 µA I would reject as unsuitable. But if that cap. is doing duty as a screen grid or HT decoupler, then the leakage current requirements are far less stringent.
That is why I asked "what job does it do in the overall scheme of things ?"
Al.
