HF Decoupling Caps questions
I can see many tube preamp with and also without HF decoupling caps between PSU and the circuit.
What is the rule?
When need I use this caps, and how can I calculate the value of this caps?
What type is the best? Sometimes I see big Elkos and sometimes small foil caps
I know I have to mount the cap as close as possible to the tube being decoupled.
By "between PSU and the circuit" I presume you mean from B+ to common, also after various decoupling (series) resistors, where such points usually have electro's to common.
Firstly one should not go overboard with decoupling measures that are only required in short wave radio (several MHz). But it is wise to put such caps in parallel with said elco's, simply because they are cheap and can do good and no harm. (Modern small electrolytics are quite good on their own.) It is possible, depending, for funny feedback to cause supersonic spurts of oscillation because of the finite nature of series wiring. (Nothing is a short at r.f.)
One does not really calculate a value, one uses as high a value as is practicable. For tube technology 47nF to 470nF is mostly used; I find 100nF small, cheap and handy. In infrequent cases of persistent tendency to instability, a larger value might be required, but at that point the remedy is usually to rather start checking the layout/wiring for unnecessarily long leads, poor earthing, etc. That is a subject on its own, more frequently a problem with semiconductor circuits.
The best type is ceramic, but at usual tube voltages they are difficult/expensive. Polyester or polycarbonate is more than enough.
Caveat: You will know not to mount such capacitors with overly long leads to the bypassing points. That defeats the purpose by introducing series inductances. But as said, with tube impedances and at audio frequencies, 'r.f. suitable' practices should not be that necessary.
I usually also use 47nF-100nF.
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