Hi Phil,
> After converting and canabalizing numerous industrial amps that
> are VERY overengineered due to their continuous duty requirements,
> I came up with a rule of thumb for plate blocking cap values without
> having to do a lot of computations each time you "roll a new one."
> I have used 10 pf per meter. IOW, 100 pf for 10 meters and 1600 pf
> for 160 meters. I agree with Tom and Marv, and this also is probably
> a bit of overkill, but it is simple to remember, and errs on the
> conservative side. As previously stated, one needs to check the
> cap for heating after a healthy smoke test on 10 meters..
The problem is generally current rating, not lack of capacitance.
The amount of capacitance is set by the operating impedance that
appears at the anode of the tube and the lowest frequency. As long
as the capacitive reactance is much less than the anode
impedance, the only consideration is dc voltage breakdown and
current rating (often limited by temperature rise).
If the anode impedance is 3000 ohms, even 300 ohms of Xc at the
lowest frequency offers plenty of headroom. The result of this would
be 300 pF at 160 meters minimum C for proper performance with
plenty of headroom. Even 150 pF would work.
It isn't a big engineering task to check the size required and be
sure you have more than the required amount of C, so you can
concentrate on something important like the current rating.
I've seen a lot of people waste time and money with needlessly
large coupling capacitors that had poor current ratings, because
they carbon copied other values without knowing WHY they were
selected.
Contrary to folklore, that cap has little to nothing to do with
efficiency. Spend your time looking at something useful, like the
current and voltage rating.
> (((73)))
> Phil, K5PC
>
>
>
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>
73, Tom W8JI
w8ji@contesting.com
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