[Amps] plate bypass caps

[Phil Clements]

> ...OK guys! Just please remember: the RF current at the choke's base can
> be> quite high, so RF withstanding current must be quite high!
> Apart from that, it's better to use different kinds of capacitor in
> parallel, rather than one alone, except if it is made especially for high
> current duty: one value in the tens of nanofarads for the very low
> frequencies, one or two ceramic units for the higher frequencies. I
> measured
> RF currents in the Ampere range on a 1200W PEP amplifier.

That measurement is meaningless, unless you specify the frequency and the
value of the RFC. For instance, the commonly used B&W 800 and 802 Chokes
have a high current at the base when used on 160 meters. So much so that
without enough C at the base (c. .01mfd) 160 meter RF can actually destroy
the filter caps. and other components in the power supply, and backtrack
through to the mains wiring. When I used to service/convert Alpha 77SX's,
the addition of a .01 mfd @ 6-10 kv disk ceramic cap. In parallel with the
existing .001 mfd at the base of the B&W Choke prevented a great portion of
the 160-meter RF from entering the power supply section. (160 meters was
obviously an after-thought in the design of the 77SX.) On the other hand,
the RF Parts/Ameritron 225 uh chokes are twice as good on 160m as the B&W's
in choking RF on its way to the power supply, but still require proper
values on bypass C to be effective. I have found that a 1000 pf doorknob
cap. paralleled with a .01 mfd @ 6-10 kv disk ceramic cap. at the base of
the choke will do a great job on all 9 HF bands. If you are only interested
in the higher HF bands, you can disregard the above advice!


(((73)))
Phil, K5PC