> Subject: [Amps] RF Chokes
>
> Eimac, in their book "Care and Feeding of Power Grid Tubes", available as
> a free download from their site, offers as a rule of thumb that to limit
> current in the plate RF choke to one percent in a Pi-net amp, the choke
> inductance should be 100 times that of the Pi-net coil. Few amplifiers
> meet this spec, because series resonances arise in typical solenoid wound
> chokes long before the required inductance is achieved. The use of
> permeability tuned chokes is not new, having been implemented in the
> commercial and amateur worlds. Pundits are quick to point out that while
> ferrite loaded chokes look good on the test bench,they will go into core
> saturation when DC is passed through them. Through judicious choice of
> core materials and attention to physical dimensions, it is possible to
> build chokes that provide high inductances without series resonances
> through the HF range.
Amen, Norm...
Harris has been doing this for years in their amps. The RF-110, (pair of
4CX1500B's) has a plate choke that is a very small ferrite loaded unit that
is clean from 1.8-30 mhz. This was the ideal fix for adding WARC bands and
160 meters, but the amateur amp manufacturers did not pick up on this. Even
the RF Parts choke, which is used in the Ameritron amps is not continuous
coverage. 11-12 mhz must be avoided. This is fine for most of us, but the L
is not close to optimum for 160 meters, and it is physically much larger
than the Harris choke. I get by with the RF Parts unit on 160m by using .01
ufd @ 10 kv bypass caps at the base. This keeps most of the RF out of the
power supply; not so important with oil-filled filter caps, but very
important to protect electrolytic capacitor banks found in most amateur
amps.
(((73)))
Phil Clements, K5PC
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