[Amps] Plate chokes (was: Re: placement of RF choke bypass cap.)

[Vic K2VCO]

On 3/24/2010 5:18 AM, DF3KV wrote:
> I tried that [RF Parts] choke in one of my amplifiers without testing it first with the
> GDO and it just turned into smoke on 10m.

I've been working on a choke for a new amplifier project. I've noticed some things which 
experienced builders probably already know, but I thought would be worth mentioning:

a) Even if the gdo has decent calibration to begin with and even if you are careful to use 
it properly (move it away from the choke to get the smallest perceptible dip) the 
frequency is pulled a lot near a resonance. I've found it necessary to use it together 
with a counter to get accurate, repeatable measurements.

b) It makes a big difference when the choke is mounted on a chassis (and probably when it 
is inside a box) compared to sitting on a wooden workbench.

I am trying out an idea as follows: I wrapped the choke with Teflon tape and made a copper 
band about 1/4" in width which I can slide up and down. The plan is to test the choke 
installed and wired with everything mounted and use this to fine tune it before applying 
power.

For what it's worth my choke is 3-7/8" close wound no. 28 on a 1" diameter ceramic form. 
I've used the RF Parts choke before successfully, but why pay for something that you can 
make for nothing?
-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/