A lot of people think that the choke is there as a safety device in case the
plate blocking capacitor shorts. Most chokes across the output are too small
in current carrying capacity to be an effective safety device should the
plate blocking capacitor short.
The real reason for the choke is to keep the output tune capacitor from
charging to some DC level and causing the tune capacitor to arc constantly.
Most load capacitors are close spaced and only rated for a few hundred
volts. Even if a vacuum capacitor is used as a load capacitor the voltage
rating is often less than the plate voltage supply.
If there is no DC path to ground on the output of the amplifier then that
puts the plate blocking capacitor in series with the load capacitor and the
DC plate supply can charge the tune capacitor if there is any slight leakage
in the plate blocking capacitor.
Another way the load capacitor can charge is from static if an antenna with
no DC ground across it is connected to the amplifier. This will cause the
load capacitor to continually charge and arc.
If you hear a constant timed "pop" in your receiver at regular intervals,
maybe a few seconds to several minutes apart, you may have an open choke on
the output of the amplifier.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Kathy Bookmiller
> Sent: Tuesday, March 27, 2012 5:11 PM
> To: Amps@contesting.com
> Subject: [Amps] question for you experts...
>
> I was doing some checking out everything on my amp. (NCL2000) and
> noticed that the 2.5mh choke that is after the loading capacitor in the
> amp. was open-connection/wire is gone. Looking at others amps., I see
> they all have them-it would be rfc3 in a SB200, 1.1mh., for example.
> Since they are in all amps, at least tube ones, what is it for? My guess
> it is to short out the HV if the coupling cap. shorted coming from the
> plates, but if I'm right, it doesn't seem heavy enough for a good power
> supply not to just burn it up and keep on putting the HV on the
> antenna/output.
> I think I can save the one in my amp. if I can get the paint off the
> wire by scrapping or chemicals and reconnect it, but by doing this, I'll
> eliminate one of the 4 sections on the choke, but doubt that change in
> value would be a big problem?
> Enlighten this poor QRPer,
> Kathy W2NK
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