[Amps] plate bypass capacitor

Gary Schafer garyschafer at comcast.net
Sat Aug 8 20:59:20 PDT 2009

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Roy
> Sent: Saturday, August 08, 2009 12:46 PM
> To: amps at contesting.com
> Subject: Re: [Amps] plate bypass capacitor
> Concerning,
> "...At 80 meters, 100uH has a
>  reactance of 2.2 kiloohms. At 1000V RF, that would make 0.45A of RF
>  current going through the choke, and this is the current that the bypass
>  cap has to conduct to ground, unless there is something else
> helping...(etc.)"
> Plate solenoid choke rated values of inductances are measured at a low
> audio
> frequency. At RF frequencies they exhibit much, much higher effective
> impedances of up to over a megohm at their first fundamental parallel self
> resonance, which is ideally put into use for highest efficiency.  At
> frequencies lower than the first parallel self resonance, they still
> reflect
> typically ballpark Zs of about 30,000 ohms or much more to the plate tank
> circuit. This is due to the principle of many self-parallel resonances
> within the windings. So on 80 meters at 1000 volts RF, current through the
> bypass cap would be on the order of only 30 mA or 0.03 A for a 100 uH
> choke,
> maximum...even smaller currents are likely in practice due to surrounding
> grounded surfaces lowering the primary resonant point. Stay away from
> fundamental series resonances. References are from Ohmite plate RF choke
> engineering papers.
> 73,  Roy           K6XK

Choke values are not so different at RF than those measured at typical audio

What happens on 160 meters and even 80 meters with a choke that is "too
small" for the frequency involved is that it is operated in a parallel
resonant mode with the plate tuning capacitor in order to provide a high
enough impedance.
Using a choke like this (too small) requires more plate tuning capacitance
than it normally would if the choke had enough inductance. The choke uses
part of the tuning capacitance and the tank circuit uses another part of the
capacitance to resonate each.

This is why many amplifiers have seemingly small values of choke inductance
on 160 meters but still work well. The Q of the choke must be sufficient to
prevent high losses and choke fires.

The self resonance of the choke is not what is relied on for this type of

Gary  K4FMX

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