[Amps] Plate Choke question

[Hardy Landskov]

But the caveat to this whole question was he stated that the amp was to be 
used on 20 to 10 meters only. But excellent comments about low band 
operation. I have been "burned" by inadequate RF choke XL in the past.
73 Hardy N7RT

----- Original Message ----- 
From: "Leigh Turner" <invertech at frontierisp.net.au>
To: <amps at contesting.com>
Sent: Saturday, November 09, 2013 5:06 PM
Subject: Re: [Amps] Plate Choke question


>
> Good comments and analogy here Peter...many homebrew tube amplifier hams
> (and the occasional commercial amp designer) often deploy inadequate plate
> choke RF bypassing / decoupling on the B+ line...especially inadequate on
> the 160m and 80m low bands...also, PS electrolytic caps do not like RF
> blowback on them :-(
>
> Leigh
> VK5KLT
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of peter 
> chadwick
> Sent: Sunday, 10 November 2013 7:45 AM
> To: Carl; Steve Flood; amps at contesting.com
> Subject: Re: [Amps] Plate Choke question
>
> BUT
>
> at 3.5 MHz, that 50 microhenries has an inductive reactance of 1100 ohms,
> and needs an extra 50pF of plate tuning capacity. Now let's assume a plate
> voltage of say 2500 volt and a minimum  plate voltage at the bottom of the
> swing of 500 volts. That's 2kV peak or 1400 volts rms of RF across the 
> choke
> or roughly 1.27 Amps of RF current in the choke on 80m - in addition to 
> the
> DC. Now let's assume a good choke with a Q of 150. That's about 7 ohms of 
> RF
> resistance and thus about 12 watts dissipated in the choke. Of course, it
> could be worse....
>
> Now look at the bypass cap at the HV end of the choke. Assume a 0.005 
> mFd -
> not an untypical value. That has XC = 18 ohms at 3.5 MHz so we have around
> 23 volts of RF across the bypass cap - which needs to be rated for 1.3 
> amps
> of RF..
>
> All of which is why you really need a choke - capacitor - choke - 
> capacitor
> - HV feed arrangement. OK, hams have got away without for years. So have a
> lot of drunks who drive.....
>
> I believe you need a much bigger choke or possibly a couple of chokes in
> series......
>
> 73
>
> Peter G3RZP
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