So it's still about 320mA and 1.5 volts across the bypass cap. If Q =150,
3 watts in the choke, Q=75, 6 watts in the choke, etc.
Actually, depending on the bypass cap and the length of leads, it could be
rather more.
All of which is why you really need two chokes in a L-c-L-c arrangement.
The second choke is almost anything that can handle the DC.
73
Peter G3RZP
========================================
Message Received: Nov 10 2013, 12:31 AM
From: "Hardy Landskov" <n7rt@cox.net>
To: "Leigh Turner" <invertech@frontierisp.net.au>, amps@contesting.com
Cc:
Subject: Re: [Amps] Plate Choke question
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@frontierisp.net.au>
To: <amps@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@contesting.com] On Behalf Of peter
> chadwick
> Sent: Sunday, 10 November 2013 7:45 AM
> To: Carl; Steve Flood; amps@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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> Amps@contesting.com
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>
>
>
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