[Amps] Plate Choke question

[Leigh Turner]

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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