[Amps] Plate Choke question

[Carl]

I much prefer tossing the so called VHF choke and replacing with a wirewound 
glitch resistor. The WW has inductance and serves a dual purpose wheras Ive 
seen a few amps where the "VHF" choke cause problems.

Carl
KM1H


----- Original Message ----- 
From: "peter chadwick" <g8on at fsmail.net>
To: "Hardy Landskov" <n7rt at cox.net>; "Leigh Turner" 
<invertech at frontierisp.net.au>; <amps at contesting.com>
Sent: Sunday, November 10, 2013 5:22 AM
Subject: Re: [Amps] Plate Choke question


> 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 at cox.net>
> To: "Leigh Turner" <invertech at frontierisp.net.au>, amps at 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 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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> >
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