Assuming you mean a vacuum tube amp, on 50 MHz I usually use a pi-net with a
small vacuum variable for plate tuning (2-30 pF, 10kV), 1/4" copper tubing
for the inductor (typ 3t, about 1.5" inside diameter, spaced over ~2.5 or 3"
long), and a 150pF or so air variable for the loading cap. Seems to work
for 2x 4CX250's, 3x 4CX250's, single 4-1000, and most recently, a pair of
8122's, all equally well with good efficiency and little to no heating of
any of the components when used at legal limit power (1500W PEP).
On 144 MHz, I usually use a 1/4-wave stripline in air, tuned to the chassis
with a teflon-coated metal adjustable vane or flapper, and output link
coupled with a small variable tuning cap at the ground end of the link. But
usually on 144 MHz, I use 2 x 4CX250's in PP, which makes it easier because
the old "plumber's delight" all-copper tubing construction, which tunes the
tank using rotating discs to change plate-to-plate C near the anodes, and
link output, works so well and is very easy to build in a small chassis (the
pipes are about 12" long and in parallel, and cost maybe $3 at the plumbing
store).
I don't know what you mean by a variable capacitor between the anode and the
antenna. How does that transform the high plate impedance of the tube to
the low line impedance of the load?
WB2WIK/6
"Each success only buys an admission ticket to a more difficult problem." --
Henry Kissinger
> -----Original Message-----
> From: Alfredo Rosati I5UXJ [SMTP:alfredo@cln.it]
> Sent: Monday, December 03, 2001 2:58 PM
> To: amps@contesting.com
> Subject: [AMPS] a question for the EXPERT
>
>
> in VHF Amplifier
> which is the best way to make the output:
> 1) Variable capacitor between Anode and antenna
> 2)Link with variable capacitor near the end of the line
>
> is there a difference in efficiency???
> i think that the link mode , is the clearest and attenuate armonicas ,
> but i do not am sure if it efficient same the the capacitor directly on
> plate.
> hello i5uxj
>
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