My guess: Design the plate output circuit to resonate with a bit less
than the rated output cap of the plate plus strays. Then put a series
capacitor in the plate to tank coil lead to reduce the effective cap
by the "caps in series" formula. This enables the 813 to operate at
the highest possible frequency but grossly limits ability to band
switch to lower frequencies ( where the series cap becomes
much more significant impedance). It doesn't matter if
the plate output is PiNet, tapped coil, link coupled. This approach
allows the design to eliminate the traditional output tuning cap to
ground and use tube cap instead.
Disadvantages include:
Inability to operate at lower frequencies,
improves operation at frequencies the 813 probably shouldn't
even attempt to use,
requires a rare variable cap with both both terminals not
grounded,
extra circuitry in the plate lead means more stray cap to
ground, somewhat defeating the purpose of the idea.
An alternative exists as shown in some 6M amp designs...use
no plate cap to ground, no variable series cap. Instead tune
the plate tank coil as a variable inductor, implemented by
swinging a shorted turn in-out or around in the coil. Another
builder showed a coil that used variable compression of the
turns.
Other traditional techniques include:
operate in push-pull
use a halfwave tuned circuit in the plate
implementation in lumped elements left as exercise for student
Have fun. And if you get 813's on 2M, send pictures.
jeff
----- Original Message -----
From: "Randall D. DuCharme" <randyd@specent.com>
To: <amps@contesting.com>
Sent: Thursday, October 12, 2000 8:45 PM
Subject: [AMPS] Series Tuned Tank?
>
> Greetings,
>
> I'm a new ham eager to build his first amplifier. Done lots with tube
> audio in the past but RF is definitely a different animal. For my first
> project I intend to use 813s. They were cheap, I have an abundance of
> them, and if I zorch a few in the learning process, I won't be too
> upset.
>
> My question is this.... In going over the RCA TT-5 manual I found in the
> section on tank circuits that you can use a 'series tuned tank' as a way
> of defeating high output capacitance. It doesn't give formulae however,
> for calculating the values the tank's members other than to state that
> you should use a series tuning capacitor with a value several times
> larger than that of the tube's capacitance. Nor does it say anything
> about the effectiveness of the tank in filtering unwanted emissions. I
> have a couple of Bill Orr's books and a few Arrl handbooks but can't
> find anything on this type of tank circuit in them.
>
> Has anyone even done this? Where can I find out more about it?
>
> Many thanks.
>
>
> --
> Randall D DuCharme (KD5IZI) Proud to be 100% Microsoft Free!
>
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