In a series fed PP circuit the Tune cap has to stand off the full DC voltage
plus the Peak RF. On AM that is 4X the DC voltage for spacing. Altho cap
values in PP are lower they are dual sections with very large spacing at
times and quite long. The Tune cap in my first PP 250TH rig in 57 was about
12-14" long so I converted to swamped grid parallel with a PiNet with much
smaller physical caps with the one I built about 15 years ago. With the pair
of 6C21's on AM I went with vacuums.
The plate choke can be whatever you want that has enough XL to block any
stray RF from a not perfectly balanced series fed tank circuit.
In a shunt fed circuit the voltage rating is the same as in a PiNet but now
choke resonances apply.
Plug in grid and plate coils are still readily available on Fleabay and
elsewhere for multiband operation. Just remember to turn the voltages off,
even a few hundred volts from the bias supply can wake you up.
Carl
KM1H
--------------------------------------------------
From: "KA4INM" <ka4inm@gmail.com>
Sent: Monday, January 26, 2015 11:02 AM
To: "amps" <amps@contesting.com>
Subject: Re: [Amps] push pull or parallel ?
On 01/26/2015 10:41 AM, Carel, pc5m wrote:
160mtrs contest past and we need more power... Plan is to add 3 dB by
throwing in another tube (2.5kw anode dissipation tetrode, grid driven).
I could go for push-pull of just parallel the tube's. I think push-pull
would be nice to get less even harmonics, but seems to be a big hassle
and
just put the tube's in parallel is much easier. Or am I missing something
?
In my opinion push-pull class "B" amplifiers are the the most overall
efficient and stable type of amplifier. The low idling current and
easy cross neutralization make common cathode operation very favorable.
The disadvantage is the double tuned plate circuit, but the need for
the familiar plate RF choke is optional. Band switching a P-P
amplifier is daunting, but for single band operation it is sweet.
Using a loop of coax as the output coupling (in the center of the
center tapped plate coil reduces the harmonic emissions even further.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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