The RF PA is class C p.p.; When cathode emission dropped the RF final
PA tubes were moved to modulator use, not the other way around.
Vintage p.p. RF PA rigs are out there and still in use. The Globe
King 400 is one example. These rigs usually had triodes and had to be
neutralized with a pair of Cn caps. The output network was balanced
and used plug in coils. There's nothing wrong with any of this except
that 2nd harmonic attenuation isn't up to modern standards, but most
operators modify the rigs to feed unbalanced line to an additional
matching network such as a Johnson Matchbox or low pass L network
which knocks down the 2nd harmonic. What did in this design was
television and TVI. Manufacturers needed to make and sell rigs with a
lot of shielding and they wanted operators to be able to operate the
rigs from the front panel controls which led to bandswitched pi
output networks and unbalanced final PAs with tetrodes and pentodes.
Also, safety was starting to get some consideration. But the p.p.
final and balanced output network has some advantages, such as a
relatively trivial plate choke, usually a dinky pie wound job on the
cold center tap of the output coil. Another nice thing is that
there's no need for a DC blocking cap because B+ is stopped at the
output coil gap where RF is coupled to a swing link that performs the
PA loading. You can have a single ended PA and still employ a
balanced output with all of the advantages if you don't mind swapping
plug in coils.
73
Rob
K5UJ
> On 1 Nov 2019, at 20:05, Ron Youvan <ka4inm@gmail.com> wrote:
> I have seen plenty of push-pull "Class B" RF amplifiers, being a worn
> out broadcast engineer. The Raytheon Company use to make low power am
> transmitters and they were superior to the common RCA and Gates units.
> The 250. Watt transmitters use two RCA 810 tubes (triodes) in push-pull
> "Class B" modulation amplifiers and two RCA 810 tubes in push-pull
> "Class B" AF amplifiers. For one kil-a-Watt transmitters they used two
> RCA 833 (triodes) in push-pull "Class B" modulation amplifiers and two
> RCA 833 tubes in push-pull "Class B" RF amplifiers.
> In the Raytheon transmitter these tubes had a reputation of lasting
> three years as the RF amplifier and when the emission was down they
> moved the modulators to the RF stages and installed new tubes in the
> modulator stage and got another three years.
>
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