As a First Radiotelephone holder since 1956 and currently keeping a vintage
Bauer 707 running on the BCB I certainly agree.
That TX uses PP 4-400A's for RF and Audio and tetrodes/pentodes are NEVER
used as Class B modulators. Instead they run AB1 for minimum distortion.
IMO there is no absolute cast in stone point for Class B or AB2 be it for RF
or audio; grid driven or GG for RF.
I run GG triodes as dedicated,or bias switched, AM linears but biased closer
to Class B in order to reduce idle current and increase efficiency a bit. A
Spectrum Analyzer is used to set the point where distortion just begins to
show.
A NCL-2000 is also used with low power AM exciters. It is swamped grid
driven now in deep AB2 and bias can be set for a clean 1200W PEP thanks to a
very efficient/strong forced air cooled blower system. Gotta love those high
gain 8122 tetrodes!
Carl
----- Original Message -----
From: "Rob Atkinson" <ranchorobbo@gmail.com>
To: <amps@contesting.com>
Sent: Sunday, November 03, 2019 8:38 AM
Subject: Re: [Amps] SB-220 HV Transformer Wanted ; thread stolen for
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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