On 11/1/19 12:30 PM, Jim Thomson wrote:
## when was the last time you saw a push-pull class B
RF linear amp....using tubes ?? I saw one when I was a kid,
and it was not linear. 2 x tubes in P-P is a complex mess
for RF HF freqs..esp multi band. They draw lots idle current.
Same 2 x tubes in parallel, in AB for ssb
linear use.... or class C, for CW, FM use is even more eff.
## GG triodes cant be used In class B on CW.... u will get key clix.
A simple PI net will kill even and odd harmonics. Typ P-P Ham xcvr
requires 7 x LP filters to kill even + odd harmonics. GG triode does
not require a LP filter.....none.
I cannot call 10 to 20 % idling currents lots, like .200 Amperes for a
3.0 Ampere plate current per each tube in a 25,000 Watt am modulator.
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.
RCA and Gates got one year out of their parallel RF amplifiers.
The reason we don't all use push-pull "Class B" RF amplifiers is our
propensity for multiband operation. Before 1960 many hams had push-pull
"Class B" transmitters that used a pair of RCA 811, 812 or 813 tubes and
1 kW B&W plug-in coil sets. (all grid driven with 100. W. B&W plug-in
coil sets.) If there is any instability in a push-pull "Class B" RF
amplifier they are easily "cross neutralized."
All of our "rice boxes" use a "Class B" RF amplifier" as the final
amplifier, driven by a beefie class "A" transistor.
If you consider the amount of time the devices are idling, class "B"
amplifiers are the overall most efficient linear amplifier that there
is. Don't think two parallel devices is the same as a push-pull "Class
B" RF amplifier.
--
Ron W4BIN - Understanding is much better than
knowing how.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|