"Push pull" or "single ended" does not define the class of operation.
Because a pair of tubes in an RF amplifier are operated in push pull, it has
nothing to do with what class they are operating in.
It looks like this is the source of some confusion here.
A typical AM plate modulated transmitter has its final tubes operate in
class C. The RF amplifier tubes may be operated in push pull or in single
ended parallel. The main advantages of push pull operation is higher plate
load impedance in the tank circuit. A smaller tune capacitor can be used.
Much more difficult to change bands.
Otherwise not much difference. The complexity outweighs a single ended
circuit.
The typical AM plate modulator usually employed a pair of tubes in push
pull. Push pull tubes in audio have a lower second harmonic content which is
desirable as that harmonic falls within the audio range.
These modulators were typically called "class B modulators" as the idle
plate current was fairly low so the tubes operated close to class B. But
they are not true class B as they did have some idle current (conduction
angle greater than 50%.
The transistor finals in our typical ham transceivers are usually run in
push pull to deal with the extremely low impedance present.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Ron Youvan
> Sent: Friday, November 01, 2019 1:05 PM
> To: amps
> Subject: Re: [Amps] SB-220 HV Transformer Wanted ; thread stolen for
>
> 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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|