Date: Fri, 1 Nov 2019 06:45:49 -0400
From: Ron Youvan <ka4inm@gmail.com>
To: amps <amps@contesting.com>
Subject: Re: [Amps] SB-220 HV Transformer Wanted ; thread stolen for
bias question
> True or (almost) class B amplifiers are only found in push pull audio
> amplifiers where one tube amplifies the positive half of the drive signal
> and the other tube amplifies the negative half. Even then often the tubes
> are biased for a small amount of idle plate current.
************
I must respectfully disagree. The: "plate current flows over exactly
only 50% of the cycle of the input" is the "book learning" explanation
(I went to a better school than that) and true (actually used) class "B"
operation is in no way restricted to: "class B amplifiers are only found
in push pull audio amplifiers" as many 250. Watts to 50,000. Watts am
radio transmitters use class "B" audio modulators and many RF amplifiers
also, including almost all solid state 100. Watt transceivers and many
MOS high power amplifiers use true class "B" final amplifiers - with an
elevated bias for a tiny 100% (degrees) current.
That does not make them class "A" or class AB."
## better ask for your money back. A class B AM modulator in
an AM broadcast TX IS a push-pull audio amplifier. 100 watt xcvrs
use push-pull RF transistors as well. Both draw idle current. If they
draw idle current, they are not class B.
## AM broadcast RF pa runs class C. PP modulator runs class AB.
FM broadcast uses class C.
The proper definition of class "B" amplification is not:
"exactly 180 degrees of conduction" it is:
"one device amplifies the positive half of the drive signal and the
other device amplifies the negative half" also not:
"often the devices are biased for a small amount of idle plate current."
But: "class "B" linear amplifier devices are ALWAYS biased for a small
amount of idling plate current to cancel the crossover distortion."
The only possible exception is where non linear RF generator is being
use to heat something such as plywood glue setting and such. Frequently
50 kW.
Class "B" final amplifiers are the most overall efficient form of high
power linear amplification if you take the idling power into
consideration, the power consumed over a long period of time, such as a
week or month.
Additionally they cancel out their second (the big one and all even)
harmonics.
I imagine class "B" final amplifiers are more linear than most single
ended amplifiers driven into grid current. (I have no proof.)
## 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 lotsa 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.
Jim VE7RF
--
Ron W4BIN - Understanding is much better than
knowing how.
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