In a message dated 2/16/06 10:01:12 PM Central Standard Time,
amps-request@contesting.com writes:
<< s it possible and/or feasible to put a grounded grid linear into Class
C? Reason I ask is my old Tempo/Henry 6n2 is used almost exclusively
for WSJT modes and they all do fine on Class C since they send single,
sequential tones. Amp is configured as push-pull on 2m and parallel on
6m. Current biasing arrangement is single 10v, 50w zener in cathode
circuit. I would think the efficiency would be much better in class C
if that is possible and would seem to help the 3CX400A7/8874's along.
Searches and old handbooks haven't been much help. Plenty of info out
there on tetrodes and biasing but not gg triodes.
Thanks - Bruce, N5SIX >>
Bruce:
This is certainly possible to do. There are several high power short wave
transmitters that I am familiar with that have a GG class C final (that is
plate
modulated). There is an old radio Handbook construction article that shows
4-400A grounded, both for DC and AC. Given the schematics of these 250 kW short
wave transmitters I referred to, the PA tubes used therein are nothing special
in the sense that they can operate at other common amplifier modes as well. I
cannot see why virtually any tube could not be used in a GG configuration as
long as the grid was grounded for RF but not necessarily DC. the 250 kW
Continental Electronics 419C uses two 8388 triodes and here, the grids are at
RF
ground but not DC ground. The bias supply connects from control grid to ground.
The general Electric 250 kW BT-250A, uses two ML-7482 triodes, and is similarly
configured in that the control grid is grounded at RF and not DC. BTW, both of
these transmitters have modulated driver stages.
The physical layout of your ham rig may or may not allow GG operation
especially if the grid goes to ground at RF _and_ DC, In which case, grounded
grid is
only applicable of the tube specs allow it. Of course, you can raise the
cathode above ground by the class C bias amount, but this gets cumbersome with
a
tetrode. Remember too that according to the Care and feeding of Power grid
Tubes, there is a formula that relates cathode inductance to its apparent DC
resistance effects seen from the perspective of the control grid. I'm not sure
how
that concept would change in a GG configuration. My guess is the amplifier
would be hard to drive because of all of the inductances from all of the
isolation circuitry for the power supplies.
Along the lines of the GG class C, I have done some mathematical analyses on
the 250 kW short wave rigs I mentioned and it appears that, unlike class B,
the driver looks like it does not have to be modulated to 100% for the PA to
make it to 100% modulation. I'm wondering if there is anyone here that might be
able to look over my calculations and tell me if I'm right. My calculations
suggest that if the envelope modulation of the driver falls below plate current
cutoff, the PA will be able to go to 100% negative modulation even though the
driver might not be. The positive modulation ability of the PA of course being
dependent on, among other things, the amount of RF drive it gets at 100%
positive crest. I also find that if the IPA and PA are separated by a simple
tapped
inductor IPA tank, that the PA will be severely overdriven at 100% peak
positive crest. Therefore it would appear that the driver should not be driven
to
100% positive to keep the drive to the PA below the point where the PA grid is
overdriven. True the grid is grounded, but some of that RF cathode current
from the IPA lands on the grid and most of it is passed on to the PA to add to
the output. But, with the IPA 100% modulated, the operating line is pushed
right
off the tube constant current chart at peak positive modulation crest.
I discovered that the problem could greatly be reduced by putting a 90 degree
PI network between the IPA and the PA (because of its voltage to current
conversion capability) the overdrive of the PA was better controlled. The
retired
transmitter TWR engineers on Bonaire that used the 419C on a daily basis,
mentioned that its modulation was very much dependent on how everything was
tuned.
And when I say tuning, it goes very much beyond the usual simple precautions
of making sure the PA is properly loaded.
All of the books that I have seen discuss grounded grid amplifiers, but only
class B GG amplifiers. The only mention about the extent to which the driver
has to be modulated was in Bill Orr's Radio Handbook where he mentioned the
driver modulation being only 70% but with no detailed explanation as to why.
The
GE transmitter manual mentions the PI network between the IPA and PA, but
there is no mention about its phase shift or what exactly it is doing.
It does not make sense to me that he driver would have to go to 100% negative
modulation unless the modulated PA was class B, but why would you run it
class B? Yes, the driver output is in series with the PA, but the PA tube is
not
conducting over 180 degrees, but less than that in class C. Flywheel effect
would not be able to make up that difference between 180 degrees conduction
angle
for class B and the 120 degree class C conduction angle. A thevinin voltage
source is replaced with a short when its output voltage is zero in the case of
a linear circuit, which would then allow the driver signal to pass straight
through, but is that theory only true in a linear case? When the tube is cut
off
it's not passing current, period. It is therefore acting like a switch and it
is "open" when not passing current. True? Not true? If true then how would
that driver signal make it through to the antenna if the PA is biased beyond
cutoff?
What are your thoughts on GG class C and plate modulation?
I would be interested in corresponding with any of you that would have some
insights on plate modulated class C grounded grid RF amplifiers. The GG
amplifier was of course used in the short wave transmitters because this
configuration does not need neutralization; a significant advantage for
something high
power and tunable from 3.2 to 22 MHz.
Jeff Glass, BSEE
WB9ETG
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