Yup you are right, I didn't look at all the bias circuit connections.
So my technique would be: on receive, turn the driver and PA bias pots
to zero. Turn down the RX audio (to minimize the supply current) and
measure the total radio current. Turn up the driver bias for a 30 ma
rise in total current. Go to transmit after turning the front panel
drive control all the say down, by closing the key or the lock switch.
Turn up the pa bias for 130 ma rise in total current. No wiring breaks
required. These will be off a little because the bias current will rise
and it comes off the +12 all the time, but good RF transistors have a
current gain of at least 100, so that change in current won't be
detectable on the half amp supply meter. Besides the bias to the driver
is shunt regulated by the diode junction drop of Q4 (which I presume
samples the driver heat sink temperature for temperature compensation of
the bias) so that circuit is not going to change supply current itself.
There will be more change at the PA bias circuit because of that 100 ohm
resistor to ground. But not MUCH. .6 volts across 100 ohms would mean a
current change outside the PA by 6 milliamps. More than the PA
transistors would draw bias current with a beta of 100, like 1.3 ma for
raising the collector current 130 ma.
Then I'd check the output keying envelope rise and fall times for key
clicks with my handy scope and the drive turned up for normal operation.
You might have someone fairly close by listen for key clicks but that
can be subjective modified by a receiver with too fast attack audio
derived AGC.
In a zero bias tube, like a 3-500Z, 572B, T40Z, or 811A, it does take
negative bias to be in class C, zero bias gives class B, a bit of
positive bias would give class A, with AB somewhere in between A and B.
A better definition of Class C is that the output terminal current
(plate, drain, or collector) flows for less than half a cycle. A class C
stage is most efficient if that output current can be made into a
saturation current with a flat top for significantly less than half a
cycle of drive. The 304TL was especially good at that, and some fixed
frequency transmitters had distorted the drive to add some third
harmonic to square up the drive with some third harmonic impedance in
the output side to accept that harmonic current. That's called third
harmonic peaking and can make a plain class C stage do 85% efficiency
from DC to RF where without it the efficiency might be more like 70 to
75% (except for the 304TL, a pulse tube that can do 85% without third
harmonic peaking).
A better definition of Class B is that the output terminal current flows
for one half cycle.
The best definition of Class A is that the output terminal current flows
throughout the cycle. It changes but never cuts off.
In class AB that output terminal current flows for more than half a
cycle but not the whole cycle. The device cuts off for part of the
cycle. Sub groups of that for tubes only are AB1 and AB2. In AB1, the
tube is never driven to grid current. Collins 32S transmitters control
the ALC by detecting grid current. In AB2, some grid current is allowed
and RF drive peaks. The 4CX1000A is a class AB1 tube, no grid current is
allowed, its grid would overheat if there was any.
Its not such a good definition to define these classes on the input
bias, because different devices act differently. Zero bias tubes usually
take zero bias for class B and if run AB would need positive bias, but
triodes that have that characteristic usually aren't run in AB modes.
Some tubes, like 807 and 6146 need negative bias for all the classes,
just they get more negative for the shorter conduction angles. And the
drive power is greater driving the grid into conduction on the positive
going peaks but that's not a requirement for class C, just typical of
tubes in common use. Then bipolar transistors can run with zero bias for
class C, but need a little forward bias for class B and more for AB and
A. Some FETs need bias just like a tube, but some enhancement mode
(think mostly power MOSFETs like the IRF510 that works at RF) needs a
little positive bias for class C, more to reach the threshold of class
B, and more yet for AB and for A, yet if it draws gate current (besides
that for charging the internal capacitances which can be significant)
its blown on that cycle because to much voltage has blown the gate
insulation layer good for only 20 volts for a standard level FET and 10
volts for a logic level FET that's turned full on at less than 5 volts
on the gate. And to add to the confusion there are N type and P type
FETs where the normal terminal voltages of the P type are inverted from
the N type.
73, Jerry, K0CQ
On 1/30/2011 9:18 PM, Bwana Bob wrote:
Yes, the note from G3VTT was typed on the inside front cover.
In the Century 22, with key up, the finals do not receive bias voltage.
The final bias is only applied when the "T" voltage is present, but the
driver bias is applied all the time. After I replaced the finals but
before touching the bias adjustments, I measured the current into the
driver/final board as 93 mA in receive and 159 mA in transmit with no
drive. More investigation is needed. Maybe somone on the list can make
some measurements on his Century 22.
Correct me if I'm wrong, but I now recall that class C uses negative
bias; class B uses zero bias. Class A uses positive bias to put the
device into the middle of its linear range. I'm not sure about class "AB".
73,
Bob WB2VUF
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