I received this interesting inquiry this week from Bob W4ZST about the
history of grid and plate current monitoring in grounded grid
amplifiers. Bob writes, /“Who came up with the idea of metering grid and
plate current in the B- circuit? I remember way back that many folks
just put their plate current meter in the HV line and (hopefully)
protected the meter from access by a hand or other body part. I remember
some that had their meters behind a glass or plexiglass panel. I was
guessing that it might have come out of Collins but haven't any info on
that. If you know, I'd like to hear as it has just been one of those
things I'm curious about and haven't been able to find.” /
I don’t have a definitive answer to Bob’s question, so I’m
polling the group for comments and suggestions. Here is my personal
historical take on the issue: I built my first GG amplifier in high
school in the late fifties (a pair of 813s) and, like everybody else in
those days, I grounded the negative HV power supply connection, which
was in a separate enclosure from the RF deck. I remember that vividly
because I almost killed myself when I unscrewed the coax connector from
the RF deck without realizing that the coax shield was the ground return
for the HV supply.
I dragged out my old 1956 ARRL Handbook for elucidation. Turns out the
power supply circuits there always show the negative power supply output
tied to chassis ground. Interestingly, the ’56 Handbook discussed the
neutral line appearing on “newer” house wiring, but the instructions
were just to ground the neutral wire to the chassis in an HV power
supply. Many amplifiers of that era, both homebrewed and commercial, did
the same. Today, of course, that practice would violate every electrical
code in the nation.
In the 1950s, most HV power supplies used full-wave rectifiers with
choke input filters and the CT of the plate xfmr was grounded to the
chassis. (I remember reading about measuring plate current in the CT of
the transformer lead, but I don't believe that idea ever really caught
on.) Even then (say 1960 or so), nobody thought of lifting the B- return
from chassis ground. The Johnson Ranger transmitter measured plate
current with a panel meter in the HV line, common practice in its day,
but a spectacularly bad idea.
I think there were several technical advances which lead to the practice
of separating B- from the chassis. One was the growing popularity in the
mid 60s of grounded grid amplifiers. In those days, triodes like the
3-1000Z weren't yet available, and hams usually just made do with
surplus tetrodes, like the 813, 4-400A or 4-1000a. One could turn these
tetrodes into makeshift triodes by tying together their control and
screen grids.
These new grounded grid amplifiers, presented several design
challenges: one was how to provide operating bias for the control grid
when it was securely anchored to the chassis. A workaround, (used in
some Henry amplifiers?) was to lift the grid to DC and bypass the RF to
the chassis with capacitors - a workable but clumsy solution that was
also prone to instability.
Another challenge was how to drive a grounded grid amplifier
with tubes lacking independent cathodes. At first, low capacity filament
transformers provided the answer. These isolated the filament from the
transformer primary, allowing the filament to be driven with RF without
being shorted to ground through the filament transformer. That was
another workable but awkward solution that was quickly abandoned when
ferrite filament chokes were invented.
A big technical advance was the realization that cheap capacitor input
filters using series-connected electrolytic capacitors could replace
expensive and heavy filter chokes and their accompanying oil-filled
capacitors. Once this idea took hold, inexpensive voltage doublers in HV
power supplies became feasible. This innovation became quickly popular,
since plate transformers were cheaper to make without a center tap and
with half the number of required secondary turns. I remember struggling
to understand how the voltage doubler worked, since the circuit has no
natural ground point. To me that was the first time I realized that
"ground" and "B-" were separate concepts, and that the B- power supply
return didn't have to be anchored to the chassis ground.
For me, that realization was a breakthrough, as it was for many others,
and it quickly led to today's practice of grounding a grid directly to
the amplifier chassis and lifting the B- above chassis ground by the
operating bias voltage. (The confusing issue to many builders still is
why only grid current flows between B- and ground, while only cathode
current flows back into the power supply. A related point of confusion
is the polarity of the safety diode between B- and the chassis, and how
it protects the amplifier in the event of a flashover in the high
voltage circuit. Understanding current flow in a grounded grid amplifier
requires some careful study!).
I don’t know when this circuit innovation first appeared, but I
suspect it might have been the in the wonderful newsletters published by
Eimac and authored by Bill Orr (SK) W6SAI and Bob Sutherland (SK)
W6UOV/W6PO. Maybe somebody on our reflector has a more informed
understanding.
73,
Jim W8ZR
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