Was it not an FCC requirement to meter the plate current of the final
amplifier in series with the high voltage and plate of the final
amplifier on broadcast transmitters?
So it was a TRUE reading for plate input power?
And yes, they were set behind the panel under plexiglass or other clear
non conducting plastic. All of the FM broadcast transmitters that I saw
in service in the late 70s and through the 80s were all done this way. I
don't remember if AM transmitters were this way or not. But the
modulation also added to the AM power of course.. I never got to work on
AM transmitters, other than re-lamping the towers.
One could also look in all of the old ARRL, and Editors and Engineers
(AKA Bill Orr) handbooks and see when the move to lifting the ground of
B- and inserting a meter here was on the timeline.
The theory is that the B- metering is actual the cathode current -grid
current. Grid current was measured between the cathode meter and the
center tap of the filament transformer, so the cathode current actually
measured the cathode current minus-grid current, so essentially plate
current.
I cant help but think that it was a Bill Orr "invention" as stated by Paul.
The ol' 120 volt motor (and the florescent panel lights) between one
side of the 240 volt line and ground was popular in clothes dryers?
Seems to me the dryer recepticals changed from 3 wire (120V X2 and
ground) to 4 wire (120V X2) plus ground plus neutral for properly wired
120 volt devices in the dryer.
Have to hand it to Heathkit (and probably others) for coming up with 120
volt fan across 1/2 of the primary filament and plate transformers when
the SB-220 came out in the 1960s so no neutral was required, and the
ease of which to change the supply between 120V and 240V supply.
I added some carriage returns in Jim's original text so it was easier to
read.
Great discussions!
Merry Christmas
73
Jim W7RY
On 12/12/2020 12:12 PM, Paul Christensen wrote:
An interesting subject. I went into the QST archives after reading Jim's
post...
The first reference of any kind to "grounded grid" amplification appears on the
02/1948 issue of QST where the concept is used in a 6m tetrode amplifier. As Jim pointed
out, tetrodes were used extensively in GG applications and that concept remained in a
large number of amp projects that appear in QST all through the 1950s.
In the 08/1961 issue of QST, Eimac's Orr, Sutherland, and Rinaudo discuss a GG
amp and mention the difficulty of measuring grid current. Their solution in
that project was to use a 1-ohm carbon comp resistor in the grid lead with an
RF bypass cap to the chassis. The voltage dropped across the resistor is then
used to report grid current.
But in the next QST issue (09/1961), Sutherland writes an article on what may
be the first implementation of B- isolation above chassis ground. See p. 14 of
the September issue for a PS schematic that resembles what we expect to see
today. Anyway, that's the first appearance I see in QST but B- chassis
isolation may have already been published in application notes by Eimac or
Machlett. Possibly B- isolation was published earlier in IEEE or even IRE,
its predecessor.
Regarding the joining of electrical circuit ground and neutral in an amp, that
practice lasted at least through the early 1980s. ETO's 70/77 series amps use a
120VAC blower that tap one side of 240VAC and <gulp> the chassis. Through the
production life of the 77 which lasted into the early/mid 1980s, the effect of such
wiring is to ground the neutral in the equipment. That's not strictly an NEC
violation since the NEC is a premise wiring standard and not a products wiring
standard. Still, the practice would violate UL and other product certification
standards.
Thankfully, the fix is easy in the 70/77 series. One simply cuts a jumper on
the Cinch-Jones power plug. The 3-wire power cord is discarded, replaced by a
4-wire power cable. Not even the top cover of these amps requires removal for
the change.
Paul, W9AC
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Jim Garland
Sent: Saturday, December 12, 2020 10:57 AM
To: Amps group <amps@contesting.com>
Subject: [Amps] History of Grounded Grid Amplfiers
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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