[Amps] History of Grounded Grid Amplfiers

Sat Dec 12 13:12:11 EST 2020

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 at contesting.com> On Behalf Of Jim Garland
Sent: Saturday, December 12, 2020 10:57 AM
To: Amps group <amps at 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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