[Amps] History of Grounded Grid Amplfiers

[Joe Subich, W4TV]

On 2020-12-12 10:15 PM, Jim wrote:
 >
> I don't remember if AM transmitters were this way or not.

The RCA BTA-1R out in my garage certainly has the plate current
meter recessed (behind glass).  Schematic shows late current meter
in the B+ line to the finals (parallel 4-400).

73,

    ... Joe, W4TV


On 2020-12-12 10:15 PM, Jim wrote:
> 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 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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