The plate current meter in most AM TXs cannot be in series with B- side of HV
power supply if it provides multiple elements and modulator.
Often the screen grid bias is provided thru a series dropping resistor from the
modulated B+ and the modulator is also powered by the same power supply.
73
Bill wa4lav
________________________________
From: Amps <amps-bounces@contesting.com> on behalf of Joe Subich, W4TV
<lists@subich.com>
Sent: Saturday, December 12, 2020 11:08 PM
To: amps@contesting.com <amps@contesting.com>
Subject: Re: [Amps] History of Grounded Grid Amplfiers
CAUTION: External Sender
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@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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