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[Amps] Re: History of Grounded Grid Amplfiers

To: amps@contesting.com
Subject: [Amps] Re: History of Grounded Grid Amplfiers
From: Chris <chris@chrishays.com>
Reply-to: Chris <chris@chrishays.com>
Date: Sun, 13 Dec 2020 17:33:20 +0000
List-post: <mailto:amps@contesting.com>
I suspect there aren't too many FM broadcasters opting for the indirect method these 
days, relying instead on the power output metering of the manufacturer.  I'm 
not  sure if the requirement for calibration of such meters every six months is 
still in the rules, but that would require an expensive calorimeter load to do it 
properly, which as a requirement I think is a bit silly for a specification for the 
station allowing for 5 percent over and 10 percent under authorized power.

The other problem, not surprisingly, is solid-state FM transmitters are much more common these 
days, and they don't have a plate to have voltage and current measured on.  In fact, they 
are generally a bunch of individual modules combined together.  Thus, the only practical 
way to measure power is to trust the calibration of the manufacturer's output meter. 

AM is still direct  using base current in the case of a non-directional station, and common point 
current in the case of a directional antenna.  Its much easier these days thanks to Delta's proprietary 
and patented modulation cancellation circuit in their toroidal sampled RF ammeters.  They produce a rock 
steady reading of the carrier level regardless of the modulation.. I once gave a tour to an rf engineering 
class, and the first thing the instructor asked was why the current wasn't bouncing with modulation.  I 
had to explain the specialized meter that only reads the carrier power.  Of course the text books in use 
still proclaimed the only way was with a thermocouple type ammeter, which would follow modulation.  I 
wonder if the book was ever updated.

By the way, engineering texts may not call it AM, but rather "double-sideband, large 
carrier."

Chris

------ Original Message ------
From: amps-request@contesting.com
To: amps@contesting.com
Sent: 12/13/2020 09:00:02
Subject: Amps Digest, Vol 216, Issue 7

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Today's Topics:
 
  1. Re: History of Grounded Grid Amplfiers (Paul Christensen)
  2. Re: History of Grounded Grid Amplfiers (Fuqua, William L.)
 
 
----------------------------------------------------------------------
 
Message: 1
Date: Sun, 13 Dec 2020 00:30:05 -0500
From: "Paul Christensen" <w9ac@arrl.net>
To: <amps@contesting.com>
Subject: Re: [Amps] History of Grounded Grid Amplfiers
Message-ID: <000401d6d111$0488ed20$0d9ac760$@arrl.net>
Content-Type: text/plain;       charset="utf-8"
 
"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?"
 
Although possible, I don't recall that as a requirement because AM broadcast station 
power in the U.S. is determined by the so-called "direct method."  The direct 
method of power is determined: (1) directly from measurements of RF voltage, RF current, 
and phase; or (2) by calculating the product of the licensed antenna or common point 
resistance at the operating frequency and the square of the unmodulated antenna current 
as measured at the point where the resistance has been determined.  The latter method is 
the type most commonly used.
 
In a prior life, I recall calibrating our remote antenna current ammeter by 
walking out the to the ATU, killing modulation, then breaking the knife switch 
across antenna RF ammeter long enough to take a reading and match it to the 
remote meter.
 
The Indirect method of using the more common power formula of P(out) = Ep x Ip 
x F is only temporarily allowed when the direct method cannot be used.   The 
FCC gives some latitude as to the computation used for indirect method 
measurements.  By contrast, nearly all analog FM broadcast stations measure 
power by the indirect method where F (percent efficiency) is determined by the 
transmitter manufacturer during testing.
 
Paul, W9AC
 
 
 
------------------------------
 
Message: 2
Date: Sun, 13 Dec 2020 09:42:52 +0000
From: "Fuqua, William L." <wlfuqu00@uky.edu>
To: "Joe Subich, W4TV" <lists@subich.com>, "amps@contesting.com"
        <amps@contesting.com>
Subject: Re: [Amps] History of Grounded Grid Amplfiers
Message-ID:
        
<BN7PR03MB4499FF874FB852120A4B1265CBC80@BN7PR03MB4499.namprd03.prod.outlook.com>
Content-Type: text/plain; charset="iso-8859-1"
 
 
 
________________________________
From: Fuqua, William L. <wlfuqu00@uky.edu>
Sent: Saturday, December 12, 2020 11:44 PM
To: Joe Subich, W4TV <lists@subich.com>; amps@contesting.com 
<amps@contesting.com>
Subject: Re: [Amps] History of Grounded Grid Amplfiers
 
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
 
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