[Amps] Re: History of Grounded Grid Amplfiers

Sun Dec 13 12:33:20 EST 2020

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

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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.)

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Message: 1
Date: Sun, 13 Dec 2020 00:30:05 -0500
From: "Paul Christensen" <w9ac at arrl.net>
To: <amps at 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

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Message: 2
Date: Sun, 13 Dec 2020 09:42:52 +0000
From: "Fuqua, William L." <wlfuqu00 at uky.edu>
To: "Joe Subich, W4TV" <lists at subich.com>, "amps at contesting.com"
	<amps at contesting.com>
Subject: Re: [Amps] History of Grounded Grid Amplfiers
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	<BN7PR03MB4499FF874FB852120A4B1265CBC80 at BN7PR03MB4499.namprd03.prod.outlook.com>
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________________________________
From: Fuqua, William L. <wlfuqu00 at uky.edu>
Sent: Saturday, December 12, 2020 11:44 PM
To: Joe Subich, W4TV <lists at subich.com>; amps at contesting.com <amps at 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 at contesting.com> on behalf of Joe Subich, W4TV <lists at subich.com>
Sent: Saturday, December 12, 2020 11:08 PM
To: amps at contesting.com <amps at 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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