[Boatanchors] Simple Relative Power Meter Circuit Needed

[hwhall at compuserve.com]

Point taken on the non-coincidence of plate dip & max power out.  That's one operating clue to a neutralization that needs touch-up.  But I still don't see the real value since there's certainly not a few dB difference in the power obtained tuning one way or the other.  We're taught that milking the last few watts into the feedline isn't making a difference at the distant receiver but it might shorten the lifespan of the amplifier tubes.  But if it's being done for the sake of reproducing the old days at the AM station, then I'm not addressing the actual motive.  :-)

WayneWB4OGM

-----Original Message-----
From: Michael D. Harmon <mharmon at att.net>
To: hwhall at compuserve.com; Boatanchors Mailing List <boatanchors at mailman.qth.net>
Sent: Mon, Aug 8, 2022 7:49 pm
Subject: Re: [Boatanchors] Simple Relative Power Meter Circuit Needed

 Hi Wayne, Thanks for the note. The method to my madness dates back to one of my previous lives as a broadcast engineer at a 100 kW FM station.  When you are dealing with a transmitter with that kind of bandwidth, you can't "dip" the plate current on the PA.  You will go stark raving cuckoo trying to find a dip.  The way you tune a transmitter like that is by power out.   Part of the reason is that the PA tuning is done by a Jennings vacuum capacitor with a crank on it.  You have to crank several revolutions to get from one side of the passband to the other.   The "traditional" way to tune up an HF transmitter has always based upon dipping the plate (cathode) current.  That works on an HF transmitter because the passband of the typical HF SSB/CW transmitter is so narrow that the dip is visible.  Tuning for power out works the same on an HF transmitter as it does on a big FM BC transmitter.  I've been using that approach for lots of years.  Having a built-in RF detector in the PA cage just eliminates an additional piece of hardware. Try this:  Put an RF wattmeter on the output, set your panel meter to read plate current, and tune up the transmitter.  You will find that the max power out will almost exactly correspond to the plate current dip. Since my current project happens to be a Swan 350, I'm quoting here from the Swan Tube Transceiver Compendium, compiled by Robert Balonis NB3W.  In Section 2.4.1 (Tune-Up Essentials), the following paragraph is referenced: Tune to RF Output Maximum rather than Cathode Current Dip. The most efficient operation is achieved when the PA is adjusted for maximum RF output power using an external wattmeter (or the relative power out­ put meter function on later transceiver models). The cathode current dip is really only an approximate indication of proper PA stage tuning, as the dip will only coincide with the maximum RF output power setting if the PA tubes are perfectly neutralized. Since neutralization does not track exactly from band-to-band, some inconsistency between maximum power output and PA cathode 
  current dip is both normal and expected.  So, when tuning-up using an external wattmeter, the cathode current reading is essentially ignored and, most likely, the current dip will not correspond to the minimum current dip. 
  Hope this clarifies my request. Mike, WB0LDJ
  
 On 8/8/2022 6:26 PM, hwhall at compuserve.com wrote:
  
 
I'm unclear why you need an external meter for tune-up. The plate current provides for tune-up measurements, doesn't it? 
  Wayne
 WB4OGM
 
 -----Original Message-----
 From: Michael D. Harmon <mharmon at att.net>
 To: boatanchors at mailman.qth.net
 Sent: Mon, Aug 8, 2022 4:51 pm
 Subject: [Boatanchors] Simple Relative Power Meter Circuit Needed
 
 I have an old Swan 350 that I'm rebuilding.  The meter has two scales - 
  receiver "S" meter and transmitter cathode mA.  I'd like to add a simple 
  RF pickup inside the cabinet so I wouldn't have to use an external 
  wattmeter for tuneup.  I figure some kind of a simple diode pickup in 
  the RF cage and a sensitivity control fed to a switch on the front panel 
  so I could switch the meter from "Plate Current" to "Relative Power 
  Output" mode.  I don't need a calibrated wattmeter - basically just a 
  field-strength meter feeding the meter switch.
  
  Does anyone have a better idea?  What about the guts of an old Heathkit 
  field-strength meter repurposed for the task?
  
  Suggestions and tips welcome.
  
  Mike, WB0LDJ
  
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