[Boatanchors] Simple Relative Power Meter Circuit Needed

[Michael D. Harmon]

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 achievedwhen 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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