[BC] WANTED - an FM tx.

[Michael Patton]

I once bought and installed a late-model (G-series) CCA 20 KW FM 
grounded-grid rig, and to my horror, I found that the "grid current" 
meter was simply set to measure and show a percentage of cathode 
current.  I tried to show the CCA folks the problem but they were never 
very interested in my rantings; good design was not a strong suit of the 
revived Atlanta-based CCA.

This grated on me more and more as time went by, and since I was the 
contract engineer for the station and had ongoing access to it, I 
devised and implemented a mod to return the HV negative to the cathode, 
not to ground, as Richard noted, leaving a 1 Ohm resistor between the 
supply return and ground, across which I was able to derive a very nice 
grid metering sample.  I was able to verify that it was indeed actually 
grid current by running the driver only, among other tests.  I very 
carefully noted the mods on the schematic and wrote up a narrative for 
the book.  Unfortunately, some years after that, there was a failure in 
the transmitter, and the mods got the blame--of course.  By then, all my 
careful documentation had been ruined by a leaky roof, and the station 
got Jerry Meier from the remnants of CCA to come put everything back to 
"factory" status.  Why they couldn't drop a dime on me--an hour away--to 
put it back to (really) rights is beyond me, but I guess they had their 
pride.

One thing I have learned over the years, is that there is a time to 
modify and a time to leave alone (I guess Solomon missed that one).  
Most equipment designers are very competent at their trade, and their 
decisions are made from sound premises--even the compromises.  However, 
that's not always true, and I have surely seen circuits--even in gear 
from major players--that cried out for a few common-sense mods.

Mike Patton, owner, Michael Patton&  Assoc.
12231 Industriplex Blvd, Ste C, Baton Rouge, LA  70809
225-752-4189 ofc     225-266-9745 cell     www.michaelpatton.com

On 11/10/2010 1:18 PM, RichardBJohnson at comcast.net wrote:
> A properly designed grounded grid transmitter can still measure the current going to the final amplifier plate circuit when measuring the negative lead, even though the cathode (filament) contains both plate current and grid current. The metered HV supply negative lead is simply returned to the cathode circuit, not to ground -trivial.
>
> Some companies were never able to figure that out!
>
> Cheers,
> Richard B. Johnson
> Book: http://www.AbominableFirebug.com/
>
> ----- Original Message -----
> From: "wpio fm 89.3"<wpio at gate.net>
>
> WOW, interesting that the FCC had a lot to do with plate current getting
> measured in the hot lead.  Seems it's not as accurate to sample from the
> low side.  It was always a mystery how, CCA for example, knew what to
> set the variable resistor at because the grounded grid is getting some
> of that current.
>
> My guess was that they temporarily put a DC current meter in the plate
> lead, and used that figure for the low side xmtr plate current panel meter.
>
> Randy
>
>