Central Electronics Broadband "No Tuning" Grid and Tank Circuits...

[WD5JKO]

Gito,

    Your comparison to audio amplifiers is valid, but misses an important point. These RF amplifiers with broadband couplers are single ended designs that are biased class AB1. If you did this with an audio amplifier, the tube conduction would be much less than 360 degrees and therefore distortion would be significant unless you go push pull.

    The Central Electronics couplers are low Q and broadband resonant. The RF waveform from these amplifiers is a very good sine wave.

Regards,
Jim
WD5JKO

[W1AEX]

I wonder if you can measure the inductors and read the cap vlaues?

Grant, thanks for posting the picture and the schematic. So... what exactly do we have here? A series LC circuit joined to a parallel LC circuit. Perhaps each is resonant to favor one end of the band? The arrangement of the two inductors in the picture almost suggests coupling between them as well. Frank's questions regarding what the efficiency looks like across each band, and at the band edges, is a good one, as well as the curiosity regarding the values of the components for the unit you have displayed there. It certainly looks simple enough to mess around with. I would imagine the components would have to be quite strapping in a class C plate modulated final.

Rob

[Steve - WB3HUZ]

If you have access to old QSTs, check out this article.

"Applying Band pass Couplers to Amateur Transmitters" by Clinton DeSoto, QST, May 1938.

[nq5t]

I wonder if you can measure the inductors and read the cap vlaues?

as well as the curiosity regarding the values of the components for the unit you have displayed there.

Unfortunately I am not in a position to look at either the high res original image or the actual coupler for a couple of weeks.  It does look like the two caps on the smaller of the two windings are 500pf.  Can't read the print (from the posted pic) on the larger ones.  I can measure the inductors, but won't be able to get to it until the end of the month.

Grant/NQ5T

[W1AEX]

Thanks for the information on the QST article Steve. That's an excellent analysis of the broadband coupler concept. It looks like CE was very successful at applying their own design and producing the desired amount of coupling to cover each band with acceptable efficiency in the units they built. It sure beats making 5 or more adjustments when sliding down the band on 75m or 40m! Of course when moving to another band it gets even more involved!

I have a little 4-65 deck in need of a new tank circuit. It might be fun to play around with this stuff a little bit when I am looking for something to puzzle over.

Rob