[HomeBrew] She's a "whooper". . .

[Philip Atchley]

Hello all,

Today I did the first preliminary tests on my "Rockin' '50s MOPA" 
transmitter (6X4, 12AU7 and 6AQ5).  Not having any "proper" rocks, I 
used a 3579.5 KC colorburst Xtal for testing.

I got about 3 Watts out on 80M (80M Loading capacitor needs tweaking) 
and 5 Watts out on 40M.  So that's good.

HOWEVER, Every time I key the transmitter, the B+ takes a "dive" from 
about 360 Volts static to 250 Volts (loaded to 40mA 6AQ5 Plate current). 
  Since the same B+ point is used for both Oscillator and final, the 
frequency takes a bit of a "dive" along with the B+.

Not as bad as a few Homebrewed transmitters that I've heard out there, 
but unacceptable to me!  At least it's "somewhat" stable.  At normal 
keying speeds the chirp isn't too obnoxious, it's that initial "swoop". 
  To be honest, I didn't expect the B+ to drop that much under keying 
(88uF of filtering.  NO room for a filter choke etc).

I'm not sure if it's the high transformer secondary resistance, (340 
Ohms in each leg) or the plate resistance of the 6X4 rectifier (which 
tests good).  Perhaps going to silicon rectifiers will reduce this 
effect, but I REALLY wanted to keep the set entirely '50s vintage!

Anyway, troubleshooting and further work will have to be put aside for 
the moment as I have a friends Collins R-388 receiver here that needs to 
be recapped and overhauled.  That'll give me time to think the whole 
thing through. . .

73 de Phil  KO6BB
RECEIVERS:
    Hammarlund SP-600 (Circa 1952)
    Heathkit Mohican (Circa 1962)
    Icom IC-751  (Circa 1982)
    Grundig S350 (Circa 2004)

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QSL GALLERY: http://photobucket.com/albums/y123/KO6BB/
Merced, Central California,