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[AMPS] Alpha 78

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Subject: [AMPS] Alpha 78
From: (Rich Measures)
Date: Sun, 4 May 97 08:11:48 -0800
>Hi guys:
>       While watching the alpha 78's manual, I realized it has some resistors 
> in 
>parallel between the rf from the exciter and the 8875s Can't remember how 
>many, but the value was 15 ohms. I wonder what purpose they serve, with 
>such a low value and also wonder if they can be removed, to increase drive 
>=> output power??
>       besides, anyone can provide any hints about this fine amplifier. I'm 
> only 
>able to get 1,1 KW (with 100 W drive) and thought it should be at least 
>1.5 KW.
>       Thanks for any answer.
>Ernesto LU6BEG  
Negative feedback resistance in the cathode-input path increases drive 
requirement and reduces IMD.  The drive requirement for three 8874s is 
around 80w.   With 100w of drive, 15 ohms may serve to aborb the extra 
drive--and prevent the cathodes from being overdriven.
  The key-down cathode current rating is 0.35a per 8874---or for (3) 
8874s, 1.05a.  As long as you do not exceed this rating, changing the 
resistance to other than 15 ohms should be ok.  However, if 8874s are 
tuned up with a tuning pulser, and used only on SSB, the maximum cathode 
current rating increases to 0.5a per 8874.  However this current can not 
be measured directly on the meter.  To convert the meter indication, 
divide the reading by the duty-cycle of the tuning pulse.  For instance, 
if a CW keyer sending high speed dits is used for tune up, the duty-cycle 
is 50%---which would effectively double the meter indication.  
 If you find that you can not obtain maximum rated current, the 8874s' 
cathode emission may NOT be worn out.  Instead, you may have gold 
contamination on one or more of the cathodes.  Gold poisons emission.  
This condition can be semi-fixed by tapping the tube in the vertical 
plane to knock the gold melt-balls off of the cathode.  See "Parasitics 
Revisited", Sept./Oct. 1990 QST magazine for more information.  

R. L. Measures, 805-386-3734, AG6K   

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