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