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[Amps] ALPHA 77

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Subject: [Amps] ALPHA 77
From: (Phil Clements)
Date: Sun, 4 Aug 2002 13:01:54 -0500
> Why would one need to tune for grid-current ?  Minimal grid current does
> not give a correct tune.  Maximal grid-current does not give a correct
> tune.  A correct tune is max out with the drive needed to produce 1A of
> anode current per 8877 -- which coincidentally results in c. 100mA
> grid-current per grid, which could be seen on a 0-500 scale.  However,
> there is no need to measure  grid-current on a cathode-driven amp.
> Grid-driven, yes.

When I tune an 8877 amp, I tune everything for maximum power out.
I then tweak the PA loading cap slightly to give a needle-width decrease
on the wattmeter. I need to see the grid current at this time to make sure
it goes down and not up when making this adjustment. This slight reduction
in suds makes a nice increase in clean. Also, I find that when running 2200
watts output per 8877, the grid current is only 60-65 ma per tube, and the
anode current is well under 1 amp. Just a better all-round environment for
expensive tubes. 8877's installed here in the '70's are still 100% using this
tuning method.  My grid-trip circuits never allow over 100 ma per tube under
any circumstances; the cathode current is also limited to 1 amp maximum.
This assures that antenna/feed-line/tuner/human error mishaps do not cost
me a tube.

Phil, k5PC

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