Hi Bill,
The GI-7b has one side of the heater tied to the cathode.
Feeding bias thru the transformer CT results in modulating
the amplified signal with 60 Hz.
A friend did the same thing. Look at http://gi7b.com/
for how to apply bias.
vy 73,
Bryan WA7PRC
----------------------------------------------------------------------
Date: Sun, 20 Apr 2014 11:34:07 -0400
From: "Bill & Liz" <magoo@isp.ca>
To: <amps@contesting.com>
Subject: [Amps] WEIRD ONE!
I just finished a GI7-B conversion of an FL2100 to 6M. Everything works fine
with good output and no anti-social habits to this point. BUT there are two
issues the solutions to which have eluded me so far:
1-Upon key-up and with no drive applied, I have almost no idling current on the
meter (between B- and ground across a shunt). The tube is cut off in RX and in
TX a variable bias is applied via a bias board to the CT of the filament
transformer. The resistor shorting relay works and the bias changes from about
?22V to around ?6V (it is variable). I should see about 40 ma of idle current,
not the width of the meter needle.
2-With about 250W output measured on a Bird with 500W slug 25-60 mhz, I see
only about 100 ma on the meter when, for that output, I should be seeing
perhaps 250-300 ma. The meter shunt is good and the meter/shunt combination
has been tested in my meter test jig. A 0-300 ma meter was substituted (with
no shunt) with the same result.
Now, one clue may be that the fan slows down as I tune the amp, suggesting that
RF is getting into either the fan or its 12V power supply (possibly the meter
circuit too???). I am working on that with more by-passing and shielding. But
what could account for the almost nonexistent reading of idle current in a
no-drive condition?
Any ideas here? What have I missed?
Bill VE3CSK
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