I have an RCA 2X 3-400Z's G-G, for 1 KW out. I was having a problem with power
output. It seemed that all the power I could get out was 500 watts and that was
on a good day. After some investigation, (the head banging kind), I found that
the filament voltage was about 3.8 VAC!! Some one before me had apparently used
18 ga. wire to run to the RF choke. The voltage at the transformer was the 5.1
VAC which would make the 3-400's very happy. The distance between the filament
transformer and the RF choke was about 18 inches. The repair was to replace the
wire with 12 ga. wire that made the voltage at the filament pins, (not the RF
choke), 4.8 VAC. My output went to 1 KW and has been running fine since.
Mod-U-Lator,
Mike(y)
W3SLK
I'm not sure if this has been brought up before, but wouldn't the best way to
get a truly accurate reading of filament volts on a glass tube like a 4-1000
would be to solder small wires directly to the filament pins and run them to
an accurately calibrated AC voltmeter? There can be a small amount of
resistance where the socket grips the tube pins and in a high-current
filament circuit even a small resistance can drop the filament voltage some
fraction of a volt? Is it feasable to measure the filament voltage at the
pins in an operating GG amplifier where the filament circuit is above ground
hot with RF? I would think using a pair of small RF chokes at the pins would
isolate the AC voltmeter from the RF but could they cause problems with
making the circuit unstable? Has anyone ever done this? Thanks for any
comments. 73 Todd Roberts WD4NGG .
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