[Amps] WEIRD ONE!

[Paul Hewitt]

Hi Bill
The meter between B- and chassis is the grid current meter and there should be no reading with no drive.  Zero signal Idling, (Anode) current, (ZSAC) normally refers to plate current, the meter in series with the B- line.

73, Paul


Paul Hewitt
WD7S Productions
QRO Homebrew components
http://home.earthlink.net/~wd7s/contents.htm

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Bill & Liz
Sent: Sunday, April 20, 2014 8:34 AM
To: amps at 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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