[Amps] WEIRD ONE!

[Jim Garland]

Bill,
I'm not familiar with the circuit of the FL2100, but your symptom sounds like there may be an unintended extra shunt that's throwing off your meter readings. Sometimes that's caused by a protective diode between B- and ground intended to safeguard against flashovers. In most grounded grid configurations with the grids tied directly to the chassis ground, the resistor between B- and ground just monitors grid current. The B- return to the power supply from the tube cathodes has to be completely isolated from ground, except for the grid current monitoring resistor. Just a thought.
73,
Jim W8ZR 

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Bill & Liz
> Sent: Sunday, April 20, 2014 9:34 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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