[Fox_Tango] Re: [Amps] FL-2100F/B

[Dr. Gerald N. Johnson, electrical engineer]

Ordinarily the grid bias on a tube is negative with respect to cathode.
So any capacitor shorts would lower the bias voltage and raise the
idling current. The tubes commonly used in GG linears are designed to
run with very little negative bias under drive, often are rated at zero
bias. Though with higher than normal plate voltage, its handy to add a
few volts negative bias to keep the tubes a little cooler. Sometimes the
bias is raised beyond tube cutoff during receiving times to prevent the
tubes hiding desired signals with shot noise.

I'd be extremely suspicious of the metering circuits (having seen them
20 or 30% out of tolerance from resistor drift). You can check the meter
circuit calibration by connecting a good milliameter between plate and
filament (with power off and disconnected). Then apply a small voltage
across the power supply filter, limit it with series resistance if
necessary, and compare the plate current meter with the added test
meter.

Old abused tubes will have lower plate currents, first on peaks. When
their emission gets low enough to limit the idling current, the peak
plate currents will be the same as the idling plate current.

Likely there's some bias developed from grid current, and the
electrolytic in the grid circuit is to keep those changes to a syllabic
rate.

Check all resistors for change in value, the common carbon composition
resistors drift with time, humidity, and heat and sometimes voltage.
Generally (unless IRC) rising in value with each exposure, sometimes
returning but never to the initial value.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.