Alpha Power ETO 91B - Grid Current Monitoring and Indicators; Tune-up Procedure

Any linear amplifier must be adjusted for optimum efficiency and linearity at a specific
power level. If operation at higher power is then attempted without appropriate readjust
ment, the result will be flattopping, "splatter," and (usually) excessive amplifier grid current.
If operated at a much lower power level than it has been adjusted for, the amplifier's effi
ciency decreases considerably.
Grid Current
The ETO 91/3 operates in Class AB2 when delivering maximum output power consistent
with excellent linearity. A small amount of grid current flows and the green GRID MIN LED
lights as drive approaches the optimum level. The green GRID LED should flicker on some
SSB voice peaks, and light under CW/SSTV/RTTY carrier conditions. As overdrive ap
proaches, grid current increases rapidly and the red GRID MAX LED lights. At maximum
output and efficiency, the red LED lights dimly; full illumination of the red LED indicates
overdrive and must be avoided. If the red LED lights before the desired value of plate current
and/or power output is reached, readjust amplifier loading before continuing. On SSB,
optimum output consistait with good linearity occurs when the green GRID LED lights on
most voice peaks and the red LED flickers dimly only on the highest peaks.
Excessive grid current results from overdrive and/or inadequate loading. The solution is to
reduce drive, and/or increase amplifier loading. The 91B/s 4CX800A/GU74b tubes are well
protected and these adjustments tend to be less critical than in many other amplifiers.
ALC
The
91/3 grid current limiting circuits provide substantial tube protection against possible
damage. The ALC voltage generated by the 91/3 cannot control the amplifier itself and so is
not applied internally. If ALC control is required, a connection must be made from the 91f3 to
the transceiver ALC input. (See Sections 3 and 4.4)
Tune-up Procedure
NOTE: The forward and reflected power LED bar graphs and GRID LEDs are peak-respond
ing.
BEFORE INITIAL TUNE-UP, MAKE SURE A SUITABLE ANTENNA OR 50 OHM
DUMMY LOAD IS CONNECTED TO THE 91/3. Leave the amplifier off or in standby and
apply exciter power to make a forward vs. reflected power check. If reflected power is less
than ten percent of forward power, the VSWR is lower than 2:1 and it is safe to prooeed with
tuneup.
1. Turning On the Amplifier
* Place the OPR-STBY switch in STBY (standby).
* Rotate the multimeter selector switch to HV.
* Depress POWER/ON.