Grounded-Grid Amplifier Tune-up
Linear amplifiers are like induction motors--they are designed to run
fully-loaded. If your grounded-grid amplifier's instruction book says to
reduce drive power during tune-up--and most of them do--it is not giving
you correct information. In order to be linear, amplifiers must be
tuned-up with the same peak drive-power level that they will be driven
with during actual operation. Reducing drive power changes the output Z
of the amplifying device to sornething other than the tank circuit was
designed to rnatch. Thus, the tune and load settings with low drive will
be wrong when norrnal drive is applied.
Tune-up rnethod #1: Set the amplifier's HV supply to the CW-Tune/low-Vtap.
If you are not sure where to preset the Load control, set it to
>70% of rnaxirnurn loading [30% of CJ to be safe. Apply the drive level
that you intend to drive the arnplifier with during actual use.
Alternately adjust the arnplifier's Tune and Load controls for rnaxirnurn
relative power output. The whole process should take less than 6
seconds. It rnay sound brutal, but this tune-up rnethod results in good
amplifier linearity and it won't darnage the tubes if the rnaxirnurn anodecurrent
rating is not exceeded. If the anode-current is excessive, the
resistance of the cathode, RF negative-feedback resistor needs to be
increased slightly--or the PEP adjust control in the transceiver needs
to be turned down.
Tune-up rnethod #2: [not for FM, A0, and RTTY operationJ To reduce the
stress on an amplifier during tune-up, use a reduced duty-cycle driving
signal. This can be accornplished by keying the transceiver, on CW rnode,
with a CW keyer, set to send approx. 50wprn dits. CW dits have a 1/2-on,
1/2-off, or 50% duty-cycle. Using this rnethod, the amplifier rnay be
tuned-up, again for rnaxirnurn power output, in its higher-voltage, SSBrnode.
Keyers that have a weighting adjustrnent can be set to produce a
light dit that has a duty cycle of about 30% instead of the norrnal 50%.
Another device for reducing the duty-cycle during tune-up is a tuningpulser.
If you want to operate with reduced power during good band conditions,
first tune up your arnplifier with norrnal drive power, then turn the
rnicrophone gain down to reduce power.
[ 此帖被BA7KW在2014-09-05 10:39重新编辑 ]
