Copyright Donald W Thomas, K5ZRQ
8
5.0 CIRCUIT DESCRIPTION:
The LA-600 is a classic TV Sweep Tube Amplifier using five 8950 tubes. One of the tubes is a grid
driven driver stage and the other four are the PA tubes in grounded grid final operation. It has a low
and high power output setting and features a pre-amplifier which provides approximately 6 db gain on
received signals.
Figure 5-1 Chassis Layout
In the figure above, you can see the Driver Section in the lower right corner. The variable capacitor
C30 to the right of the Driver tube is for tuning the Driver output and is accessed thru a small hole in
the side of the top cover. The four PA Tubes are just above the driver section. You can see the Plate
Cap connections with their Plate Parasitic Suppressors. Above them you will find the Plate Load and
Plate Tune capacitors used for tuning the finals for proper operation. In the bottom center, K1 is the
main relay for switching the PA in and out of line. K2 is mounted on the PCB and it functions to switch
the Preamp in and out of line as the PA is keyed. To the left is the main Power Transformer. Above it
are the four capacitors that make up the high voltage power supply filter. The Fan in the center of the
chassis runs continuously as long as the Amplifier is turned on providing air circulation for the tubes.
