Copyright Donald W Thomas, K5ZRQ
15
9.0 POWER TRANSFORMER:
USING TRANSFORMER 271-025 (10 LEADS) WITH LA-550, LA-600, and LA-650
The original power transformer used in the LA-550, 600 and 650 series linear amplifiers was the 9
or 8 lead version of the 271-025. If the linear uses 6LF6 tubes it has the 9 lead version, because of
the requirement to provide 6 volts to the filament of the driver tube. If the linear uses 8950 tubes the 8
lead version may have been used as there is no requirement for a 6 volt center tap. The 8950 has a
12 volt filament.
Because the 10 lead version of 271-025 has no 6 volt filament tap it cannot be used to replace the
transformer in a linear with 6LF6 unless a simple modification is performed. This consist of replacing
the driver tube with an 8950 and re-wiring the filament pin (pin #1) to the 12 VAC filament source. The
other socket pin connections remain the same. The driver stage must be re-aligned and re-
neutralized because the internal capacitances of the two different tube types are not the same.
Also when using the 10 lead transformer, the two primary windings must be wired in parallel. For
the 110V AC source the 2 windings are (1) orange and brown and (2) yellow and green. To wire them
in the proper phase the orange and yellow leads are both connected to the unswitched line cord lead
at the term strip (this is the line cord lead that is not connected to the fuse holder). The green and
brown leads are both connected to the adjacent terminal. This terminal has a wire leading to the
power switch.
10.0 REPAIR TIP:
SWITCH REPAIR TIP
If you have a switch like this and it is being intermittent or not working at all, it probably just needs
cleaning. This is a DPST switch and if you cannot find a suitable replacement, this is how to
disassemble and clean the old one. When removing switch, make sure you remember to identify
which wire connects to which terminal. Word of caution, the AC POWER and the PREAMP switch are
used to secure the front panel to the chassis.
Figure 10-1
