[Amps] Need Smarter Than I Am Help.

[Kim Elmore]

I did this once on a single-tube 3-500Z amp (an AL-80A) and raised the 
B+ about the same amount. A few back-of-the-envelope calculations showed 
that I needed more L in the tank circuit. It used a Pi-L configuration 
on the lower bands. All of my changes were fundamentally empirical.

Originally, the B+ was 3100 V no-load, 2700 V loaded. It is now 4000 V 
no-load, 3600 V loaded. At 2700 V, the plate impedance is about 3500 
ohms, but at 3600 V it�s about 5000 ohms. To maintain the same loaded Q 
the output network needs more inductance. For all bands except 160 m, 
coil taps can be moved but for the 160 m band there are no taps. Thus, a 
small powdered iron toroid of approximately 10 uH has been added to the 
160 m inductor. I mounted this toroid onto an insulating pad underneath 
the 160 m coil. I moved coil taps around until the load control was 
about in the middle of its range on all bands. 15 m and 10 m didn't need 
much (if any) change, but I added inductance on all other bands. It was 
easier to drive and, by the book, had slightly better 5th order IMD at 
the higher B+. Make sure cooling is sufficient because you'll stand to 
make a fair bit more heat.

A drawback is that the RF voltages on the bandswitch are elevated quite 
a bit, so the amp will be very unforgiving of light loading. I never had 
a problem, but I always tuned the amp carefully after that making sure I 
was never in a lightly-loaded configuration. Overall, I realized a 
whopping 1.5 dB increase in power output from my mod, but I gained a LOT 
more than 1.5 dB in knowledge!

Kim N5OP

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