Hi Ian and Everyone,
Just wanted to put in my 2 cents worth. The Alpha 77 was a nice amplifier
until things went dreadfully wrong. I have modified two Alpha 77's to SX
types. The amp works great on 160, 80 and 40 meters. However, on 20, 15 and
10 tuning is almost impossible.
Here's what I found out by measurement:
1) The tubes mounted in their sockets measured 26 pF each to ground.
2) The vacuum variable tuning capacitor is mounted too close to the power
supply divider wall that even when the vacuum variable is tuned to minimum
capacitance there exists 50 pF to ground.
3) The rf choke is capacitive at 28 MHz and adds an additional 3 pF to ground.
Therefore, the minimum tuning capacitance for this amplifier is 105 pF!
I am sure someone else has had this heartburn and I just told you why.
73 Hardy N7RT