> 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
I have been there, done that also, Hardy....in addition to your list:
The tank circuit was designed for a Q of 10-12 with an anode Z of
2200 ohms. Putting two tubes in lowers the Z to 1200 ohms or so.
The B&W plate choke has far less L than required for 160 meter operation.
Same for the bypass capacitor at the base of the choke..not enough C
for 160m. Another choke can be switched in for the low bands, to give
a total of 200uh plus, and turns can be adjusted on the original choke
to raise all resonances above 30 mhz, along with eliminating the 3 pf to
ground.
We have all seen the drift from those door knob padders on the loading
cap. when they get hot. There is room to put in larger doorknobs to cure
this. Larger door knobs are also required on the blocking cap. assembly
to handle the additional 10-12 meter current.
The original vacuum variable can be replaced with a 7-1000 pf unit to
cure the tuning problem. Due to their construction, 1000 pf and larger
vacuum variables have a lower minimum C. ( usually 5-9 pf)
Adding a filament choke helps keep the 160 meter RF out of the
transformer and the a.c. mains. Those puny primary fuse holders
need to be replaced with larger units that hold FN style fuses.
There are several power supply and glitch protection issues to
address also.
That is why I advise my friends to buy a clean 77DX and keep it
a DX....hard to tell the difference on the RX end due to the poor
HV regulation when running two tubes.
(((73)))
Phil, K5PC
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