[Amps] Alpha 77D some confusion with bandsswitch indexing

[Martin Sole]

Hi,

 

>From some reading I know about some of the differences between the earlier
PA77, 77D and later Dx and Sx versions of this amplifier but I would like to
understand something that has so far eluded me. My 77D has 160m fitted, I
have no need for that at this time but just want to be clear about the way I
think it is configured. I cannot find an online copy of the 77D manual or
schematics, the earlier PA77 is different to my amplifier as are the later
Dx and Sx manuals.

 

When I started the rebirthing process this amplifier S/N 2709 had been out
of service for 20 years or more. The band-switch rotates from B at the most
anti-clockwise position through A and then up to 24-30MHz at the most
clockwise position. This seemed strange as I noticed a high input SWR and
poor output tuning and efficiency. Moving the band-switch knob such that the
most anti-clockwise position is A and 

With the band-switch arranged as I think it should be seems to give more
reasonable performance, the load capacitor is positioned better, not fully
meshed, output and efficiency seem better and the contacts on the band
switch seem more appropriate as to where it switches in the second load
capacitor and fixed value capacitors. Inputs SWR is good too. But in this
position B is the most clockwise position and connects with a tap between
the plate end and the 10m position of the tank coil. Was the 77D envisaged
as covering 50Mhz as well at some point! It would seem I know have things
about right, just confused about the switch positions, the markings and the
taps. Unfortunately the manuals I have provide no indication other than to
suggest B should cover 1.8 to 2MHz with additional padding. Hmm not in my
77D it seems.

 

Would be interested to hear how it should be from someone who might know,
also any source of documentation that might be more correct for this
amplifier.

 

Thanks

Martin, HS0ZED