Hi folks,
Thanks to all that responded to my original "PA-77 musings"
query. I'm back, though, with a few other questions for the
panel. :-)
I've decided to move the B+ supply and mains/step-start
contactors into a separate cabinet, largely to make room for
a new controller and low voltage supply. For those that
aren't familiar with it, the PA-77 is a rather massive single
3CX1500A7/8777 grounded-grid HF amp, built by ETO/Alpha in the
70's. That's the context, here are the questions:
(1) I haven't located a suitable tapped-primary filament
transformer yet. (120VAC) I've checked both Dahl and Hammond.
We have our own transformer vault, so the line voltage is
both high and stiff. I -could- use a big rheostat, but
would much prefer taps on the primary. Any sources/ideas?
(2) With 240.8 VAC on the primary, (Peter Dahl A77S) 32uf
filter, and 300k bleeder resistance, the B+ runs just about
exactly 4KVDC with only bleeder current. Since the original
150k bleeder resistors are both old and no longer available,
I'll probably use three 100k 50-watt Ohmite type 270's. Should
I put caps across them, and if so, what?
(3) The original bridge uses four "block" rectifiers,
so I think I'll build a new bridge using 1N5408's. I was
thinking nine diodes per leg, for 9KV @ 3A. Sound reasonable?
(no caps or resistors)
(4) Assuming a standard metering design, the only thing holding
B- and ground together is the grid meter shunt and (new) 3A
glitch diode(s). Given that the RF deck and power supply cases
will be bonded with a copper-braid strap, is that enough for safety
purposes?
(5) I'm a little nervous about running the tube right at 4kv,
which happens to be Eimac's specified maximum anode voltage.
The bleeder current is already a bit higher than average, so
I'm not too excited about increasing it, thus heat and
dissipation in the supply. Any comments?
(6) Last, I'm considering putting large 120V MOV's across
each of the mains legs to the neutral. Any reason I shouldn't,
or should do it differently?
Thanks,
Jim, N7CXI
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