For those interested, I've begun to document the filament mods conducted on
my Alpha 77Dx. Recall, that I wanted to (1) decrease the filament voltage
to the 8877; and (2) float the filament above ground potential.
>From a historical perspective, it's interesting to note that ETO/Alpha
floated the filament in the original 77 model. This was the first Alpha amp
to use the 8877 and the amp is enclosed in a smaller cabinet, but still
resembles its bigger brothers, the 77Dx and 77Sx . Why they decided to
ground one side the filament in the 77Dx/Sx is a mystery, especially since
Eimac recommends a floating, ungrounded filament.
Note in the links to photos below that I added a 160m-10m filament RFC from
RF Parts, even though I decided against tying the cathode to the filament as
done in the Ameritron AL-1500. In that amp, one common RFC is used to
isolate the cathode and the filament. Since the 77Dx already incorporates a
separate cathode choke, I decided to utilize a separate choke only for the
filament. Should I decide to tie the cathode to the filament in the future,
this can now be easily accommodated. Also added where three Bourns gas
discharge tubes (GDT) at the tube socket to ground -- two for the filament
(one on each side), and one for the cathode.
Finally, you'll see a series-limiting wirewound resistor to get the filament
voltage from well over 5.25VDC down to a target value of 4.85VDC. The
reason for high filament voltage is that my AC utility mains runs about
247VAC at this location. Before wrapping up the project, I may replace the
resistor with a 25W @ 0.5-ohm rheostat and mount it with an aluminum angle
bracket to the removable plate for the 2nd 8877. The soldering to the
resistor is temporary in the event I replace it with the rheostat.
You'll also see new #14 filament wiring. I stuck with red coloring as per
Alpha's use, but I would have preferred to use a yellow pair to distinguish
it from HV cabling.
All mods are reversible and holes are drilled in the replaceable plate for
use with a 2nd 8877. As long as I am owner, I will never install the 2nd
tube in this amp for several reasons. Reversing the mods requires only
replacement of the plate.
Photo 1 shows the overall component changes with new filament RFC in the
back, GDTs, filament resistor, filament bypass caps, and new wiring.:
http://216.229.20.37/images/77DX-1.jpg
Photo 2 is a close-up of the tube socket showing the GDTs and connection to
the new filament choke.
http://216.229.20.37/images/77DX-2.jpg
Photo 3 shows the temporary Dale/Vishay wirewound resistor mounted to the
removable plate, 0.02uF @ 500VDC disc bypass caps on a new terminal strip,
and a better view of the new filament wiring.
http://216.229.20.37/images/77DX-3.jpg
As always, comments and suggestions welcome!
Paul, W9AC
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