Just thought about another possibility...
If I add an RF Parts bifilar filament choke to the 77Dx, there may be just
enough series IR loss in the choke to get the 8877 back into the specified
filament operating range of 5.0V (+/- 0.25V).
The 77Dx grounds one side of the 8877 filament to the chassis. If I add the
choke (and use balanced bypass caps to ground), I would like to lift the
ground (like done on the Ameritron AL-1500) and run a dedicated wire to the
other side of the tube's filament connection. Is there any benefit to
grounding one side of the 8877 to the chassis in GG configuration? I always
thought that the AL-1500 was designed better than the 77Dx in the filament
area.
Paul, W9AC
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: "Amps Amps" <amps@contesting.com>
Sent: Thursday, January 22, 2009 1:45 PM
Subject: Alpha 77Dx Filament
> Looking for opinions on the subject of 8877 filament voltage...
>
> Due to high AC line voltage here (~ 127/254 VAC), my 8877 filament sees
> approximately 5.75 Vrms (as measured on a Fluke 189 True RMS DVM). The
> 8877 is rated by Eimac to run at 5.0v (+/- 0.25V). Eimac specs filament
> current at 10A.
>
> Since the filament current is significantly less than it would be on a
> directly-heated cathode, I am considering bringing the filament voltage
> down 0.75V by using a series 0.075-ohm @ 20W wirewound resistor with
> integral chassis-mount heatsink on the secondary side . The resistor
> would bring the filament down to 5.0V and dissipate about 7.5 watts of
> heat in the process. However, if I can find a nearby mounting hole to add
> an angled aluminum mounting bracket, I could use a 0.1-ohm @ 25W rheostat.
>
> I suppose yet another option involves series limiting the common
> HV/filament primary side to bring the line voltage back down to 235VAC or
> so. Whatever I use, I need to make the mod reversible without hacking
> things up and affecting the re-sale value of the 77Dx amp.
>
> Paul, W9AC
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