Hi Paul,
I was always under the impression that the 8877 should have the filament
tied to the cathode in the amp as there is little tolerance in the voltage
that the tube can withstand between cathode and filament.
An arc could raise the cathode well above filament if not tied together and
cause a short filament to cathode. I think W8JI has discussed this in the
past.
Another thing to check if you are not going to tie filament to cathode is if
your gas tubes can handle the energy available from the power supply in the
event of an arc.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Paul Christensen
> Sent: Saturday, January 31, 2009 10:16 PM
> To: Amps Amps
> Subject: [Amps] Alpha 77Dx Filament Mod Update
>
> 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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