Hi Rich
For the 8877 at HF you can save the filament choke, a simple gas discharge
spark gap at the cathode will fire long before an internal arc to cathode
will jump to the filament. The discharge tube insures the cathode stays no
more than the gaps voltage rating above ground potential during a HV arc to
cathode and will fire long before a "glitch diode" will turn on and conduct.
Eimac recommends this on the 8877's bigger brothers. Un-grounding the
filament secondary is a good idea also, no sense having the filament looking
like a lower impedance path to ground than the spark gap !!
Cheers
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://home.earthlink.net/~wd7s
----- Original Message -----
From: "R.Measures" <r@somis.org>
To: "Paul Christensen" <w9ac@arrl.net>
Cc: "AMPS" <amps@contesting.com>
Sent: Saturday, March 05, 2005 12:55 AM
Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
>
> On Mar 4, 2005, at 6:30 AM, Paul Christensen wrote:
>
> > Whoops - my mistake. The 77Dx's 8877 does not tie the cathode and
> > filament
> > together. One side of the filament is grounded. Would it still be
> > advisable to add the common-mode filament choke anyway?
>
> Not if you have a free supply of 8877s.
> - I would add a bifilar RFC, unground the filament/heater winding, and
> wire the cathodes to one side of the heater.
> >
> > -Paul, W9AC
> >
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