The gap I'm using is a Bourns 2035 series, 90 volt, 20KA (single pulse),
hope the glitch resistor holds !! The gap through Mouser is $3.22 single
piece price
Arc to ground or internal arc to cathode, same problem for the filament. If
the filament secondary is not grounded the cathode to filament breakdown
rating becomes 150v plus the transformer winding breakdown rating, (or
feed-thru cap breakdown rating, connecting wire insulation, etc). The
90-150 volt spark gap still saves the filament.
I have had unintentional spark gaps between the B- buss and ground of .5"
fire,(with catastrophic results) while a 1N5408 "glitch" diode sat there and
smiled at me. The spark gap is faster than the diode, race them sometime.
I now use a spark gap paralleled with the "glitch diode plus a spark gap at
the cathode and all is good in river city.
Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://home.earthlink.net/~wd7s
----- Original Message -----
From: "R. Measures" <r@somis.org>
To: "Paul Hewitt" <wd7s@earthlink.net>
Cc: "AMPS" <amps@contesting.com>; "Paul Christensen" <w9ac@arrl.net>
Sent: Saturday, March 05, 2005 8:14 AM
Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
>
> On Mar 5, 2005, at 7:57 AM, Paul Hewitt wrote:
>
> > 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.
>
> What is the peak-I rating of the gas-discharge device, the V-drop
> across it at the peak-I rating, and what is its cost?
>
> > The discharge tube insures the cathode stays no
> > more than the gaps voltage rating above ground potential during a HV
> > arc to
> > cathode
>
> The problem is not a HV arc to cathode, the problem is a +HV arc to
> chassis ground -- which in turn tries to elevate the cathode to the neg
> amount of HV. Since this is typically several neg. kV, and the max-V
> rating between the heater and the cathode is c, 150v, there's trouble
> in river city.
>
> > and will fire long before a "glitch diode" will turn on and conduct.
>
> Glitch diodes are thoroughly on at 1v per junction.
>
> > 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
> >>>
> >
> >
> >
> >
> >
>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
>
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