[Amps] Alpha 77Dx Filament

[Paul Christensen]

I bit the bullet and ordered the RF Parts filament choke for the Alpha 77Dx. 
I've determined that two of my 8877 suffer from an intermittent 
cathode-to-filament short -- which explains why there's no RF drive seen 
when the grounded filament comes in contact with the cathode.  A third 8877 
is perfectly fine.

By using the RF Parts filament choke and un-grounding one side of the 
filament from the chassis, both bad 8877 tubes should come back to life 
since the filament and cathode will be tied together.

I went back through the AMPS archives and saw a few postings from guys 
placing Bourns gas-discharge-tubes (GDT) devices between the cathode and 
ground.

http://www.bourns.com/data/global/pdfs/bourns_circuit_protection_selection_guide.pdf

Capacitance is specified at less than 1.0 pF.  I would think that's a 
de-minimis value at the tuned cathode input.  The Bourns GDT I've seen 
referenced is model 2027-09 and is rated at 90VDC @ 20KA surge.

Can anyone talk me out of using the Bourns GDT here, especially after tying 
the filament and cathode together after I install the filament RFC?

I've also ordered some 14 AWG Teflon-jacketed wire that, depending on 
cabinet geometries, will be wound around the circumference of the bottom 
'till about 0.5 VDC is dropped.   If the cabinet won't facilitate this, I've 
also ordered 0.05, 0.075, and 0.1-ohm @ 20W wire-wound resistors with 
integrated chassis-mounted heatsink.

Tnx!

Paul, W9AC