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
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