> 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.
Gary,
I agree with that assessment. And that probably explains why the AL-1500 is
tied since (I believe) Tom designed that amp for Ameritron.
As to how important this is in real operation, I am not sure. I have seen
opinions range from "pro" to "con" for the tying of the two elements
together. However, neither the Alpha 77, 86, 77Dx, nor 77Sx tie the
filament and cathode together. I suspect that other GG amps in the Alpha
product line do not tie the two elements together.
Personally, I have no preference either way. The most important aspect for
me was to get the filament return off the grounded chassis. The chassis
should form the basis of a safety ground and not be used to carry between
10A-20A (e.g., 2 x 8877) of AC current. I have a real problem with using
the chassis in that manner. I have no issue with single-point grounding
(i.e., if necessary, tying the filament to get it at ground potential), but
not when the chassis is being used as a high-current conductor of
electricity.
Presumably, a fast rise of the cathode could couple to the filament. I have
not seen the inner mechanics of the 8877, but I imagine the physical
distance between the two elements is very close.
The GDTs I'm using are made by Bourns and are rated for a surge of 20KA. If
I end up tying the cathode to the filament, then three GDTs would be in
parallel to ground. The capacitance of the GDT is less than 1pF. At 10m, I
am not sure if the addition of that little capacitance would create a
problem. In any event, two of the GDTs could be easily removed, if
necessary. Also, the 77Dx uses a series of 1N5408 diodes from the HV(-)
terminal to chassis ground.
No matter what, I now have the filament RFC in place, and the filament
floats above the chassis, pursuant to Eimac's recommendation for
indirectly-heated power tubes. I am waiting on a new 8877 to compare with
my old tube that has an intermittent cathode/filament short. I'll post a
final report after I run some tests.
Speaking of the chassis conducting current, the Alpha 70A, 70V, 77, and
77Dx/Sx are safety hazards in their company-produced form. Each of these
amps uses a 120VAC blower with its neutral tied to chassis ground. Should
the neutral lift between the amp and the serving load center, the chassis
will conduct current for the blower through the BNC, SO-239, RCA connectors
and find any way possible to complete the neutral path through any of the
above grounded connectors.
Thankfully, the fix is simple and doesn't even require removing a single
screw from these amps. The chassis and neutral can be un-linked by cutting
a jumper at the Cinch-Jones power plug. Then, the three-wire power cord is
replaced with a four-wire cord in a (L1-L2-N-G) configuration. Now, neutral
and chassis ground are safely disconnected and meet NEC criteria. I am
using #10/4 flexible power cable, purchased from Home Depot. This is a
mother of a cable to handle, but it still feeds well into the amp's
Cinch-Jones power plug.
Paul, W9AC
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