>Looking at the bottom of my 3-500z, it looks like a pentagon with the "lone
>pin at the
>top. It and the 2 pins below it on both sides check with continuity with
>all three
>pins. And the 2 pins that are the closest on the bottom also check with
>continuity.
>Would this tell me if the tube is good or not?
>Thanks....Steve
No. To test a 3-500Z, one needs a high-pot. tester. This is how I test
them:
1. Gas check. (cold) Apply 8kV (current limited) between the anode and
the grid. If there is a blue glow inside the tube, the tube is fatally
gassy. Normal anode/grid leakage is under 10uA at 8kV.
2. Bent filament helix check. Apply increasing potential between the
filament and the grid. Normal breakdown is 6kV to 9kV. If the breakdown
v is lower, the filament is probably bent. When hot, the filament to
grid clearance decreases, so a 3-500Z with a bent filament is likely to
short when the filament is hot. IMO, 3-500Z filaments can be shorted to
the grid by two things: 10Gs in a centrifuge for about a minute with 6v
on the filament - and by intermittent vhf oscillations. If you find a
3-500Z with a bent filament, it might be a good idea to measure the
resistance of the anode, vhf suppressor resistor.
- IF the tube passes the above tests, it is probably safe to test its
emissive ability in amplifier.
Rich---
R. L. Measures, 805-386-3734, AG6K
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