There are 2 x chinese factories. One has a 12% failure rate.? The other has a 70% failure rate.? ?The good 3-500Z's all have the white base...and not the brown base.?
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The 3-500Z's that Tom at CTR sells have gone through some exhaustive tests...to weed out the duds.? The tubes should have C measured from? anode to grid...and also grid to cathode, and anode to cathode.? ?Then hi-pot tested from anode to grid..and also grid to cathode.? ?Then the emission testing..to measure grid current vs anode current etc. .? Other tests include measuring cathode current (typ 14.5 amps).
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The issue with chinese tubes is.... they typ hi pot test between grid to cathode...like 700-1000 vdc.? ?A good eimac? will hi pot test 5 kv? from grid to cathode.? ?A good tube should? hi-pot test 12-16 kv? between anode to grid.?
Typ chinese tubes have higher C between grid and cathode.... due to narrower spacing...hence the lower hi pot test results.? In typ operation, there is not a lot of peak V on the cathode.? ?100 watts pep into 50 ohms is only 70.7 vrms / 100 V peak.
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The good chinese tubes will last a long time.? ?CTR tests em correctly.? The other tube dealers just sell em.... and it's? up to the end user to evaluate em.? ?That's fine too..as long as when there is an issue, the end user can return em / exchange em.
