See my similar comment from 2006 here:
http://amfone.net/Amforum/index.php?topic=6841.0
> > > A "bad report"
> > > A pair of supposedly matched Chinese 3-500Zs sourced from the same
> > > supplier showed vastly different grid and plate currents at zero bias and
> > > the same HT. We tried several sets before we found anything remotely
> > > matched. The glass envelopes were also rather odd shapes so the air flow
> > > through cooling chimneys was uneven. The pins in the sockets were not
> > > aligned, causing us to have to modify the sockets to take them. The tubes
> > > seemed to glow MUCH brighter than the Eimac tubes, perhaps indicating a
> > > mis-spacing of elements internally and perhaps a thinner anode meaning
> > > that they had nowhere near 500 watts dissipation.
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