Found a gem - brand new in the box B&W 2500A one of the last ones built,
power cord was never attached and sealed manuals in factory bag with
receipts. Owner apparently had a stroke as it was in shipment, been in
storage since. Couldn't pass up the deal from the estate.
Slowly re-formed (American made) caps and replaced bleeders with metal
oxides at 3x the value - didn't want to burn 50 watts in the chassis for the
originals. All is well.
It has "new" 3-500ZG's from RFP. They seem to have very good gain, low grid
current at rated output on my amp (PT-2500A, pair of 3-500s). About 1300
watts out with ~65 in, over 1500 with 95 in.
I am rather familiar with the Eimac 3-500Z's with the standard anode, but
have never seen these ZG Frankenstein laboratory anodes before under load
and lit up.
Here is the thing. the std Z's normally start glowing rather symmetrically
all around and vertically (not perfectly but.. ) and these ZG's seem to
start glowing from the bottom of the tube up. They also seem to start
glowing more slowly, as if they have much more mass.
What I mean is, when the bottom 3 "biscuits" of the anode are orange, the
top "biscuit" is still dark without color. Very uneven thermal
distribution. Is this normal for these suckers or do I have an issue with
the tubes. Or just run'em until I smoke 'em?
Another detail - while new tubes never plugged in, they have been in the box
since '96. I "re-generated" them on low anode voltage and de-tuned the amp
to provide maximal dissipation in the tubes to allow the "gettering" anode
to do its thing while glowing a dull red at the lowest power level. Did
this for about an hour after running the filaments alone w/o plate voltage
for about an hour (probably didn't do anything, but it couldn't hurt and I
wasn't in a hurry).
Ya gotta love the feedback from glowing tubes - makes the 87A rather boring
by comparison. and that instant on too!!
-bob, w9zv
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