A quick hv leakage test will tell you a lot about the tube(s). This is done
without filament power, measured between grid and plate.
It is important to make sure the current is limited and one easy way to do
that is by using a hv vom such as Triplett 630 set on 6kv scale in series with
the hv test circuit. You have to subtract the meter reading from the hv
source voltage to derive actual applied voltage.
As you increase the hv, the indicated current will suddenly begin to rise
and this is the point the tube is just beginning to show leakage current. A
good
tube will reach six to eight kv or even more if it has a very good vacuum.
I have seen n.o.s. 8877 tubes with leakage onset as low as 1600vdc. You can
be assured of a big KAPOW if such a tube is fired up without conditioning.
Conditioning consists of running the tube filament only (with cooling air)
for a couple of days followed by slowly bringing up the plate voltage over a
several hour period. With patience, an otherwise unusable tube can often be
recovered to full performance.
Hope this helps.
73/k5gw
In a message dated 7/31/2008 12:32:16 A.M. Central Daylight Time,
klahaie@centricata.com writes:
Lucky Me! (Maybe?)
Received a pair of Eimac 8877's from a friend, condition unknown but
initial checks show no shorts, and physically one appears to have very
little use (lettering still bright, just some evidence of light dust in
the cooling fins) and the other could pass for NOS. Of course,
appearances mean nuthin'
They are both date code 8750, and I assume this means 1987, which rumor
has it was a bad year for these. There is an engraving in the top of
the anode compartment, CYR-058 and CYR-068, so these appear to be
practically identical, certainly from the same run.
I don't have anything that runs the 8877, but I would love to build
something to use them. But, before I start procuring the necessary
sockets and chimneys etc. to incorporate these into a project, I wonder
if there is a somewhat definitive test to see if these are good or just
pretty paperweights!
I suppose I will get lots of offers to send these to you for an
in-amplifier test, only to find out that they aren't worth the return
freight hihi.
I do have access to a professional HiPot tester, and would like to know
if it is advisable to do that, and if so would you test between the grid
and anode? I hear the grids are pretty fragile.
Any further ideas? Also, if anybody has a socket and/or chimney system
available for the 8877, I would be building a single holer so would
appreciate knowing what's out there. Contact me off list on that.
As always, any info much appreciated.
73 Kevin K7ZS
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