High voltage testing is excellent advice and especially for the large tube
pulls that are unknown,or any high voltage component,vacuum variables,vacuum
relays,diodes,even doorknobs.Trying an unknown tube in an amplifier can
destroy the plate choke,bias,metering,power supply ,etc..I first test with
my hipot for any leakage between grid and anode..This can be confusing if
there is any kind of conducting contamination on the ceramic or glass.Be
sure to clean these areas first.A good rule suggests nearly twice rated
plate voltage without current draw for an unshorted tube.
Filament/heater testing&conditioning is next and then on to slow burn in
with controlled airflow, hv,etc as suggested by the previous poster.
If you don`t have the equipment and are dealing with an expensive tube,then
I suggest contacting a tube rebuilder such as Zonum Industries.They can do
testing&burn in for a very reasonable fee..And for those building around an
unknown tube...Peace of mind! Jim,K7RDX..
----- Original Message -----
From: <TexasRF@aol.com>
To: <klahaie@centricata.com>; <Amps@contesting.com>
Sent: Thursday, July 31, 2008 7:41 AM
Subject: Re: [Amps] 8877 Advice
>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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