[Boatanchors] testing 3-500Z tubes for shorts

[Rob Atkinson]

I'd take one of the amps you have and I'd try the following:  I'd put
the your AC service 240 v. hots to the high voltage transformer
primary on a 240 v. variac and leave that off.  I'd plug in the tubes
into the amp sockets and connect the anode heat sinks and turn on
filament voltage and blower.   It doesn't seem to me that the grids
are your problem unless the tubes have been run for long with near
maximum or too much grid current.  After around an hour, I'd start
bringing up the high voltage until you see around 1 KV on the Ep
meter.  If you still have not had any problems I'd key the amp into
transmit with no drive and let the bias idle current heat up the
anodes.   The idea is to degas the tube by getting some color on the
anode to getter the tubes before putting them on the full B+.  There
should be some color showing after the amp has been put into transmit
for a few minutes.  You should be able to leave it like that for some
time.  The time varies with the amount of cooling the amp has for the
tubes.  You'll have to watch them but the plates should be okay
red/orange.  After you have been through a few key/cool down cycles of
this you can try going up to 1.5 KV and so on.  Actually depending on
how the fil. supply is wired to the cathodes you could try this one
tube at a time.   Something similar is my plan for putting some
4-1000As into operation eventually.  They have not been used in
several years, maybe even 15 or 20, are untested, so I am working out
a way to gradually get their feet wet again.   If I missed something
with this plan let me know.   Your tx idle Ip for a pair of 3-500s
should be somewhere around 120 to 140 ma.

73

Rob
K5UJ

On Wed, Jun 1, 2011 at 6:26 PM, John King <k5pgw at yahoo.com> wrote:
> I have 5 amplifiers that use two 3-500Z tubes in each. Over the years, I have had those tubes go bad and short, blowing the chokes and caps from grid pins to ground. I normally just replaced the tubes and associated grid components when that happened.
>
> Now I have a number of pulls and am trying to determine if those have grid to plate shorts or grid to filament shorts. I have been using my 8025 Fluke digital meters to try and detect shorts. So far none have tested shorted with the multimeter.
>
> I have decided that some may be not shorted but "nearly shorted" and the shorts don't occur until the tubes are heated up with the filaments and the plate voltage is applied. I suspect that after heating up, that the grid will sag into the plate bringing about a "big bang".
>
> If you have experience with these tubes, I would like your input for testing without a "big bang" and if there is a "near short" that rears its' ugly head only after being heated up, how would I correct the sag and near short. Thanks and 73, John, K5PGW
>