[Amps] re: baking out tubes

[R.Measures]

On Oct 26, 2004, at 1:15 AM, Steve Thompson wrote:

> On Monday 25 October 2004 12:44, Traian wrote:
>> Hi Bill,
>>
>> May a rather expensive power tube to not have an internal gettering 
>> sytem?
>> The getter is used even for the cheapest small signal and low power 
>> glass
>> tubes, so why to not be provided for a $500 (or maybe $5000) power
>> transmitting tube?!(Unless, it include a vacion device, of course).
>>
>> Details about vacuum tube conditioning can be found in the Eimac's
>> AB21 Application Bulletin and in the newer edition of Care and 
>> Feeding...
>>
>> I have made experiments for some tube conditioning and I have already
>> posted some results and personal observations, see:
>>
>> http://lists.contesting.com/archives/html/Amps/2004-05/msg00030.html
>>
>> From then now, I have also verified six 8122's and other two GU43B
>> tetrodes with the same positive results. The breakdown voltages are
>> increasing by "debarnacling" for the NOS tubes but it seems that is 
>> not
>> for the heavy used ones? (at least not for a used 4S040 and 4CX1000 I
>> have tried).
> For what it's worth, my theory is that a well used tube with an oxide 
> cathode
> has minute amounts of cathode material spread around the inside, and 
> this
> leads to a leakage level that won't reduce.
>
-  I agree Steve.  And, there's another problem:  Since barium-oxide 
has a lower boiling point than strontium-oxide, the barium tends to 
boil off, and most of what evaporates condenses on the much-cooler 
grid.  Thus, when a higher-mileage grid becomes warm from the flow of 
normal grid current, the barium coating on it begins emitting electrons 
in the Wrong stinkin' direction - and the anode-current starts 
decreasing.  The barium that sticks to a gold-plated grid of an 8877 
can apparently be partly dislodged by vertically and firmly  tapping 
the top of the tube with a 4oz hammer -- which is the same procedure 
that is used to dislodge gold meltballs from the cathode and from the 
anode insulator.
    note --  Barium evaporation can be minimized by operating an 
oxide-cathode tube's heater at the minimum rated V.
-  Historically worthless trivia:  The Ham who reportedly discovered 
that hammering an 8877 can reduce gold meltball leakage lives in Ohio 
and he owned a Dentron DTR-2000.  Apparently DTR stands for Dead Tube 
Radio (freq. amplifier) because he had witnessed the destruction of 
more than several 8877s in it.  As the story goes, one day, right out 
of the blue, he decided to try hammering the top of a dud 8877 to see 
if he could dislodge whatever was causing the leakage and internal 
flashovers.  Apparently, the technique was somewhat successful.  When I 
tried the same technique on a pair of leaky 3cx800A7s out of a 
Ten-Tec™Titan, the leakage current in one tube decreased from c. 55uA 
to <5uA, and the leakage-current in it's sister decreased from a bit 
over 130uA to <5uA.  The shocking thing was how much hammering it took 
before the leakage current stopped decreasing.  These tubes were then 
tried in the Titan, and it worked normally.
--  The Titan uses gorgeous, silver-plated strap for its 
parasitic-suppressor inductors.   

-  cheerz - ...

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org