[Amps] 3-500Z cool down time

[Jim Thomson]

Date: Tue, 6 Jun 2017 12:45:26 -0400
From: "Carl" <km1h at jeremy.qozzy.com>
To: "Kimberly Elmore" <cw_de_n5op at sbcglobal.net>, "Catherine James"
<catherine.james at att.net>, "Amps group" <amps at contesting.com>
Subject: Re: [Amps] 3-500Z cool down time

<The initial use was for ISM amps.

Except for a short time in the late 80's Eimac 3-500Z's had no seal problems 
and I still have a few with 70's date codes that are fine.

The Chinese versions have been OK so far.

Another tube that eventually became a leaker is the 4-1000A.

Eimac and Chinese 3-500's are always prone to outgassing from the anode 
which has absolutely nothing to do with seal leaks to the outside air. 
Unless you are prepared to go thru a regular regettering process then it 
becomes more of a ues them or lose them scenario.

Carl

##  4-1000s  are leakers.  I had 3 of em turn into an internal smoke bomb, with
just the fils turned on.   White clouds of smoke, 1 sec after the fil was turned on. 
They had gone ..up to air. Lost most of their vac.  Toast. 

##  Chris, AB6QK, has come up with a pretty good procedure.  But Carl is correct, the
problem with 3-500Z is outgassing from the anode, not seal leaks.  You cant do anything about
seal leaks, the fil will burn up real fast if the vac is lousy. 

## A  much safer way to  getter the 3-500Z is to temp apply POSITIVE  bias to the tube, then  you can draw
loads of plate current, with very little  plate voltage.    Like  900-1400  vdc on the anode. 

##  I used my lab supply, which is a switcher, and is adjustable from 0-60 vdc.....and  also has current limiting 
up to 5 Amps.   Red + Green banana jacks bonded together on front of supply.... and strapped to chassis.
Black goes to cathode.   Set current limit for say 800- 1000 ma.   50 K resistor used for  cut off bias is temp
shorted out.  Turn on fill xfmr to normal fil V, and turn on fans or blower.  Increase B+ with variac till you get 
900-1400 vdc.    Increase vdc on adjustable lab supply  till  you get some plate current flowing.   Increase
till tubes show some color, or anode diss is maxed out, or within the capability of the cooling setup.   If you
cant get the idle current high enough, then increase the B+ a bit more. 

##  By using Positive bias on the tube, it will draw stupid amounts of idle current..with very low B+ voltages. 
The beauty is... you dont ever have to apply drive ! 

##  let it sit like that  for 1-4 hrs.  By that time you will have gettered the 3-500Z.  The getter on the 3-500Z is
just Zirconium slopped onto the anode plates.   The gettering process is done via getting the anode to show some color.  
Unlike metal triodes like the 3x3, 3x6 and 3x10, which have internal puck shaped  getters, that are heat activated by
the directly heated fil. 

##  air leaks on the seal can be discovered with the use of a hi-pot tester.  That procedure really should be done prior to the
above gettering procedure.  If it fails the hi pot test, the tube is toast.  A good  3-500Z should hi pot test to well over 8 kvdc. 

##  when the tube is driven in normal operation, the developed  RF voltage in the PI net  tank will back feed..via the plate
block caps..and superimpose itself onto the anode.   RF is just high freq AC.   That AC voltage is in addition to normal
B+ voltage.   Typ, the total  voltage on the anode, when driven, is a little less than  double the loaded  B+ voltage. 
Hence the reason the  hi pot tests are typ in excess of  double the rated B+ voltage of the tube...on a good tube.    

Jim   VE7RF