[Amps] 3-500Z cool down time

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Tuesday, June 06, 2017 1:40 PM
Subject: [Amps] 3-500Z cool down time


> 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.
>

** I have one of those on display


> ##  Chris, AB6QK, has come up with a pretty good procedure.

**  It needs a bit of editing


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 suggested that over on the other site decades ago and still use the 
same ancient HP lab supply from a hamfest. At 1000V it doesnt need much.


>
> ##  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 !


**  RF just aggravates potentional problems but sometimes you use what you 
have when not doing it repeatedly.


>
> ##  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.

** Most of the time the internally released gasses will cause  the Hi Pot to 
fail. Some of the early editions of the 3-500 and 572B would fail at ~ 3kV 
and less.
You want at least 4X the DC HV hi pot for AM use, and Eimacs tested at 12 kV 
out of the box.

>
> ##  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


>
>
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