----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@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@jeremy.qozzy.com>
To: "Kimberly Elmore" <cw_de_n5op@sbcglobal.net>, "Catherine James"
<catherine.james@att.net>, "Amps group" <amps@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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