Good info Ben, almost identical to what the old time tube companies wrote up
in the 30's when seal technology wasnt the best. I even use the process with
rx tubes such as UX 280's and similar oldies.
One thing that should be added is a HV surge resistor, around 25 Ohms 50W
and a real wirewound; no cement.
That will limit arc current to a safe value and minimize the chance of tube
damage.
Once cooked it is a good idea to hi pot them; I built my setup around a neon
sign xfmr and can variac up to 13KV. Hi potting also helps cull out bad
Chinese 3-500Z's and similar; Ive returned several back to RF Parts.
Thankfully TH/TL Eimacs were built into the 70's and maybe later and the
those havent deterioated much if at all. My NIB 810's are USA JAN Cetrons
with 1990 date codes.
Carl
KM1H
----- Original Message -----
From: "Ben Dover" <[EMAIL PROTECTED]>
To: "Discussion of AM Radio in the Amateur Service"
<[email protected]>; "Discussion of AM Radio in the Amateur Service"
<[email protected]>
Sent: Wednesday, November 14, 2007 9:24 AM
Subject: Re: [AMRadio] Difference and use of Eimac TH and TL tubes
-----Original Message-----
From: david knepper <[EMAIL PROTECTED]>
Sent: Nov 14, 2007 7:19 AM
To: Discussion of AM Radio in the Amateur Service
<[email protected]>
Subject: Re: [AMRadio] Difference and use of Eimac TH and TL tubes
Carl, the problem is that many of these old bottles are gassy.
There are some techniques to rejuvinate them, like reduced plate voltage
to
burn off the gas.
Gassy? Indeed they ARE!!!
I'm a big fan of the 304TL. The vast majority of the tubes out there now
are
military JAN jugs left over from WW2, and you can count on 'em being
gassy.
There ain't no such thing as a perfect vacuum seal, and these bottles have
had over 60 years to leak.
I was rather amused by some of the postings about 304TLs on a "triode
audio"
web group... imagine using this huge old war horse of a tube in class A
to
produce 5 or 10 watts of audio!
One dude who was trying to really get serious with the 304TL warned
everyone
that the MAXIMUM plate voltage used shouldn't exceed 2000 volts, otherwise
the tubes would flash over internally. Not surprising, if you haven't done
a
bit of conditioning before putting 'em into service.
The whole point of conditioning is to get the tube HOT... hot enough to
activate the remains of the getter that sucked up the last of the gas when
the tube was made.
My approach in the past has been to hook up the tube to a filiment
transformer,
bias supply, and a BIG plate supply with a variac on it. Thus set up, I
apply
maybe 500 volts to the plate, and adjust bias for 200 or 300 MADC. Then,
just let
the beast sit and cook for at least a day.
After that... start raising the plate voltage slightly to increase the
plate
dissipation a bit, and let it cook some more... maybe another day, max.
Finally... on the third day, increase plate voltage until the tube is
pulling
300 watts (the tube's rated plate dissipation), or if you're feeling
lucky, go
perhaps 350 - 400 watts and let it sit some more... maybe 6 - 12 hours.
BTW,
when you've reached this point, it's a good idea to deal with the normal,
Eimac
defined tube cooling methods to keep from overheating the plate and grid
seals.
Air thru the hole in the base wouldn't hurt either; the filament seals
need some
cooling too.
After this, you're ready to test.
Change the bias voltage to completely cut off the tube, and keep it cut
off. Now,
start increasing plate voltage SLOWLY, and in steps with a resting period
when you
reach your intended step... maybe 2 or 3 minutes.
Sometimes, as the plate voltage comes up, you'll get a brief, minor flash
inside the
tube. These are usually split second duration... and not a big deal. If
you should
strike a solid, sustained arc in there, cut B+ and try cooking the tube
some more.
If you can get to the rated Eb (3000 volts) without an arc,
Congratulations! I don't
run 'em at that high a plate voltage, so I usually stop in the 2000 - 2500
VDC range.
If you can't get to your desired Eb, even after repeated cooking,
Congratulations! You
now have a pretty glass paperweight/conversation piece for the shack!
<<GRIN>>
Once you have the tube at the plate voltage you want, it's a good idea to
adjust the
bias to bring up plate current in small steps, and let the tube cook for a
while at each
step. I don't find it necessary to take it all the way to max plate
dissipation; I just
go for maybe 1/4 or 1/3 of it, just to get the plate heated up good, and
let it sit for
a half hour or so.
Bear in mind that when the newly zapped tube may or may NOT be good in AM
service; remember
that with modulation the instantaneous plate voltage is gonna go up. The
only way you can
know if the tube's OK with AM is to try using it.
Mr. T., W9LBB
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