[AMRadio] Difference and use of Eimac TH and TL tubes

Ben Dover quixote2 at ix.netcom.com
Wed Nov 14 09:24:32 EST 2007

-----Original Message-----
>From: david knepper <collinsradio at comcast.net>
>Sent: Nov 14, 2007 7:19 AM
>To: Discussion of AM Radio in the Amateur Service <amradio at mailman.qth.net>
>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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