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

jeremy-ca km1h at jeremy.mv.com
Wed Nov 14 10:39:03 EST 2007

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 

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.


----- Original Message ----- 
From: "Ben Dover" <quixote2 at ix.netcom.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>; "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
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 <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 
>>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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