[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 
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" <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 
>>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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