-----Original Message----- >From: david knepper <[EMAIL PROTECTED]> >Sent: Nov 14, 2007 7:19 AM >To: Discussion of AM Radio in the Amateur Service <amradio@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 ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
