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I have a Drake transceiver. I want to replace the valves in the final output stage. The valve in question is a 6jb6.

I was told by elmers that the valves need to be preheated (cooked) before replacement.

What is the logic behind this, and is it essential?

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  • $\begingroup$ What is "preheating" in this context? $\endgroup$ – jippie Oct 4 '15 at 7:59
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    $\begingroup$ Every valve circuit has a heater, and if it has a valve rectifier it's going to get preheating anyway for 30-40 seconds until the rectifier warms up. The fact is that vacuum tubes are an astonishingly reliable technology given their nature. It's not uncommon to find 60-70 year old valves working perfectly despite many decades of God knows what abuse. They're even rather hard to break if you drop them. There are plenty of stories and videos around of guitar amplifiers being thrown down stairs and then working perfectly. They also haven't told you how to actually do it. Sounds like drivel. $\endgroup$ – EJP Oct 4 '15 at 9:02
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    $\begingroup$ @EJP Yes! If a valve amp or radio is faulty, suspect the capacitors. Then the switches and pot wipers. THEN the valves... It may be that "elmers" meant some form of "reactivation" : running heater current above normal to de-gas the valve after years of storage. But I thought this didn't apply to "new" valves... pa0fri.home.xs4all.nl/Lineairs/Tubes%20reconditioning/… $\endgroup$ – Brian Drummond Oct 4 '15 at 11:50
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The process is called degassing. This is done to prevent arcing in high voltage transmitting (power amplifier) tubes. It is not as important for lower voltage tubes but I am sure if done correctly no harm should result.

To degas you preheat the tube when it has been inactive or in storage for a long period of time. Residue of gas is removed or absorbed by getters (elements such as zirconium placed in the tube to absorb gasses). As a result of degassing, the cathode regains its optimal emission. If that is omitted, there is a greater risk of arcing (flashover) between anode and other internal connections. Preheating can be done using the tube's heater filaments for a period of time prior to applying high voltage to the anode.

There are a number of resources on the Internet that describe the methods and why it needs to be done. I myself have actually never done it. See W8JI's web site: www.w8ji.com.

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