Tremaine's Tube Cooking

[Jonte Knif]

From Audio Cyclopedia:

"What is the purprose of pre-heating vacuum tubes?

-It is the custom in most plants empolying a large number of vacuum tubes in critical places such as recording and reproducing equipment, to preheat tubes and cook them for at least 100 hours before putting them into service.

This is accomplished by connecting the plate, screen grid, suppressor grid, and control grid to one side of the filament or heater circuit. The heater circuit is supplied from a source of voltage regulated to within 1 percent of the rated heater voltage. No plate voltage is used during the cooking period.

After removal from the preheating process, the tubes are individually tested for internal shorts, noise and leakage, then selected in pairs for devices such as compressors, noise-reduction amplifier, and other equipment requiring matched pairs of tubes.

It has been found from experience when equipment is operated 24 hours a day, tube life is prolongened, internal noise reduced, and the general characteristics are stabilized."

In general it is obvious that burn in makes sense. But could somebody explain the details.

-Why no plate voltage?
-Why the cathode is not connected to anything?
-Is the 1% tolerance for heater voltage really necessary?

I'm about to build this kind of burn in jig for ca. 20 tubes and will perhaps include the possibility to apply anode current too. But should I if Tremaine's method is adequate or even better in some mysterious way?

-Jonte

 

[tv]

No voltage is perhaps to avoid any cathode stripping?

 

[burdij]

I believe the method cited is not very accurate. Most burn-in operations I have seen or worked with put the tube in a circuit with appropriate voltages applied. The accuracy of the filament regulation is a bogus requirement as long as you don't exceed the operating limits. One of the basic things that burn-in does is activate the gettering processes built into the tube and helps to sequester any remaining gas molecules. Also the process stabilizes the emission coating on the surface of the cathode.

Here is a burn-in rack at the new Western Electric. Note the lightbulbs used as plate loads: