Preparing Transmitter Power Tubes for Use after Storage
Still many of us are using tube power amplifiers and rely on NOS or "pull" tubes that have often been manufactured many years ago and stored since. With a bit of preparation that is not a problem, but unfortunately this is not always done! What to do to avoid losing perfectly good tubes in a flash is described below and in the separate compiled PDF-document. It is a recommended read before using such valuable tubes.
Two main areas need attention:
Especially tungsten filaments in directly heated tubes (3-500Z, 4-125A etc.) change their structure and become brittle. Application of full power leads to destruction of the filament. Slow ramping up of heater voltage and limiting the current is required to reform them. This is best done using a regulated power supply. Filaments should then be run at nominal value for e.g. 30 minutes. For ceramic tubes (4CX250B/F etc.) this might be all that is needed. Refer to the PDF-document for details.
Important: remember that many tubes need cooling body and pins with an air-flow even with only filament power applied, so a fan is needed to avoid e.g. damaging the pin seals. Note that it is also useful to reform the filaments before shipping to make them more shock-resistant.
This is not about air-leaks through pin seals, but deterioration due to diffusion of very small impurities in the metals and ceramics used. The vacuum in the tubes is not what it should be and they will thus not be able to stand the high plate voltages applied. Luckily this can usually be corrected, how it is done is described in the compiled PDF-document that also adds more background to these matters.
The PDF-document here is compiled from previously published work by SM5BSZ, SM6EYH, ON4ADN, ND2X, UR4LL, YO9FZS, G3SEK covering this subject. It is hoped that it helps to keep those tubes alive and the power amplifiers running!