Blackened GZ32 glass envelopes

[Klimon]

Just curious; I just came accross this explanation for the blackening of a tubes glass envelope: http://www.r-type.org/exhib/aab0024.htm, something I experienced with Mullard GZ32 in very soft conditions (270V @ 60mA), dual mono and both tubes turned dark without further consequences. Any explanation for this? -- the link doesn't provide much info on rectifier blackening. fwiw sovtek nos gz32 stays perfectly translucent in the same circuit

Simon

[ulibub]

Hi Simon!

The blackening in some tube bulbs is the result of minute amounts of metal evaporating from the hot electrode system (cathode, heater, heater insulation) and condensing on the cool glass bulb. On longtime used tubes (each type, not only rectifiers), this can even form a really metallic shining coating. I've also found that this depends upon the manufacturer of the tubes. I found for instance that Tungsram tubes often get very much obscured and even "metallised" on their bulbs, whilst brands like JJ don't show any darkening on the same types even after long term usage. I have an EZ90 from Tungsram in my preamp which is almost completely dark/metallised after > 10,000 hrs of use, but which still works perfectly. I can tell the same thing from ECC and EF - types.

I think this results from small impurities of (comparatively) volatile metals (copper....) in the electrodes - sometimes also some traces of free aluminium in the Al2O3 heater insulation or some Barium evaporation from the cathode oxide surface.

I don't think the explanation of the darkening by "sputtering" (which is a well-defined technical term - deposition of metal ablated by arcing/gas discharge) in your link to be substantive. Such a process would definitely damage the tubes almost instantly - which cannot be the case in longtime used, darkened but still perfect working tubes.

In my experience nothing to really worry about!

Best regards

Uli

[Klimon]

Hello Uli,

That makes alot of sense! In restricting the phenomenon to the tubes itself it's a comfort to know that I'm not abusing the Mullards. I also noticed the silver spots (near plate-hole) on many Tungrams, this seems to prove your point.

Regards,

Simon