A little more detail
This is not a commercial valve but a product made to specification by Telefunken exclusively for two companies
Agilent Technology and Hewlett-Packard
it was used in avionics and in the electromedical sector.
It met a series of qualitative requirements as restrictive as never before had been required.
In addition to the absolute constancy of production, to the best materials available at the time and to the most qualified workers, the two companies required absolute not microphone sensitivity and RFI / EMI shielding characteristics.
In particular, they demanded impeccable constancy of operation with controlled emission over time and the least possible noise generated.
A special treatment of the glass was therefore developed by the German manufacturer consisting of a black compound containing damping and shielding materials, applied at the end of the production process which covers all the exposed part of the valve with the exception of the space inside the fixing terminals, therefore, the exclusive <> symbol that distinguishes all Telefunken production is visible
The production was carried out in batches of 12 identical pieces electrically together. We found two of these packs.
In fact, this exclusive production is superior as parameters to the ECC802S from which it derives.
It is practically impossible to find new specimens, those that have appeared on the market over the years bear the manually written initials and no longer have the original label applied on the glass. Normally they come from old dismantled aircraft so it is difficult to determine the real state of wear.
It is true that these are tubes that could work for 40,000 hours so even if you use the measurement parameters they will always be excellent
We have 12 perfect pairs at the moment, never used, placed on a base only for our test.
An expensive but unique opportunity to get the best ECC82 ever.
Best,
-JP