Testing 5963 tubes on Amplitrex AT1000

Yesterday I received a batch of tubes from France. It was a box with 5963 tubes, a special quality ECC82 or 12AU7 or 5814. These tubes 'made in USA' NOS (stated by seller), were used in the 1960's as computerparts, longlife 10.000 hours usually made for Hewlett Packard. These tubes are fairly unknown to the broader tube community as a very good replacement of types as ECC82 or 12AU7.

However, I started testing the tubes - as I Always do when receiving new tubes- on my Amplitrex AT1000. I used the standard 'fixed bias' on the Amplitrex. The results were not very good. 13 tubes very 'dead'. Hmmm. Need to contact the seller on Ebay for a refund…

But before I was going to report back to seller, I first did check all tubes again on the Amplitrex, but this time I was using the 'autobias' and manual test time. Why ? I read somewhere, the 5963 tubes have a very thick cathode, but are also build to withstand a total cutoff, something 'normal' tubes would destroy.

Measuring the tubes in this way I got the results I expected to get: balanced triodes around 11.0 mA. Yes! Now this batch became one of the best test performing batch of tubes I had the pleasure of testing!

Checking further on this tubes, they seems to be Sylvania's Goldbrand with 2 mica, 17 mm long grey plates, with Sylvania logo (so no modern Philips Sylvania issues) with a round getter.