A couple of weeks ago I bought a box of 200 6N3P tubes from 'Reflector' made 1992 in Russia NOS. As always, buying a large box of tubes I was a little bit anxious about the result.
Experience tells me, buying tubes in Russia is not without risk. Sometimes you buy tubes and the are DOA or are measuring bad, for from NOS. So it's a gamble buying this tubes.
Yesterday I started testing these tubes.... The first 10 where OK... but then it happend.... 4 DOA (dead on arrival). After measuring and testing 40 tubes on the Amplitrex AT1000 10% was dead. Hmmm. Need to check with the guy who sold the tubes.....
But also, within 40 tubes I found 4 matched pairs 1% both on gain and balanced triodes! So this is positive and can only be achieved when buying large quantities of the same tube and matching them on the Amplitrex AT1000.
Also I saw the measurements I found had a wide range: gain from 2,0 to 6,0 mA and GM a range between 3,0 and 5,60 mA/V.
Can't wait to test the next batch of 40 tubes...
When testing such a large quantity of tubes their are Always tubes who above the median average of the group of tubes. I saw a tube measuring 6,0/2,6 mA. 6,0 mA means 130+ % of NOS. That seems OK, but 2,6 mA on the other hand in relation to 6,0 mA doesn't. So this is an example of a tube that isn't perfectly suited to sell. It can't be matched and the difference between triode 1 and 2 is far to big.
So it's a guess how many tubes will remain after testing 200! My guess will be in the range of 152. So a total rejection rate of about 25%. I'll keep you posted!
Update July 10th 2018
- 15 pairs of matched and balanced pairs @ 1%
- Many 1% matches on the first triode
- 9 DOA's (9% of total)
- 80+% scores above 2,8 mA
So that's not a bad result for the first 100 tubes. My assumption of 25% DOA doesn't seem to get true. Also I see much better 'matches' overall on this batch of tubes then offered for example on Ebay. Most of the matches are at least 'gold', 'platinum' or 'platinum+'. One of the advantages of buying large batches of tubes. Let's see what the next 100 tubes will bring!