In the world of diminishing resources.
Especially in the area of Vintage Tubes and where certain ones from this era, are desirable and have a rarity.
I don't see many revealing their pearls to show how to select the best offerings as they appear. A knowledge of the desirability of the Tube is usually enough.
The KR VT 231 Black Glass or not is a good example from this thread, the tube is revealed, the concerns about quality are shared.
The rest is Caveat Emptor for a inexperienced or not well connected buyer.
I am such a individual with limited experience. I am fortunate to have a friend who is second generation Tube Collector and what I will refer to as a Specialist.
All Tubes Purchased are pre agreed as part of a sale to be tested by this friend.
All vendors have to agree to accepting the tube returned and monies refunded, if the tube is testing quite different to the seller supplied Test results when Tested at my end.
Fortunately this is a method that has worked out on quite a few occasions, with the odd anomaly and refund request made.
It does take a little time to assemble a selection for tube rolling purposes.
Fortunately on occasion, a selection of Tubes have been loaned by this friend, for the purpose of Tube Rolling and selecting a permutation that has proven to be quite attractive. My Two Phonostages and DAC have all been subjected to evaluation through the use of loaned Tubes.
The friend has demonstrated Tubes in use with different usage life left in them, a Tube with a reading with approx' 30% down on the Manufacturers Optimum Reading can sound quite nice, it is only when an Optimum Reading Tube is used as the comparison, does the tube get separated for not having the qualities that really show out.
In my recollection of an assessment of two same Branded/Model Tubes, a Cold Optimum Measured Tube stands out with immediacy as an attractor, much more than a Heated Tube with a 30% reduction in usage life.
This was also experienced with a Pair of ECC81's on my phonostage, I was loaned as a stop gap, two tubes from the Brand I was interested in, that according to the friend who owned them, were almost at the end of their recommended usage.
When I finally was in possession of Two Tubes very closely matched and at the manufacturers optimum measurement, the exchange of Tubes, was to say the least, Jaw Dropping, music that was present, took on a whole new voluminous presence, along with the Performers becoming quite distinguished and very much in the room.
All tubes to a certain point of degradation will produce a Sound, it is the remaining usage life within the Tube, that will allow for the Tube to be an attractor and impressive, the nearer optimum measure, is in my view, the place that can really show where the desirable qualities are to be found.
