Mullard Blackburn tubes, marked Made in Great Britain
red printing for Revere/Wollensak (3M brands in the Sixties)
noted for warm tone in hi-fi preamp sections in some audiophile circles
The tubes tested strong on the Jet-Check section of the B&K 707 tester (#1: 103 and 100; #2: 99 and 105, where 66+ rates as "good" - there is no life test applicable to tubes using the Jet-Check section as there is for more conventional tubes tested by the 707).
Tubes are tested on a B&K 707 mutual conductance tube tester recently serviced and calibrated by Mike Mabry in Woodland, WA. Mike has been working on this series of testers for many years.
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