[Test-Equipment] B&K 747 headscratcher

[Avery Comarow]

I recently sold my 747, which worked fine for me, and the new buyer is 
having problems. I'll post his description of them below with my reply, and 
would really appreciate any advice I can pass along to him. Thanks!

73, Avery W3AVE in Potomac, Md.
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[buyer's description of problems]
I got some wacky readings so I was going to try to calibrate it and test 
some tubes again before deciding whether it got damaged or not.  (Have you 
tried to calibrate it?  Is it easy to do?)  Maybe you can help me figure it 
out.  Have you ever run into anything like the following?  I tested 12+ 
Sovtek 6922's from my Power 3 amps and they tested funny--on both Test 1 
and Test 2, the reading would start out in the good range (80-90), then 
rise about 5-10 points after 5 seconds or so, then it would slowly descend 
until bottoming out after 20 seconds or so in the 40-50 range.  The tubes 
are about 2+ years old so I presumed they should be good since they 
supposedly have a 5,000-10,000 hour life.  (I tested 2 almost new tubes of 
the same type and they did the same thing, although at slightly higher 
readings top and bottom.)   I also tested my Svetlana 6550C power tubes, 
which are also about 2+ years old.  I presumed that the 6550's would test 
worse because they have a shorter life span (2-3,000 hour life 
expectancy).  But, all 16 of them scored off the meter at 120+.  I'm 
completely baffled.  Do you have any idea whether that sounds like it may 
have a worn out part, needs a calibration, or got its brains jostled during 
shipping?  Any thoughts?  I may call the B&K repair site and ask them what 
they think.

[my reply]
Calibration is a snap if you have a digital voltmeter, or even an analog 
one. It takes about half an hour, max. I did it about a year ago. I don't 
think calibration is causing the strange readings, and I'm equally baffled. 
I would have expected the opposite--that the 6922's might test high and the 
6550's would drop (because their higher filament current would load down 
the transformer, lowering the filament voltage by 10 or 15 percent--almost 
as if you were doing a life test). If you have a known good 5U4 or 5R4, 
test one and see what happens--those rectifiers also have very high 
filament current.