[R-390] Re: IMD

[Drew Papanek]

Phil,

The B&K emission tube tester will only catch gross defects: shorts, gas, 
poor cathode emission.  It will not, as some testers will, test 
transconductance, the ability of the tube to amplify.  Many of the 
transconductance testers only test a pentode such as the 6DC6 in 
triode-connected mode.  The only "tester" which will truly simulate the 
conditions found in an R-390a is an R-390a.  The shorts function of the 
emission tester will allow you to disqualify from further testing those 
tubes which could damage your real tester-the equipment that the tube will 
be used in.

Parasitic oscillation in the 6DC6 can be caused by a defective C227, that 
stud-mounted small can next to the 6DC6.  There also was a production mod 
which added a 68 pf cap right at the tube socket, in parallel with C227 thus 
ensuring good cathode bypassing.  I had trouble with C227 in my '67 EAC 
causing intermittent oscillation as the antenna trimmer knob was turned.  
Not all 6DC6's tried would oscillate.  I also noticed lower gain on the 
lower bands.  Replacing C227 (difficult because of tight space) cured the 
problems.

Drew

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