[Hallicrafters] 8122's and the SR-2000

[Jim Liles]

There is a huge group of 8122's for sale on ebay.  The test that the seller 
has performed and described means absolutely and patently nothing except 
that there are probably no shorts.  The only emissions test for an 8122 
based on the vast differences in geometry and bias charistics is at  0 volts 
applied to G1.  A momentary test at 0 volts G1 will result in 800ma for a 
good tube.  Burle color coded the 8122 to define the differences in the bias 
charistics.  Red tip 8.0 to 10.9 --- Blue 11.0 to 13.9 --- Green 14.0 to 
16.9 ---- Black 17.0 to 19.0.  The Red and Blue will work is the SR-2000 
without mods.  The Green needs Bias and neutralizing mods.  And the Black is 
a problem in that it reaches full conduction before G1 reaches 0 volts 
partially disabling the ALC detector.  The ALC system on the SR-2000 needs 
to see G1 current fluctuations which occur long after saturation with the 
Black tip 8122.  Also, the Black tip needs an incredible amount of negative 
feed back to neutralize when used with high impedance drivers.  Kindest 
regards Jim K9AXN