[Hallicrafters] The SR-2000 and sellers of "MATCHED" 8122's

[james.liles]

If you are an SR-2000 user buying matched 8122's for use in the SR-2000, please be aware of the following caveats regarding the 8122.

First, the Hallicrafters SR-2000 finals are run in the AB1 configuration with high impedance drive. 

Second, the Hallicrafters engineers lamented that they had to buy a bushel of 8122's to find a half dozen pairs that were matched AND would work in the SR-2000.

Why wouldn't any matched pair work in the SR-2000??  There are vast variations in the parameters from tube to tube and between matched pairs.  The  Burle 8122's were color coded Red, Blue, Green, and Black to distinguish the tube parameters which would help to predict the idle current bias as well as the ability to NEUTRALIZE the tubes.  The Burle tubes are the only 8122's that are color coded this way.  The Red and Blue Burle tubes will work in the SR-2000 without modifying the transceiver or power supply.  The Green Burle tubes can be made to work with modifications to the transceiver bias configuration and neutralizing bridge as well as a change to the P-2000 bias configuration.  The Black version cannot be made to work well in the SR-2000.  Requires too much bias to idle and cannot be neutralized even with mods.

Why should this affect neutralization?  I  believe the reason to be the precision that is required to accommodate the close spaced elements.  It would appear that the control grid which should be in the shadow of the screen to plate is a bit off causing the idle bias and feed back to vary wildly.  

I would guess that the Black version will work in the NCL-2000 because it's an AB2* amplifier with a low impedance input and wider bias control.  

The 8122 can be tested and profiled to predict matching, neutralizing, and output parameters to determine the suitability of a pair for use in the SR-2000.  

Kindest regards Jim K9AXN