[Amps] Matching GU-74Bs

[Carl]

Matching parallel tubes in an amp stage is nothing new. Many tubes have 
sufficient production tolerance control that matching is not needed. Others 
require it. Examples are sweep tubes and the 8122 as used by Hallicrafters 
and National. These were all static matching where the same grid bias 
resulted in the same plate idle current. Not a perfect procedure but close 
enough.

I modified the HF and 6M NCL-2000's Ive had since 64-65 for individual bias 
adjust back then. A delta of around 5V is tolerable before IMD starts to 
suffer from the lower value biased tube being easier to drive. With static 
matching and a single adjustment a pair within 1V was considered matched.

Ive deliberately kept far away from Russian tubes so cant say what their QC 
is like. However if Alpha is making such a fuss over it Id suspect that they 
vary considerably.

Carl
KM1H