[Johnson] Matched tube sets for modulators???

[Richard Peterson]

My experience with matched tubes is they don't stay matched forever. I 
actually modified a Dynaco 70 stereo amp once with separate bias pots for 
each tube (the original had one pot for the left channel, and one pot for 
the right channel, and each channel had two tubes). I could balance 
everything beautifully. There were several mils of different resting current 
between tubes before I did that, so I presumed the tubes were at slightly 
different spots on the curve.

Could I tell a difference listening? No. Not at all. Zip. Zero. Nada. But I 
did not have a distortion analyzer.

In time, tubes seem to wander away from their settings. And something tells 
me a balanced pair would do the same thing.

I would not worry about it much. If you really do worry, consider separate 
bias pots, if you don't mind a modification. But for moduatlion service, I 
don't think it is worth the trouble. (I presume the rig does not use self 
bias.)

Richard
WB5NEN