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Neither box is perfect. The 6200B is wimpy in the collector\plate current above 200 V (100 mA). The 575\122C can deliver 500 mA up to 400V, but has no higher voltage range (for tubes or transistors). That definitely limits the 575\122 when trying to characterize power tubes.
The 6200B's base step generator can deliver +- 35V max, enough for most tube control grids (but not all). The 575\122 can only deliver +- 10 or so; too low. But then the 575 is much easier modified with a tube specific adapter. Behind the normal test fixture is a big hole with easy access to the chassis internals. That makes picking off a heater supply and +300 V for a screen grid regulator possible, as well as adding a base step amplifier inside the fixture.
The 575\122 C has nice base control features such as OFF/SINGLE/REPETITIVE. The single is for high power dissipation situations. The 6200B has a base pulse mode where the base step is only on for ~200 uS at the peak collector voltage. That is Fairchild's solution to the high power dissipation situation. Fairchild's method doesn't work too well because the CRT beam isn't blanked and the trace moves all over
the screen during the switching period of the base pulser. This feature may not be working properly on mine, or as I believe, it was poorly designed.
One important detail for the advanced user is that the 575 is much more versatile in what it can display. The vertical can display base voltage or current, as well as the normal collector current. The horizontal can display base voltage or current as well as the normal collector voltage. Because of this feature all parameters such as forward and reverse transconductances and input/output resistances can be directly displayed. This is more than the Fairchild can do.
The verdict is not yet in on which is more suitable for tube testing, or which is the better box. I will have to wait and get the 575 running reasonably well.
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