Meck XB-702 restoration

[bandersen]

This set is really trying my patience. I was thinking that maybe the filament voltage wasn't dividing equally among the tubes and did some testing.
The result was low readings on all the 12SN7s (10 volts) and the 25L6 (20 volts) so I went over the schematic. It makes no sense!

If you add up the string with the 50 ohm dropper, you get 119.5. That's without the 50 ohm resistor! There is no reason for it to be there unless they intentionally want all the tubes to run lean.

The Teletone version does not have this resistor.

The other string adds up to 75.4 which should have a 139 ohm resistor to get 117. 150 if close enough.

Why in the world would they want to starve one of the filament strings?!?

[Kevin Kuehn]

With the dropper resistors in series it stands to reason that the higher filament voltage tubes will be starved of proportionally more voltage than the lower voltage ones. That being said ohms law would indicate you likely have closer to 250 ma going trough the 12SN7's, 25L6 string. But yes, I doubt they are that critical.