[R-390] R-390 and 6082 Regulator Tubes

[Drew P.]

Bernie Nicholson wrote:

(snipped)

"regarding the R390 and 6082
tubes , I repaired one here the other day, for a Friend , and 6082 s are
very hard to find in this country, but 6080 s are very cheap , Being 6v
filaments ........I just reconnected the filaments , in series and put a
power diode in series  off the 24V "

Problem is, this modification runs the 6080 heaters at excessive power. Consider it this way: Assuming that the 6080 heaters are a constant resistance load, operating a pair in series straight from a 24 v supply with no diode would be double the rated voltage, and thus 4 times the rated power. Insert the diode, and the power is then cut in half.  One half times four times is twice the rated power. 

In actuality, tube heaters are not a constant resistance, and so the actual heater power would not be quite double, but the power would still be excessive. 

Instead of just a diode, a diode and dropping resistor could be inserted, but the R-390's power transformer would be working a bit harder. Another solution would be to cut the heater voltage with an added  transformer. The added transformer would have a 24V center tapped secondary winding which would be used as an autotransformer, leaving the primary unconnected.

Drew