[R-390] R390 B+ Expected Tolerance

[Jacques Fortin]

Hi Barry,

Er... not exactly.
First, 2.7mA in 1000 ohms gives 2.7V across R625, and ~ 7.3V across R626.
The 2.7mA value came from (180V - (2 x 85V) = 10V and 10V/3700ohms =
2.7027mA.
ASSUMING that all the voltages and resistance values are exact.
The two 5651 passes the same current value, if we neglect the contribution
of R627 (85µA).
BTW, R627 is there to force V609 to strike first, then V608 will fire.
Otherwise, the output can raise up to 230V before the two VR tubes can
strike, because their maximum striking voltage is 115V...

TOLERANCES:
A "normal" 5651 should operate between 1.5 and 3.5mA going thru, with a
nominal voltage across of 87V, but the regulation spec is 3V from 1.5 to
3.5mA.
Other sources (see attached) states that the voltage across can be between
82 and 90V... (argh !)
The 85V I used in the previous calculations came from what I measured with
many similar VR tubes that I previously tested, especially European 85A2
variants.
To mix the situation even more, the 5651A is spec'd at 85.5V nominal... (GE
Essential Characteristics manual).

At the end of the day, if you find a pair of 5651s (or any other variant)
that give you a regulator output very close to the 180V target value, keep
them there !
That "selection" of VR tubes will then override any other parts tolerances
in the whole circuit.
And yes, having a regulator output at any value between 170 and 190V can be
"acceptable", as long as the regulation and output noise holds up.

73, Jacques, VE2JFE in Montreal