[Hammarlund] HQ-170A VHF VR tube??

[Roger D Johnson]

The old VR tube only regulates over a narrow range of currents. For
the miniature VR tubes the range is 5 to 30 ma. To calculate the value
of the dropping resistor, subtract the VR voltage from the B+ and
divide by .03 (30 ma). Using 250 volts and an 0B2 as an example, the 
resistor works out to be 4833 ohms! This is the value that allows
the maximum allowable current through the VR tube in the absence of
any load. As the load current increases, the current through the VR
tube decreases. Since the minimum current for the tube is 5 ma, only
25 ma of load current is permitted before the regulation starts to get
flakey.

My HQ-170 manual shows that the dropping resistor is 3k ohms. With a
B+ of 250 volts, this allows 48 ma to flow through the VR tube until
the other tubes on the 105 volt line warm up and start drawing current.
This is clearly way above the ratings of the VR tube and is probably
made even worse at startup by high line voltage and the fact that the B+ 
is probably above 250 volts until the cold tubes start drawing current.

MY HQ-180A manual shows a 2k dropping resistor and a 0A2 regulator tube.
This allows 50 ma through the regulator tube at a cold start! In the A
series, the situation is moderated somewhat due to the oscillator and
1st mixer tubes being on a separate filament transformer and energized
anytime the receiver is plugged in. This allows those tubes to draw
current immediately and reduces the current through the VR tube 
accordingly.

I must confess that I don't know if, or how much, currents above the VR
tube ratings will damage the tube. It does appear that Hammarlund sized
these resistors too low. One would have to measure tube current while
varying line voltage and switching loads on and off to determine the
proper value.

73, Roger




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