[Hallicrafters] 0A2 question

[Roger D Johnson]

wb8jkr at juno.com wrote:
>   I remember about 30 years ago, I replaced the 5U4
> LV rectifier tube in my HT-37 with a homemade
> SS replacement consisting of  4-1KV 2.5 amp diodes
> (HEP-170) and a series 10 watt 200 ohm resistor. This
> combination should have been the equivalent of the 5U4
> tube however, I remember when I turned the transmitter
> on the 0A2 would flash over for a very brief amount of time.
>  I stopped using the solid state replacement and went back to
> the 5U4, never trying it again.
>  My question is, does anyone know if the 0A2 has a certain
> finite amount of time it requires to ionize, and until the
> gas in the tube actually ionizes maybe the voltage input to the
> regulator soars too high and caused the flash over? 
>  I'm thinking of trying the same thing again, but thought maybe
> because of the above problem it would be wiser to use a
> string of three 50 volt 5 watt zeners in place of the 0A2, thinking
> the zeners would conduct faster than the 0A2.
> 
>  Any thoughts?
> 
> 73,  Mark  WB8JKR
> 
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Hi Mark...

I just looked at the HT-37 schematic. The 0A2 is feed from the 300
volt line through 4k of resistance. This means that the 0A2 draws
37.5 ma before the tubes warm up and start siphoning off some of
the current. The max current for the miniature VR tubes is 30 ma!
If the 300 volt line is higher, due to line voltage or replacing
the rectifier with solid state, the problem is worse. The series
resistance should, ideally, be sized to limit the current to 30 ma
under worst case conditions, e.g. when the rig is first turned on
and none of the tubes are drawing current. Unfortunately, this often
means that the connected tubes draw too much current and cause the
circuit to lose regulation. The VR tubes are designed to draw from
5 to 30 ma in operation. This means that the connected tubes can
only draw 25 ma in a properly designed circuit. Too often the tubes
draw more and the mfr reduces the size of the series resistor to
compensate. This means that the VR tube is overloaded at startup.
Unfortunately, paralleling VR tubes to increase current capacity
is not usually feasible due to variations in starting voltage. The
tube that ionizes first drops the voltage and the other tube never
fires. Hope this helps.

73, Roger


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