[Hallicrafters] 0A2 question

[wb8jkr at juno.com]

  Roger,

  The point I was trying to make is that until the
gas in the 0A2 ionizes the tube doesn't conduct
at all and draws NO current, during the time between
you flip the power on switch and the time the 0A2
"fires" the applied voltage soars, then when the tube actually
fires the applied voltage is high enough to allow excessive
current to be drawn by the 0A2, once the 0A2 fires and
stabilizes everything should return to normal.
 My question was, would a zener conduct faster,
or in other words clamp the applied voltage faster
than the 0A2.
 The reason I'm asking is because I want to get the B+
off the 5 volt winding due to the poor insulation and
resultant failure known to the transformers in the
HT-32/37. At the very least I guess I could do the
solid state thing to the high B+  (5R4) supply and
leave the low B+ (5U4) alone.
 I recently replaced the transformer in a HT-37, and
upon autopsy found that the insulation between the
HV winding and the 5 volt winding on these 
transformers to be VERY poor, and the unit did
in fact fail with a HV winding to 5 winding short.
 Also, the 0A2 CAN be used in circuits that draw
more than 25 ma., the 0A2 simply can't be used in a circuit
whereas the total current variation exceeds 25 ma.
 For example if the cicuit draws a total of 150 ma, but
the minimum current drawn never falls below 125 ma. or
rises to more than 150 ma. the 0A2 can be used.


Mark  WB8JKR


On Sun, 28 Nov 2004 10:31:25 -0500 Roger D Johnson <n1rj at adelphia.net>
writes:
> 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

 
> 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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> http://ussliberty.org/
>