[Hallicrafters] 0A2 question

[David C. Hallam]

Given the propensity for failure of the OEM HT-32/37 power transformers,
anything you can do to help them along is a good idea.  Replacing both of
the rectifiers with SS devices is a good idea.  In addition the 5 V filament
leads should be disconnected at the tube sockets, as a rectifier to high
voltage windings short is the usual mode of failure.

The low voltage rectifier should have a series resistance so that the
voltage remains the same as with the vacuum tube rectifier.  I have no
connection with Weber Copper Top replacement rectifiers but they make
replacements that plug in and have the resistance built in so that the
voltage drop is identical to the tube they replace.  They work well.

The 6146's can take the increased voltage of a SS replacement for the 5R4
without any problem.  The filter caps should not be a problem either.  An
increase in the anode voltage of a tetrode will have little effect on its
anode current.

Any stress placed upon other components by the rapid start of the SS devices
is secondary to the problems with failure of the transformer.

Therefore, unless you have a power transformer that has been rewound with
quality wire and insulation, I would recommend replacing the vacuum tube
rectifiers with SS devices with the caveat of remembering to control the
increase of voltage of the low voltage line.

David C. Hallam
KC2JD

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of wb8jkr at juno.com
Sent: Sunday, November 28, 2004 6:27 AM
To: Hallicrafters at mailman.qth.net
Subject: [Hallicrafters] 0A2 question


  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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