[Milsurplus] BC-342N PS Question

[Jim Whartenby]

If all that one had to worry about is the initial charging of the filter capacitor then Mike is correct.  The slow warm up of the rectifier allows the filter caps to be slowly (in a few AC cycles) brought up to working voltage.  Of course if you do this first then turn off the set for a few seconds and then turn it back on again, you lose that benefit because the rectifier cathode is still quite warm but the filter capacitor is fully discharged.

The less realized issue is the cycle to cycle current drawn from the power transformer that charges the filter capacitors back up to the peak voltage level.  This is the maximum peak plate current and it should not be exceeded for reliable rectifier operation.  If one increases the value of the filter capacitors you also increase this peak charging current.

If you look at an older tube manual, one that has any real information on the 5W4, you will note that the recommended circuit and all of the test data graphs used a 4uF filter capacitor!

If capacitor values approaching 20uF are used than one has to increase the impedance seen by the rectifier by using current limiting resistors in series with the transformer high voltage secondary winding.  

All of this is explained in great detail in the Radiotron Designers Handbook, 4th edition on pages 99 and 100.  For more information on rectifier circuits, go to chapter 30 starting on page 1161.
Regards,
Jim

--- On Thu, 8/27/09, Mike Hanz <aaf-radio-1 at aafradio.org> wrote:

> From: Mike Hanz <aaf-radio-1 at aafradio.org>
> Subject: Re: [Milsurplus] BC-342N PS Question
> To: "Mark K3MSB" <mark.k3msb at gmail.com>
> Cc: "List Milsurplus" <milsurplus at mailman.qth.net>
> Date: Thursday, August 27, 2009, 6:34 PM
> As long as you are using the original
> rectifier tube, you have a 
> built-in inrush limiter, Mark.  20uF is well within
> the tolerances for 
> such caps and should be no problem. 
> 
> 73,
> Mike  KC4TOS
> 
> Mark K3MSB wrote:
> > I need to replace the dual 8 uF caps in the power
> supply.  I read (can't
> > remember where now) that if hum still exists with the
> new 8 uF caps,  use 16
> > uF each.  That's been shown to clear it
> up.   So. of 16 uF caps aren't a
> > problem...
> >
> > The parts box is lacking 8 or 16 uF caps,  but
> has a good stash of 20uF
> > caps.   Do you think there would be any
> problems using them?    I'm a bit
> > concerned about increased inrush current,  but
> figured I could add a
> > themister after the bleeder resistor.
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