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From: scst83@csc.liv.ac.uk (Mr. C.D. Smith)
Newsgroups: sci.electronics
Subject: Voltage Multiplier Problem.
Message-ID: <C52HE5.8s4@compsci.liverpool.ac.uk>
Date: 6 Apr 93 14:54:05 GMT
Sender: news@compsci.liverpool.ac.uk (News Eater)
Organization: Computer Science, Liverpool University
Lines: 53
Nntp-Posting-Host: goyt.csc.liv.ac.uk

Hi,

   I've just built a valve preamp and use a diode/capacitor
voltage multiplier to step the 12vAC supply voltage up to approx 
260vDV. As the load resistance increases, the RC constant decreases
and hence the output voltage drops. I need as high an output
voltage as possible. (about 260v).
  The circuit consists of seven voltage multiplier stages
(ie 14 diodes and 14 capacitors) each capacitor develops about 38v
across them, and I take the output from across seven capacitors=260v.
  Each capacitor is 100mfd at 68v.
 If I increase each of the capacitors rating (220-470mfd), that will
increase the RC constant, and hence alleviate some of the problem.
These capacitors are going to be a little expensive as I need 14 of
them, So:

1. What would happen if I connected a 470mfd at 400v capacitor in parallel 
with the output (and hence in parallel with the seven 100mfd capacitors)?
  Will it, as I assume, increase the C in circuit and hence increase
the RC time constant? If it does, and my output voltage becomes more
stable by doing this, then I will have killed two birds with one stone...
as I am experiencing some 50Hz ripple (and hence amp hum), and this new
capacitor will 'smooth' this out.

and
2. Can anyone recomend a suitable value for an inductor (choke) to
be placed in the output line of the power supply to filter out
the 50Hz mains hum. Lowest resistance possible !

  I've not got my preamp with me at the moment because it is TOO MUCH
of a distraction.... (I'm at university and I've got to get my 
dissertation finished in two weeks !!!!!!!) hence I'm mailing not
playing with my solderoing iron (it'll be cheaper too in the long
run... especially if 14 new large capacitors don't work as planned!)

Email please.... Can't always read the net.....

Thanks in advance..


Chris ;-)

  +====================================================================+
  |Name : Mr Chris Smith       |     Twang on that 'ole guitar !       |
  |Addrs: scst83@uk.ac.liv.csc |                                       |
  |Uni  : Liverpool University |Quest: To build more and more hardware |
  |Dgree: Computer Science     |                                       |
  +====================================================================+

 "What ever the sun may be, it is certainly not a ball of flaming gas!"
  -- D.H. Lawrence.

 * All views expressed are my own, and reflect that of private thought. *
