[JMS] Millen 90800/90831 power supply and 90881 amp

Thu, 19 Feb 2004 09:49:23 -0700

Cal,

If you use a bridge rectifier on your power transformer you will get =
twice the voltage output but the rated continuous current will drop to =
half of the 250 ma value, so be a little careful about how warm the =
transformer gets.  You can pull only so many watts out of the =
transformer.  I would also suggest you consider making your filter a =
choke-input design with the capacitors on the output side.  That will =
limit your high voltage output to about 0.9 times the transformer RMS =
voltage less the drop across the choke provided the choke inductance is =
above the "critical" value, L(henries) =3D Load Resistance (ohms)/1000.  =
A choke-input filter will also improve the regulation of the power =
supply.

Jim Hanlon, W8KGI
  ----- Original Message -----=20
  From: [email protected]<mailto:[email protected]>=20
  To: =
[email protected]<mailto:james_millen_society@mailman.=
qth.net>=20
  Sent: Wednesday, February 18, 2004 2:33 PM
  Subject: [JMS] Millen 90800/90831 power supply and 90881 amp

  All:=20
  I am gathering the parts for the 90800/90831 combo to use with the =
90881 amp. First off, I was concerned about power supply design =
concerns. I used to think that to make a "good filter", one need only =
slap on two appropriately voltaged caps and a choke. Taint so! I found =
out that it is important that in order to have good regulation involved, =
the choke must be appropriately loaded, especially when operating CW =
(since the load is not always constant at that point). Well, with the =
600 VCT 250 mA xfmr I had on hand, I was going to full wave rectify the =
unit. That meant that the total voltage out should be 300 VAC times =
1.414. Turns out I would have 420 VDC after the two diodes. That meant =
that with a 12 H choke I had around, the minimal amount of current for =
the choke would be 35 mA (or 420V/12H). With the choke, however, I may =
be in for a slight voltage drop but regulation should be fine with the =
p.s. slightly loaded. Now with the xfmr rated at 250 mA and the choke =
rated 240 mA, that means now I have a headroom of 205 mA. In any event, =
the transmitter and modulator combo needs at least 400 VDC and 200 VDC =
for the plates and the screens. I think the supply will handle both =
units quite adequately, including the filament output of the xfmr (which =
is 6.3 VAC at 6A but I won't be using all of that).=20

  I'm also thinking about running another filament transformer =
"bass-ackwards" from the 5.0 VCT tap so I can have a bias supply for =
both the transmitter and the amp. They need -45 VDC and -48 VDC =
respectively (with the amp at class B p-p operation). However, I'll =
probably switch the bias over to something higher than -48 VDC for the =
amp during STANDBY periods. I know the grids of the 812A's will handle =
about -100 VDC so I'll switch between the -100 and -48 via a relay.=20

  The bridge rectifiers are on the way. They are rated at 6KV at about =
5-.6 amp. I'll team them up with three 560 uFd/400 VDC caps in series =
along with the appropriate voltage dividers. I'll have to figure some =
surge current protection but the ARRL Handbook as a few of these =
configurations handy.=20

  O.k. That's about it. I'll keep you apprised of the situation. Cal, =
N6KYR.=20

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