[Boatanchors] Patterson Power Supply Help Needed

[Brian Clarke]

Most of the 70 V and 100 V line audio transformers I have seen are
designed for audio frequencies and NO DC current. Using such a
transformer as a filter choke will not work well; the DC current flow
will reduce the effective inductance considerably. Further, the taps
on most of the ones I have seen are on the secondary; for a filtering
application, you should not be using the secondary taps at all.

A far better solution is to put your multimeter in series with the two
filter capacitors and turn the set on to measure the operating
current; if you are concerned that there may be a short on the HT
line, wire a 100 W incandescent lamp in series first - if it glows
very brightly and stays bright, you need to solve that problem first
before causing damage to your multimeter. Then find a 8 or 10 H choke
that will handle the required level of current. Almost any tube set
should have a filter choke you can borrow.  A filter choke is
designed for carrying DC current, where a line audio transformer is
not.

With respect to the 8 uF filter capacitors, I would check that they
are still functional. If not, replace them with modern
equivalents.Using larger filter capacitors may strain your rectifier
and power transformer.

73 de Brian, VK2GCE. 
Brian Clarke
BE, MBA, PhD, CPEng, FIEAust
MD, Clarke & Associates P/L

On Sat, 7 Sep 2013 16:41:01 -0400; Ray opined:

 I would suggest you look around for an inexpensive, maybe free, 5
watt PA systems 70.7 volt audio transformer. These are typically used
to provide a selected amounts of power to speakers in a distributed PA
speaker system.
 It is an auto-transformer with taps and should work well in place of
the original field transformer. 

 They usually come with several impedance/power taps in that range,
one of which should work OK for you. A similar 25 volt transformer
with the right taps on it would most likely serve as well. 

 Anyone working in that field would likely have a good used one they
would give you.

 I would think this would be vastly superior to a simple resistor.
 Ray, W4BYG

 On Saturday, September 07, 2013 2:57 PM, Joe said:

 I just picked up a nice Patterson PR-10 receiver but there is no
speaker.
 Originally, it had an electrodynamic speaker and a field coil. The
schematic is here: 

 http://www.nostalgiaair.org/PagesByModel/091/M0013091.pdf

 I want to substitute a PM speaker because I don't have much choice.
The schematic lists the field coil at 1.5K ohms. My plan is to change
the filter caps and substitute a 3K/10W resistor for the field coil
just to see if the receiver works. Does the sound reasonable? The two
original filter caps are
 8 mfd. Should I up the second one to 22 mfd for better filtering now
that there is no field coil or am I better off adding a second 3K/10W
resistor and a third 10 mfd filter cap?

 I've read about these types of substitutions but have never had to do
one.

 Thank you for your help.

 Joe Connor

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