[Collins] 51J-* / R-388 heat shield

[Dr. Gerald N. Johnson]

On Thu, 2007-01-18 at 10:13 -0800, C E wrote:
> One way to do it would be to remove the transformer
> and paint it white. 

An aluminum sheet with tabs to transformer or other mounting screws
would be be more effective, but can make the rectifier tube run hotter
by reflecting heat back to the tube. And might not have much effect on
the transformer from reflecting its radiant heat back to the
transformer. Both the tube and the transformer heat internally. I'm not
sure a shield will cool the transformer even if that shield is spaced
away from the transformer. 

Maybe if the shield was flat black on both surfaces and thick enough to
transfer heat to a portion that was above both the tube and the
transformer so that convection currents and radiation could cool the
extended shield both components might run cooler. A spectrum selective
surface facing the tube may accept visible more than it radiates it back
as IR, but that's a surface that is difficult to produce.

Probably some of the transformer heat comes from it being designed for
115 volts. Modern line voltage tends towards 125 volts and that can
drive the core of the carefully designed transformer into saturation
raising core and primary wire temperature. A core hitting saturation
hard takes increased primary current, with significant spikes at the
peaks of the line voltage. Inserting a bucking transformer to lower the
line voltage to something like 112 can cool the transformer
significantly.
> 
> Also, what would be the ramifications of switching
> over to a s/s rectifier? (I know the usual increase in
> voltage might task the present components but I don't
> know how significant that would be). 

Typically the high vacuum rectifier drops 40 to 80 volts. Silicon drops
under a volt. Making that change without adding a resistor to do some of
the drop does stress tubes and electrolytics. And since the silicon
rectifiers don't have the 15 watts of heater power (I'm presuming 5Y3)
the overall package is cooler. Often (in R390A) that series resistor is
200 ohms, 25 watts, but the 51J may take a little less power. It would
be practical to locate that resistor remotely to remove that power from
the immediate vicinity of the receiver, though it takes some care
because it has 300 volts or so on it and unless shielded presents a
shock hazard.

After silicon replacement of high vacuum rectifier, dropping the voltage
more with the bucking transformer is an option so long as the tube
heaters still get 6.2 or 6.3 volts. A variac works, but someone will get
their hand on the knob and crank it up where it doesn't need to be, so
the fixed wired bucking transformer is safer.
> 
> Cal, n6KYR/8
> --- Joe Eide <jeide at execpc.com> wrote:
> 
> > Hello from KB9R,
> > 
> > I am looking for a clean way to install a heat
> > shield between the 
> > rectifier tube and the power transformer on the 51J
> > / R388 series receiver.
> > 
> > Can anyone show me some pictures or links showing
> > nice installations of 
> > this?
> > 
> > Thanks,
> > 
> > Joe
> >

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer