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

[C E]

Hi Doc!
Good observations all around! I would also opt to
install a small muffin fan directly over the source of
the heat. I did this successfully with an NC-125 and
had a bonus of not only decreased heat but better
frequency stability. 

Cal, N6KYR/8
--- "Dr. Gerald N. Johnson" <geraldj at ispwest.com>
wrote:

> 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
> 
>
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