[R-390] NEED HELP R-390

[Mark Huss]

We design and build Military Electronics.  Rule#1 is to design it so 
that no matter what, when the 'Grunt' pulls the trigger, bad things come 
out the naughty end :)           As a rule, I design with 65 degrees C 
(150F) as the absolute minimum outside ambient temperature.  You would 
be amazed at how hot the inside of a Bradley can get sitting in the 
Sun!  So when I am grabbing parts for a 'quickie', the mindset is 
thinking 65C.  Add to that the heat generated by an R390A inside a rack 
samwiched between two others, and Ambient gets quite warm.  Since i 
mostly have heat-sink resistors laying around, did not want to bother 
with heat-sinking to the chassis, and it will be situated vertically 
(basically doing everything wrong;)), a 10W would derate to about 4 watt 
at 25 degrees C.  Its life would be short at anything around 35-45 
degrees ambient.  Next higher rating is 25 watt.  with that, i am 
comfortable that it will take 80 degrees C ambient at 5W no problem.  If 
you go for Ceramic Wirewound, you will still have to derate because it 
is vertical, but not near as much because it is much larger.
Realistically, my R-390A sits open so i can enjoy the glow and keep it 
cool.  so 40 degrees would be worst case.  Using a good piece of 
aluminum or mounting it to a chassis, I could drop down to 5W mounted 
horizontally on the chassis, 10W mounted vertically on a small plate. 
You haven't lived until you see the end plugs shoot out of the Dale 
Resistors on the 'Avenger' Gun Servo Board cuse you left it on for 
thirty seconds of troubleshooting!!!
Mark

Barry wrote:

>Mark,
>
>Just curious why you suggest a 25W resistor?  300mA through 47 ohms is just
>a little over 4W, right?  Bigger is better, I suppose, but you can get by
>with less, I think.
>
>Barry - N4BUQ
>
>----- Original Message ----- 
>From: "Mark Huss" <mhuss1 at bellatlantic.net>
>To: <dhallam at rapidsys.com>
>Cc: <R-390 at mailman.qth.net>
>Sent: Wednesday, June 15, 2005 6:12 PM
>Subject: Re: [R-390] NEED HELP R-390
>
>
>  
>
>>David is right.  I repeated Dallas Lankford's work and found that the
>>receiver PTO is much more stable with a 42 Ohm resistor than the
>>origional 3TF7.  Unless you have AC power that varies more than about 7
>>volts, use a 25 watt 42 ohm resistor.  It generates some heat, but no
>>more than the origional 3TF7.  And if you have a fetish about drift of a
>>few hertz, use a Voltage Regulator and a good heat-sink.  You have to
>>add a ground wire to the socket, but it was staple to the point that it
>>was unmeasurable on my equipment.
>>I tried the 12BA6 with a 1 ohm resistor in series to measure current
>>rush, and saw quite a spike in current on turn-on.  Makes sense as
>>filiment resistance goes up as the filliment heats up.  Enough that I
>>decided to stick with the resistor to keep the filliment surge down.  It
>>helps, I noticed that the PTO takes about two seconds longer to come up
>>than normal.
>>    If anybody is interested, I did the design work up to Breadboard for
>>a plug-in Voltage Regulator.  it is designed for 40Volt Peak input
>>voltage and 80 degrees C. ambiant at 20% overcurrent.
>>
>>David C. Hallam wrote:
>>
>>    
>>