[R-390] The R-390 and the 6AB4

[Tom Norris]

Another slow Monday...

So I was playing around with this one R-390/URR that was in the pile  
that I hadn't gotten around to checking out - I did the usual quick  
check of the IF followed by an RF check and noticed "this is a hot  
receiver..."

It was easily reading it's -7  volts with .1 microvolts in, in fact I  
had to use an attenuator.  With no signal in, it had no real audio  
noise nor diode load voltage out to speak of.  Pretty sensitive  
compared to the other radios that I've been slowly getting around to  
checking. (very slowly...)  Most have been lucky to meet spec at  
initial check out.

This one doesn't, really.

The gain was because the 6C4's were subbed with 6AB4's.  With the  
6C4's in place, the initial reading was about 10 microvolts for a -7  
volt reading.  So far I've not been able to make it overload or see  
any odd signals and there doesn't seem to be any unusual distortion -  
which I think I should be seeing considering the the mixers stages  
should have a nominal gain of 2 using a 6C4, best I can tell.  (In  
the R-390, the mixer gain is fixed vs the '390A)
They're subs, and they work well in most apps, supposedly, look up  
the specs - they work *really* well here, and for folks that don't  
want to go digging into their radios it may be a solution.  They made  
no difference in the mixer and rf "alinement" by the way.

I'll end up pulling the RF deck to clean the geartrain and replace  
resistors,  tempting as it is to just leave it alone and just listen  
to it - since it works.  But, it's not to spec and the  resistors  
keep getting, ummmm more 'resistor'-y.  Many of the higher value  
resistors in it's brother were 100% high.  This isn't a spring- 
chicken R-390A we're talking about.

Just thought it was an interesting checkout after I finally got  
around to it.

Anyone looking for these tubes on purpose can find them under the  
number 6664 as well.

73

Tom