[R-390] Thrifty Tubes II

[Bob kb8tq]

Hi

If you are only concerned about sensitivity then more gain may be better. If you are concerned 
about front end overload or adjacent channel selectivity, it generally is a bad thing. If you are
concerned about your front end going into random oscillations at various times, it can be a bad 
thing. The normal design algorithm has (and had back in the 1950’s) you put minimum gain 
in front of your mixers. 

Since tubes are far from uniform (even in the same type designation) in terms of gain, you can 
go overboard fretting about this. Anybody who owns a tube tester that does measure 
gain knows that a radio (or other piece of gear) can do pretty well with some “dead” tubes in it. 
Equally, putting in all brand new tubes into a working radio rarely makes a difference.  There
is a range that works ok and making sure of that was part of the design process.

Finally, not all tubes are in some hyper critical part of the circuit. There are always going to be
stages where gain could be just about anything and the radio is going to work ok. That often gets
you into “I this new tube and it worked” sort of conversations. The key question is always which
location did it work in. 

Lots of fun !!!

Bob

> On Jun 26, 2019, at 1:54 AM, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:
> 
> Yo Bubba Dudes!,
> To those who might fret about the substitutes I mentioned.  Both Art and Oscar would have used them in their sets had they been available.
> Trust me.
> You can go back to worrying is Tisha is going to make her Elton John/Liberace tribute R390A with the deep red panel, white knobs and gold lettering.
> But if you drink a warm glass of milk before bedy bye time, you'll sleep better. And try to concentrate on the perfect shade grey as well and you should have pleasant dreams. And in the morning you might be able to some new shades of crayons.
> 
> Trust me.
> Regards,
> Perrier
> 
> Regards
> 
> 
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