[R-390] tube testers

[Barry Hauser]

> RE: [I] have always been of the opinion that the best tube  tester is the 
> circuit that the tube is in

That's true, and many of the tube tester manuals say so -- another YMMV 
disclaimer.

Ken wrote:
> This is a good point.  I have a Sylvania tube tester and it works ok but 
> there are times I wonder just how accurate it is.  I found my tube pin 
> straighter, now I am going to look for  some tube extenders.

Tube extenders are well worth having, though some are a bit too big to fit 
some places in R-390's.  You might want to roll your own, but it's not easy 
to find 7 and 9-pin bases.

There are various tradeoffs in tube tester design, so they may be more 
accurate with some types of tubes than others -- and there can be errors in 
the tube charts.  The manufacturers did not necessarily calculate the 
correct minimum reading for every number.  Many of the entries were probably 
determined through extrapolation/interpolation, if that.  (Or they cheated 
and used a conversion factor on the values from some other tube tester.) 
And, of course, what's good enough in one socket in a particular piece of 
equipment might not be in another for the same tube number.  YMMV prevails.

Tube testers do have their uses, such as:

-- Sorting through batches of old tube finds.
-- Pre-testing for shorts, grid emissions, burnt out filaments -- before 
installation
-- Making relative comparisons/matching or selecting tubes -- such as tubes 
that should not be too "hot".
-- Checking for burnt out tubes that are part of a filament string.  Yeah, 
you could do that with an ohmmeter, moving the tube extender around from 
tube socket to tube socket, or pulling the chassis.  In this case (like a 
tube Transoceanic or other series filament setup), it's a time saver.

You don't want to use a good piece of equipment to find out if a tube has a 
dead short that might take out a transformer.  We are also learning that an 
NOS tube/valve is not necessarily "forever".  There could be bent, shorting 
elements that have occurred in storage, repeated shipping around, old latent 
defects, etc.  Long term air leakage around pins has also become more 
frequent, so there's something to be said for pretesting in even the 
simplest of emissions testers before plugging 'em into the equipment.

The reject values for mil testers such as the TV-7's seem to be set 
conservatively low.  Good tubes generally read well over those, except for 
rectifiers and diodes which tend to be right on.  Seems to be designed to 
avoid excess "tube jockying".

If you pay enough for a tube tester (like too much), chances are, you'll be 
more inclined to use it to save tubes than to find ones to toss out.

Not sure how popular "re-tubing" is on this list, but if you're going to 
arbitrarily replace all yours, please send the castoffs to me for proper 
disposal ;-)

Barry