[Milsurplus] Technical assessment

[Floyd Davidson]

"pete williams" <jupeter@datafast.net.au> wrote:
> G'day,
>I've been waiting for a break in the ... er ... weather, to ask what
>might be a fundamental question on tube (valve) behaviour.
>
>I had occasion to check a number of 12 SG7 on my TV7-B tester and
>found that a large number had readings of between 10 and 30 when the
>listed value was 40 as a minimum.
>
> Thinking that the test set might be defective, I then tested  a
>couple known to have readings of 100--- they came from a U S supplier
>and were marked as such. My readings  of these 2 agreed with  theirs.
>
> Strangely, the tubes I would have discarded performed OK in the radio
>with no discernable difference between them and the  high reading  "u
>beaut" tube. Tubes were part of a  IF strip.
>
>Question ... why is it so ? I am aware that the readings are a view of
>the transconductance and I have  some - not enough ?- knowledge of
>what it means.

In most applications the circuit can be designed to operate
independent of the characteristics that vary from tube to tube.

An example where that is _not_ usually true would be the output
stage of a typical ham radio transmitter, where the object is
maximum output, because several different tube characteristics
will limit maximum output.  Tube inter-electrode capacitances
also cannot be controlled exactly, and hence neutralization has
to be individually adjusted per tube.  Power amplifier control
grid bias is another value that is often individually adjusted.

But in a typical receiver IF amplifier none of the above
applies.  Instead what is desired is a specific gain from that
one stage, or a range of gains that are automatically adjusted
with an agc circuit.  The amplifier's performance is almost
totally independent of the tube's transconductance, as long as
it is above some minimal value.

Typical circuits employ feedback, some times at the signal
frequency but some times as an agc circuit, and may also choose
other circuit parameters such as plate voltage and series
resistors to make the operation independent of the not only the
tube itself, but also independent of line voltage and other
variations.

>Having gone thru' tubes over the years and  trashed those that didn't
>measure up, am I derelict in duty of care and common sense not to have
>put them to the service they were intended for ?

Yep!

>May be the cumulative effect of low reading tubes would have shown up
>in ultimate performance of the radio as there are  3 in the IF and a
>couple in the RF stages.

It might.  Might not too.  It depends on the individual circuit.

>Comment pse.?

Way back when the world (and me too) was a lot younger, I used
to be paid money to maintain tube equipment.  We're talking
telephone stuff, such as 1000 Mhz FM wideband receivers which
were 4 racks full of tubes.  Some days a person might spend an
entire 8 hour shift doing nothing but changing tubes.  Hundreds
of tubes in one day.

We did have a tube tester, but basically *never* used it.

That is because generally the performance of any given tube in
any given circuit depends on the circuit as much as the tube.
So what a tube did in a tube tester was mostly a very good
indication of... what the tube would do in a tube tester!  And
little else.

The way to test a tube is to replace it with a another one, and
maybe try two or three.

There were other tricks too.  For example these receivers were
dual redundant.  A matched pair was in use, while a matched pair
was idle and could be worked on.  Each pair was identical (and
used a different receiving antenna for space diversity reception),
and the demodulated baseband signals of the two were combined
at the output.  

The control voltage for the combiners was developed from an out
of band noise amplifier fed to a rectifier.  That was very
useful in a way the instruction books didn't mention.  It was an
FM system, so if there was no signal input, the baseband noise
output would be high.  If there was a signal, the out of band
noise would be low due to quieting.  So the way to test the
*whole* *receiver* at once, was terminate the antenna input and
measure the combiner control voltage!  If it was -45 vdc or more
(it might be as high as -60 or so), that receiver was *clearly*
plenty sensitive enough!  At -40 to -45 it might be a good idea
to check a few other things.  And if it wouldn't muster -40 vdc,
it needed work.

We were supposed to do a routine every 3 months that amounted to
spending hours verifying that the receiver was good.  We'd boiled
it down to a 60 second test...

But there were other odd things to know too.  The
characteristics (particularly filament emissions) change
drastically in the first 24 to 72 hours that a tube is used.  In
a circuit with loads of feedback, that doesn't mean much.  But
there are circuits where it does mean something.  Way back in
the good old days, you've heard of Captain Crunch and about
whistling 2600 Hz tones into a telephone?  Well one piece of
equipment designed to respond to that 2600 Hz tone was called a
X-Type 2600 cycle Single Frequency Signaling Unit.  (A @#$%^&*
piece of crap with tubes and relays in it!)  These things were
disgusting to adjust properly.  But if new tubes were used, the
unit would be a basket case in two days, guaranteed.  We used to
have a shelf of tube sockets with only the filaments wired up,
just to burn in tubes to be used in those X type SFU's.

The same types of tubes (407A's and 408A's) were also used in
teletype "modems" in 43A1 teletype carrier systems.  In those
systems the DC characteristics were not really significant, but
the absolute gain at voice frequencies was.  We found that we
needed to select tubes, because half of them provided poor
performance (at some point in the late 60's AT&T apparently
started selecting tubes just for those units, as we started
getting 407A/B tubes which were all up to snuff for the 43A1
systems).  But what we'd do is put 48 tubes into the sockets and
let them cook for 5-10 days.  Then we'd set up a scope on a test
jack on a spare TTY terminal unit, and start swapping tubes.  It
was just a matter of letting a tube warm up enough to see what
the signal output voltage was, and putting them into two
different piles.  The high gain tubes were for the 43A1 system,
the low gain tubes were for the X SFU's.  I don't think I ever
tried testing any of them in a tube tester.  No point in it.

I think that most people did replace tubes in home radio and tv
set according to how they tested though, because they didn't
want to buy a whole stock of tubes for testing spares, of
course.  But even given the price of tubes today, I think I'd
still go with a supply of spares.

-- 
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson>
Ukpeagvik "Place where people hunt snowy owls"     (Barrow, Alaska)