[R-390] Tube testers

Tue Jun 26 11:36:14 EDT 2007

I dunno Bernie ... as with anything, there are other considerations:

Those manuals were written when the gear was new and assumed it was 
within a few years old.  There are words in most of the old manuals that 
are there to avoid the wasteful tendencies of some tube jockeys.  Some 
of it was there to save the taxpayer's money.  At that time, if you 
opened the box on a new tube, it was a good bet the tube was good.  Now, 
it's a mixed bag.

Also, nowadays, most of us are trying to optimize performance, not 
operate marginally.  But there are some other considerations ...

I suppose the manly way to check a new (old stock) or used rectifier 
tube is by plugging it into the circuit.  Before you get the voltmeter 
test prods out, after having pulled the chassis or module, if the filter 
caps sizzle and explode or the power transformer burns out, that 
particular tube is shorted.  No problem, just try another -- after 
replacing the caps and/or transformer.  OK, maybe it's an audio tube -- 
ditto for the audio transformer.  IF?  If it burns out one or more of 
the IF transformers, begin again from step 1 ... after replacing the 
transformers?

Yes, one consideration is that this is not 1947 or 1957 any more.  
Fifty-sixty years later, we're gradually learning a bit more about the 
long-term shelf-life of NOS tubes.  They have often been shipped around 
repeatedly and subject to vibration and possible physical shock damage.  
I haven't found many --  but enough -- shorted ones which were screened 
out by my tube testers, to be worth a quick test. I'd rather use a tube 
tester to find out than an unobtanium power or audio or IF transformer.  
Then, even if not shorted, an NOS or used tube might be substantially 
weak or gassy.  Who was that guy with the sig line "They don't make 
tubes like they used to."  You can't trust tubes even if in the original 
box.  From time to time, we come across bad batches, or might be just 
one bad one in a sleeve of five.

Side trip:  We had the same drugstore testers here in the States.  They 
had just a few controls and lots of sockets and a roll or flip chart.  
Usually mounted on top of a metal cabinet with a stock of the most 
popular tubes.  They were very simple emissions testers.  I have a 
couple of them -- some made by "Mercury" here.  Most of the time, they 
were calibrated to err on the side of higher tube sales, but due to 
limitations, they often failed to detect defects.  (They strapped a lot 
of pins together for the test.)  They would also show bad horizontal 
output and high voltage rectifiers to be good when they were really 
bad.  I don't think there are any regular testers that do these.  Often 
they were broken down -- burned out transformer due to a careless 
customer.  The typical customer was so happy that the tube he thought he 
needed was in stock, he usually didn't complain about the inflated 
price, and might buy two or three additional tubes "just in case".

There was a different kind of "tube jockey" here as well -- The guys who 
came to the house to fix your TV set.  That might have been sometime 
after one pulled all the tubes from a set, put them in a shoebox or 
paper bag and trotted them down to the electronics or drug store, and 
found none to be bad (due to bad horizontal output, etc.) 

The visiting professional tube jockey did not carry a tube tester -- he 
carried a tube caddy -- a special leatherette covered box with a handle 
on it and about 200 of the most popular tubes.  It usually had the 
colors and logo of a tube manufacturer all over it, like RCA, GE, 
Sylvania, etc. 

He pushed the set away from the wall, but not 180 degrees, so like a 
magician, he had a "blind".  He would continue to replace tubes until 
the set worked (or it didn't and the chassis had to be pulled to go to 
the shop -- uh-oh!)  Now, if there were one bad tube, and he hit it on 
the third replacement try, the two or three he already replaced would 
stay th ere and part of the repair job, even though the odds were that 
the first two were good..  So, the bill would list the 3 or 4 tubes, 
plus the travel time and repair time and come to a tidy sum.  In 
fairness, it was uneconomic to make a house call for a $3.50 tube and 
$5.00 service call, even when gas (petrol?) was 29 cents per gallon here.

Now that was the simplest form of tube jockey fun.  There were more 
nefarious techniques.  For example, some customers learned to keep 'em 
honest by insisting on having the old tubes back, so maybe they would 
test them later or at least to foil the situation where the tube jockey 
replaced good tubes to put in the boxes that he would otherwise retain 
(in his tube caddy) to sell as new to other customers.  However, the 
astute tube jockey might keep known bad tubes to return to the wary 
customer.  It might have been dicier if it were the first time any tubes 
were replaced, where all the originals would bear the set manufacturer's 
brand.  But old tubes were usually covered in dust and the customer 
often didn't look inside the boxes.

Well, back to the subject of the value of tube testers.  Try this -- 
many of us have accumulated large amounts of various tubes -- NOS, good 
used tubes, unknown used tubes.  Some of these came from buys of boxes 
and bushels of a variety of tubes.  In many cases, we don't even have 
the gear that the tubes plug into.  But, if good, they can fetch a good 
price, whether NOS or used, but if they are tested.  Tube testers are 
handy for sorting through and grading tubes for future use or sale or 
the trash.  Also, there are some circuits which benefit from a tube with 
moderate output, and it's easier to pick 'n choose with a tester, than 
plug 'n try.  Not to mention tube-matching, as with push-pull 6V6's, etc.

Probably the greatest value is in checking for shorts efficiently.  You 
can't use an ohmmeter for that, and in-circuit testing is risky at 
best.  In some cases, they can be used to "rejuvenate" tubes.  For quick 
sortation and checking for shorts, gas/grid emissions, you don't need an 
expensive mutual transconductance tester.  In fact, there are some more 
recent emissions testers (B&K) that are pretty handy for checking shorts 
-- more elaborate "lockout" circuit for dealing with known-false indicators.

Yes, the ultimate test is in the gear, but ... there are some 
considerations. 

However, you are correct -- both down-under and up-over, we must avoid 
even the appearance of wimpiness.  After all, our respective national 
anthems include words such as "bombs bursting in air" and "you'll never 
take me alive, said he ..."  (nasty rumor he was the first toob jockey 
-- kept his caddy on a pole -- way ahead of his time. 

There seems to be some concern about which tube tester to buy.  Just 
make sure it's a manly tube tester.  In most cases, this can be 
determined by the aroma after it warms up -- a kind of English Leather 
smell, only funkier.  More meters, the better.  Painted and re-painted 
gray with stenciled lettering, even better.  Heavy? -- better still -- 
you can do weight training to build up your shoulders and biceps.  All 
those switches and levers help maintain hand strength. 

G'day mate ...
Barry

Bernard nicholson wrote:
> Guys tube testers are for wimps  the best tube tester is the circuit that the tube is working in , you can check the voltage drops across plate&screen& cathode resistors and with a modicum of mental arithmetic [remember that?] you can easily see what is happening in a circuit, in a well designed radio tubes don't work very hard at all !  The original signal Corps Specs for the RCA  AR88  calls for the set to perform to its specification with ANY or  ALL tubes at 30% emission   This is the reality ! this is the reason that tubes are rejected on the tester but the set is still working OK,  I was trained in communications in Carrier Telephony  , we used to keep detailed maintenance logs of every piece of equipment And performance checks were done regularly this gear used literally thousands of tubes  and many tubes were only changed after many years when the specs were no longer achieved, Back in these times  people who resorted to tube testers were called Valve or Tube Jockeys,  They were looked down upon and were figures of fun .   Tube testers were used in Australia as a marketing tool , you could take all the tubes from your radio or TV and test them in the shop where they could sell you some more, Very convenient !   Surely if we have Pretensions to own and Maintain an Esoteric R390A  one can learn about mutual conductance and amplification factor Ect   then all you need is a voltmeter Regards to everyone  and keep the posts coming . 
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