[Hallicrafters] JAN tube caveats

[Craig Roberts]

It's happened again -- twice!

Arriving home after a pilgrimage to Howard Mills' place last Friday, I 
was anxious to listen to my "new" Collins R-388 after "the man"  had 
played with it. It was sure that it would sound much better after Howard 
had aligned the radio (he does this in about 15 minutes as opposed to 
the hour and a half it takes me), and housed it in one of his beautiful 
powder coated cabinets (he's still got some -- and they're well worth 
the $$$$). 

I was wrong.  In fact, the radio sounded worse. It barely played at 
all.  Sure, the S-meter wiggled convincingly and signals could be heard, 
but they were very faint -- barely audible with either speaker or 
headphones. After staring under the new flip-top hood for a couple of 
minutes, I started wiggling things.  When I got to the back of the radio 
and tweaked the audio tubes, I heard an odd crunching sound from one of 
the 12AX7s.  I removed the tube shield and half the glass envelope came 
with it.  A CLUE!!  Replacement of the tube solved the problem. 

Okay, it wasn't a "real" 12AX7. It was a JAN 5751.  I had opted for it 
when retubing because the military tube costs a few bucks and is readily 
available from the guys who have inherited the vast stocks of the 
gummint surplus tubes that will outlast us all.  A "real" 12AX7 -- one 
of the Holy Grails among the audiophile/guitar amp junkie set, is now 
priced at $20 and up -- way up.

The 5751 and 12AX7 are not the same tube.  The gain of the 5751 is about 
30 percent less and the current draw a little more.  On the other other 
hand, it's allegedly "smoother" (according to famed guitarist and 
apparent vacuum tube aficionado Stevie Ray Vaughn).  I doubt that any of 
this matters much to us low-fi boatanchorites, but there it is.  Fact 
is, my 5751 broke in half for some reaon.

Then yesterday my R-388 suddenly lost sensitivity and some AVC action.  
The sensitivity issue went undetected for awhile since the bands 
yesterday (to use technical jargon) sucked, but the audio distortion was 
unmistakeable. 

Now, the old manuals always adviseus to suspect bad tubes first when 
troubleshooting.  Despite the fact that the age of our boatanchors' 
components can spell failure almost anywhere in our sets, it's still not 
a bad idea to check tubes first.  I did, and found a three-day old NOS 
6BE6 to be faulty.  Okay -- once again -- it wasn't a "real" 6BE6, but a 
JAN 5750 "equivalent".  This tube was physically intact, but betrayed an 
internal short.  Plugging in an old, "real" 6BE6 solved the problem.

A couple of similar JAN tube failures, including another spontaneous 
glass envelope cracking in an OA2WA, have struck other gear in my shack 
over the past year or so.  On the other hand, none of my "civilian" 
tubes has failed in catastrophic fashion -- ever. 

Sometimes, JAN and civilian equivalents aren't equivalent at all. (I had 
a 32S-1 that simply refused to work with JAN tubes in a couple of 
circuits). Tube data sheets will reveal the differences. And sometimes, 
I suspect, the quality control of JAN tubes suffered. In my experience 
the bad JAN tubes seem to be the ones produced in the later years (after 
the Vietnam era).  So, I would advise anyone retubing a boatanchor to be 
a bit cautious about the JAN tubes -- especially the later ones -- and 
check them first if and when something goes wrong suddenly.  Of course, 
the best course of action might be to follow the advice of many 
knowledgeable folks and not retube just for the heck of it. Y'know, "if 
it ain't broke...etc."

Anyone else had interesting adventures from the use of JAN tubes?

73,

Craig
W3CRR