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Author Topic: Vacuum tube identification.  (Read 10083 times)

Posts: 1279

« on: March 05, 2013, 09:04:17 PM »

I have a strange question. I am restoring a National NC-98 receiver. I guess I pulled a dumb one. I removed all the tubes for cleaning. The placement location is not a problem as I have a diagram, and the sockets in the chassis are all marked. Now for the problem. When I cleaned the old tubes the tube markings came off some of them. Two of the tubes are obvious as the sockets are of a different size. Some of the tubes still have readable markings. Now for the problem three of the tubes that I can not read the markings on have the same size sockets. Is there any way I can tell a tube number or type by looking at it internally? I have the schematic of the receiver so I can identify the use of the tubes, again is there any way to identify the tubes with the numbers that I cleaned off, or do I need to buy new tubes to replace the unknowns?

Any help would be appreciated.



Posts: 46

« Reply #1 on: March 05, 2013, 11:50:39 PM »

I've encountered what you're experiencing. I've found that heavy breathing thru your mouth with heavy moisture will sometimes make the tube numbers appear. It's worth a try.

Larry W0NFU

Posts: 165

« Reply #2 on: March 06, 2013, 01:19:04 AM »

The breath thing often works better if the tubes have been in the freezer for a while.

What tube types are they?

Posts: 2483

« Reply #3 on: March 06, 2013, 05:25:49 AM »

Other than what has already been suggested, I think you are out of luck.  I learned the same thing many years ago, the hard way.  Fortunately MOST small vacuum tubes are still available and MOST do not cost very much.

Dick  AD4U

Posts: 5688

« Reply #4 on: March 06, 2013, 06:13:52 AM »

**Do all of the tubes which you aren't sure about start with the same number?  That would mean same filament voltages and make it slightly less of a problem if in the wrong socket.  

**Those who really know their vacuum tubes can often study the internals to find out which tube it is, provided it is a situation like this one, in which the identity is "one of three" rather than totally open-ended.  

If one of the tubes is a rectifier, that is rather easy to identify by simply looking thru the glass at what's in there.  The same can be easily accomplished if one tube is perhaps a dual triode but another is a single.  The Beam Tetrode can often be identified by the number of connections brought to the outside world as well as the internal structure. 

**For the noob, perhaps the best recourse would be to simply order up the three tubes new from somewhere, as you are likely going to be confronted with other issues as the project continues and don't want to be encumbered with the unknown at that point.  You could be chasing in circles, not knowing if a problem would be cause by another component or is a problem just because the wrong tube is in the wrong socket.  

« Last Edit: March 06, 2013, 06:16:25 AM by KE3WD » Logged

Posts: 2085

« Reply #5 on: March 06, 2013, 10:16:10 AM »

if those were GE tubes, they always etched the EIA numbers into the glass.

for everything else, if you can determine ONE tube type, you can generally stare and compare at the internals to match the mystery tubes to it.  confirm it in the tube tester, then mark and install.

probably is not a disaster in this application... but when you get into audiofool or microwave territory, and sometimes in tube-based electronic organs, you do NOT want to randomly scramble the tube locations.  if V743 comes out of its socket for test, that tube should go BACK into V743 to avoid major realignment or possibly huge noise issues.  you also have the advantage if you scrub off the identifying marks of knowing what it is you just hosed up, so you can use a paint marker pen to relabel it.  if V743 was four stages into an IF strip, and you put it into the V627 socket as first RF, and it was a noisy microphonic tube at frequency, you would not be a very happy camper at all for a long time troubleshoot.

Posts: 4464

« Reply #6 on: March 06, 2013, 10:50:13 AM »

Every once in a while there's just enough of a difference between the printed and non-printed areas to get some idea of what used to be on the tube... Think in terms of residual surface gloss. Do no more cleaning, then find a desk lamp or similar and a magnifyimng glass. Try reading the tube backlighted, side lighted, top, bottom, or any angle that lights the labeling from the edges or inside.

Sometimes that's enough to figure out who's the 6BE6 and who's the 6BA6.

Spend enough time with BA's and you'll be able to spot a 6AL5 or 12AU7 from across the room. 6BQ7, 6C4, and 6U8, too.

The end of the world will occur on April 23, 2018 ( the day after Earth Day. Go Figure ).  If you're reading this on April 24th look for updates coming soon.  If you're reading this after June first, fuhgedaboudit.....

Posts: 3289

« Reply #7 on: March 06, 2013, 12:42:47 PM »

Here's a trick:  Get an LED flashlight with white LED's and shine the light on the tube, possibly at an angle, possibly in a dim or dark room. 

Someone suggested this tip a few months ago, and said the slight UV light that gets thru the filter in the lens was able to show the etched numbers.  A 'black light' would work as well or better.

Posts: 1279

« Reply #8 on: March 06, 2013, 04:18:13 PM »

Thank you for your suggestions. The cold tube with the hot breath did the trick and I was able to identify 8 of the 9 tubes in the radio. This left the one unlabeled tube which goes in the obvious tube missing socket. The only other minor problem which I was able to solve was two 6bA6 tubes were substitute for two 6BD6s. These are vertual direct substitutes. So all is well.

One more question. I have to restring a dial pointer cord. I have the stringing guide, but I am having some difficulty. The original string is not broken, but it seems to be difficult. Anyone out there have any suggestions on this?

Thanks again,


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