I am working on a pile of Zenith TOs. The g500 series and newer uses the 1L6 tube while the earlier versions use the 1LA6 tube.
After restoration I noted failures in the later TOs in the upper freq bands. This is a well known problem with failing 1L6s. The interesting thing is that the failure occurs after several minutes of warm-up. At first the tube will work as an osc. in the upper bands.
Testing the tube initially on a Hickok 533 indicated tubes around 300 and 240 (GM) on the osc. section. These are not really hot, but OK. At the time of failure these readings drop to around 150 to 200 (GM). Then after cooling for a couple of minutes the reading returns to 300 and the tube will work again - for a while.
Keep in mind that the 1.5V COMMON CATHODE tube never really gets warm to touch.
Gas would not do this. The tube is loosing it's emissions, probably due to age, but, that does not explain the return to functionality.
All voltages were tested and were right-on. Fil voltage drop across the tube was 1.55V.
Keep us posted, Russ, on what you find. I, too, am working on a H500 with the same tube so I'm hoping all works well. I have also read about some of the problems with this tube. My radio (see November finds) was working an all bands before I disassembled to rebuild. The radio had been worked on previously. The selenium was bridged with a silicone diode with no extra resistor added. I assume the tubes were running a bit higher filament and operating voltage - probably not good for these somewhat fragile tubes.. I'll be interested in what you find.
Is the voltage across the 1L6 filament monitored all during the good to bad and good again?
Is the B+ stable?
I have found the bias resistors around the 1L6 to move in value when this sort of fading occurs. The fix was to replace these resistors. Other resistors in thee filament string can also be moving around...GL... chas
Yes all voltages stable and at spec.
I know that a tube can act this way when it is old. The longer it runs the hotter the plate and grids get which can cause unwanted emissions.
The interesting thing about this tube is that it really never gets hot - ever - new in box (unless you put the box in the oven) or used in radio. AND the tube never stops working. The amp/mixer section is fine. The failure is that the osc just wont run above about 11mhz (oscillator) when hot. Performs just fine on broadcast band.
Replacement with a good tube cures the issue - but the old tube also tested good. So much for tube testers - right?
A possible moral to my story is: Be cautious when purchasing a used 1L6, or just buy new ones - until you can't anymore.
Anybody got a new 1L6?
P.S. These battery mini-tubes all can be a little flakey. While working on this I noted that the radio made a few notes immediately on turn-up, kind of like plucking the taunt dial string. I turned the radio over, with it on, to measure voltages and after 30 sec or so, the fil in the 1S5 went open. Replacement worked and no more "ting, ting, ting on start-up.
New at Find A Tube for 29.00. Used 18.00 pretty good considering.
Paul
Yes I saw those at FAT and was going to order a couple.
Not to be confusing - but I always think of a "converter" tube as having two elements, the osc and the mixer. in the case of the 1L6 that is also how they are tested - two values.
Hmmm. I'm in the middle of replacing things in my H500. I think I'll just go ahead and change those resistors. Why struggle later?!
FWIW I don’t know if you are dead set on keeping it original but there is a solid state replacement for the 1L6. I’ve used one myself and works great. It’s a direct plugin but ain’t cheap, like $44. There was a guy on eBay selling them when I bought mine or here’s a website selling them...http://www.solidstatetubes.com/index.html
Ron
I've got a whole box full of 1L6s I could test for you (but no way to check functionality) and see what they test at for you and I could send you some of mine as I don't have a Zenith Transoceanic radio currently that takes them.
That would work. Figure out what you need for them. I won't accept them for free.
Send me a PM when you do. We are in the wrong category to discuss it here.
Russ;
Did the 1L6 you tested give the same readings if you did not let the tube "cool" down first? Do you know if the reading dropped down, and stayed that way? I'm wondering if it might fluctuate, up and down, if left in the tester for an extended period. I'm also wondering if the loctal 1LA6 suffers from the same issue? The condition you describe, in the radio, does sound a bit like flaky carbon comp resistors, which is always worth investigating.
Regards
Arran
It seemed to go up and down, which, to the point, is the odd part.
A report says that the government stocked away thousands of these tubes for the relatively small number of sets that they purchased. I can see why.
I have been working on a few of the earlier version TOs. There does not seem to be a problem with the 1LA6 which is interchangeable with a socket adaptor.
Russ;
I wonder if the more ubiquitous 1R5 also suffers from this, they used to be notorious for refusing to oscillate if the filament voltage dropped 1/10th of a volt? I have an RCA portable, one of those late 40s ones with the aluminum case, I think it has four bands, RCA used a 1A7 for a mixer-oscilator, even though the single band models used a 1R5. I'm starting to get the impression that the 1L6 was what one British professor of engineering (architectural engineering not electronics) would call "an innovation without development".
From what I read the only reason it came into being was because Brand Z wanted to modernize their T.O chassis to use 7 pin miniature tubes, and contracted Sylvania to enter a crash program of stuffing a 1LA6 into a 7 pin mini envelope, so they wouldn't have to re-engineer the front end. Why they bothered with this makes no sense from an engineering standpoint, they never downsized the T.O series, nor the chassis to match, so it had to be for marketing purposes alone. I have one of the G500 sets and the chassis holes were punched to accept normal size tubes, so clearly it was a last minute alteration.
Regards
Arran
I Was just looking at THE BOOK (RCA Tube Manual 11/66). It says that the normal fil V. is 1.4V, which is interesting. I did try lowering the fil voltage, it just made thing worse - which is what I would expect on a weak/old tube.
The 1R5 just seems to be worse for this than the 1L6. A brand new, hot 1R5 might run at higher freq than a well used 1L6. If I get time I will investigate.
I will post a blog - mostly on quick electrolytic cap re-stuffing, but I might get into this aspect.
P.S. for anybody interested: The Captain's tubes turned out to not be 1L6s. My request to FAT as yet to be answered.
P.S.
I never heard back from Find A Tube - meaning that they probably don't have any new 1L6s. Also meaning that the posted price is irrelevant.