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 Post subject: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 12:41 am 
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Location: Peterborough, New Hampshire, 03458 USA
Has anyone had any luck or tried to rejuvenate a 1L6 tube? I have one that tests good in my Sencore Mighty Mite tube tester but hardly works at all on the upper shortwave bands on my Transoceanic. I know it's the tube because I can put a 1R5 in and those bands work fine.

But just to be sure I set my signal generator on 18mHz and used a 3' wire as an antenna. The radio was about 6' away. Nothing at all using the 1L6, but strong signal using the 1R5.

Ok - back to rejuvenation. I set the filament knob to 2 volts on my tube tester. I ran the tube 15 seconds on, 15 seconds off. I did that 5 times. I figured I had nothing to lose. The 1L6 still works just fine on the standard BC band and the lower 2 SW bands. But can't pickup the 18mHz signal from my generator at all.

That was just my idea to run the tube 15 seconds on, 15 seconds off at 2 volts. I see some people say rejuvenation in general may take an hour or two?? But I wasn't sure that applied to running 1 volt tubes at 2 volts. Maybe I should run it longer at 2 volts?

I'm kind of hung up on using a 1L6 because when it was working on the high bands it was definitely better than the 1R5.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 1:16 am 
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No hope. 1L6 has an oxide filament, once the oxide has been depleted it will no longer emit.

Test says it is good on both tests?

If so, consider the bias resistors across the filament of the 1L6 in the radio may have changed value. That can cause a greater portion of the filament current to pass through the resistors and not the filament. Also check the overall filament string voltage and other resistors across filaments. You may be surprised. The difference with 1R5 may also be an alignment problem...

YMMV

Chas

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 1:24 am 
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Agree with the above. Do you have another Transoceanic to try the 1L6 in? It seems odd that that the 1R5 would work well on the upper frequency shortwave bands. Do you notice the stations coming in at different points than what the dial indicates when using the 1R5, normally a significant alignment shift is required for a 1R5 to be even close on the high SW bands?

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 1:31 am 
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Increase filament voltage a little in your radio. This can be done by paralleling a resistor on the filament dropping one. Even increasing by .1 volts can make a weaker 1L6 oscillate on higher bands. Also true of 1R5 tubes. A little higher filament voltage starts oscillation.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 3:07 am 
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Location: Long Island NY
Converter tubes like the 1L6 can be difficult to test; what they measure at 60-Hz on a tube tester is not an accurate indication of what they’ll do at 18-MHz. Trying the tube in another radio would be a more positive way to test it.

Aside from filament voltage as mentioned above, the other thing I’d be concerned with is the state of alignment of the TO. If somebody realigned it with a 1R5, it might not work so well if you stick a 1L6 back in the socket. The tubes were close but not identical by any means and if somebody optimized the adjustments for a 1R5 you may need to re-align if you want to go back to a 1L6

If the 1L6 really is weak, there were some AM portables and direction finders which used 1L6s but didn’t tune much over 5-MHz. You might be able to continue using it in one of them if you don’t “rejuvenate” it further.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 4:07 am 
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NOPE.


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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 12:54 pm 
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Based on your feedback I could have caused this problem myself. Last week I replaced the selenium rectifier with a 1N4007. I ended up adding a 68 ohm resistor in series to adjust the filament voltage at the 1L6 socket to 1.328 volts at 120VAC on my variac. I was using a 1R5 to check/adjust the voltage in case something went wrong. Then I put the original 1L6 back in to do alignment. I didn't notice anything strange while doing the alignment on the upper bands. The alignment must have been close, because I didn't have to move any adjustments very much.

But now the radio doesn't pick up much of anything on the upper bands with the 1L6 installed. The upper bands seem just fine when I install the 1R5.

I could kick myself for not checking the filament voltage before I replaced the selenium. Maybe 1.3 is just too low for this particular 1L6? Actually, I never did check the filament voltage with the 1L6 installed. I'm going to do some experimenting today and first of all check the filament voltage with the radio connected straight to AC line voltage without my variac. It normally runs around 122VAC. Depending on what that shows I may try bumping the filaments up to 1.4.

Another question - shouldn't my tester show emission problems if the 1L6 is weak? My mighty Mite shows 100, which is the middle of the "good" scale on my tester. My 1R5 shows 92 for emission quality. 70 to 125 is "good" on this tester. Or is it just not possible to test 1L6 emission very well?

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 2:41 pm 
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moallen wrote:
Another question - shouldn't my tester show emission problems if the 1L6 is weak? My mighty Mite shows 100, which is the middle of the "good" scale on my tester. My 1R5 shows 92 for emission quality. 70 to 125 is "good" on this tester. Or is it just not possible to test 1L6 emission very well?

Sounds as if a element as maybe shifted causing change in the interelectrode capacitance. Some issues cannot be tested.

Basically if radio works OK on another 1L6 it's a bad tube, if OK on 1R5 & neither 1L6, it's a radio issue.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 5:10 pm 
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Update - I measured the voltage across all the filaments with the radio plugged directly into the AC line, which was 121 VAC. The 1L6 was 1.378, and the total across all the filaments was 7.89. That was letting it warm up for 5 minutes. Then I tried the same measurements with my 1R5 installed. The 1R5 was slightly lower at 1.354 with a total of 7.82.

Then I checked the 3 resistors in parallel with some of the tubes, including the 100 ohm across the 1L6 filament. All of these resistors have 10% tolerance and are at the high end. The one across 1L6 is slightly out of spec at 112.8 ohms. Jumping the gun, I quickly changed it to one that read 100 ohms exactly. Just as quickly I realized that took things the wrong way and dropped the voltage on the 1L6 to 1.309.

Next I changed the 68 ohm resistor I had installed in series with the new 1N4007 to 50 ohms. Whoa - that kicked the filament voltage up to 1.438 on the 1L6 for a total of 8.12. I decided to leave it that way and put the radio back together since I didn't have any way to bump the 50 ohms up a click or two.

Low and behold the 1L6 works on the high bands at that higher voltage. I checked the data sheets for the 1L6 and 1R5. Both rate the filament voltage at 1.4 volts at 50mA. Same for the 1U4's and 1U5. But I'm sure you guys already knew that.

Kind of risky but, I had installed a 9.1 volt zener diode across the filament string as suggested in the H500 restoration guide. Hopefully that will help with any voltage spikes that come along.

Anyway, I'm guessing this means I have a weak 1L6 that works ok on a slightly higher voltage??

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Wed 13, 2019 5:21 pm 
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Yes, that's exactly what it means. It will probably work fine for quite a while, and the added zener protects the entire string if voltage ever goes too high.

Running it at 2 volts even for a relatively short time can also affect the emission at the normal voltage, but not in the manner that you were thinking. Often tubes with that style of directly emitting filament tend to lose emission capability at their rated voltage if they have been operated at a higher voltage for more than several seconds. Hopefully yours hasn't been seriously affected.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Thu 14, 2019 12:11 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Consider that the 1L6 filament temperature is maintained by a current flowing
through it. However there are other currents flowing from it.

These currents are due to the positive voltages shown in the
inset below, and are proportional to the overall B+ rail.



So, with the silicon replacing the selenium mod, are all the B+ voltages correct ?
Attachment:
ONE L 6.JPG
ONE L 6.JPG [ 45.02 KiB | Viewed 371 times ]

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Thu 14, 2019 2:16 pm 
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radiotechnician wrote:
Consider that the 1L6 filament temperature is maintained by a current flowing
through it. However there are other currents flowing from it.

These currents are due to the positive voltages shown in the
inset below, and are proportional to the overall B+ rail.



So, with the silicon replacing the selenium mod, are all the B+ voltages correct ?
Attachment:
ONE L 6.JPG


That's a good question. I was so focused on the filament voltages I haven't checked B+. I'm going to take the radio apart again today and see about dropping the 1L6 filament voltage a tad below 1.4 volts. I'll check B+ at the same time.

Some H500 schematics show 90 volts for B+, some show 94 volts. I assume that's while operating on 117 VAC?

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Thu 14, 2019 3:34 pm 
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1st 1U4 plate voltage is 85.8 volts, 1L6 is 95.1, and 2nd 1U4 is 95.1. B+ at the 1st electrolytic is 104.1 volts. So, it looks like B+ is relatively close.

For the record - I did change the 100 ohm resistor that was reading 112.8 ohms across the 1L6 filament to a new one that reads 98.1 ohms. With the 50 ohm resistor in series with the 1N4007, 1L6 filament voltage dropped and now reads 1.366 volts at 120 VAC. Needless to say, the 1L6 no longer picks up much on the high bands. It won't even pick up my signal generator at 18mHz.

But my 1R5 works ok on the high bands at that filament voltage. I guess I'll have to give up on the 1L6 until I luck into a good replacement or breakdown and buy a solidstate replacement.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Thu 14, 2019 9:38 pm 
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If your 1R5 works on the higher bands it may well be a 1L6 in disguise. Some 1L6's were marked as 1R5's, I think this was due to a shortage of 1R5's at one time.

DM


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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Fri 15, 2019 1:21 am 
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Location: Powell River BC Canada
One trick is to have a fresh 1.5 volt battery (I used D cell)
available to connect across the 1L6 filament pins with the correct polarity.

This is a good diagnostic trick. Also the radios came with a spare set of tubes
held in clips inside.

This always ensured that the tube filament got the correct voltage, no matter
what the rest of the radio, or the line voltage was doing.

Modern restorers have access to capacitors that are far better (lower ESR)
than the ones used in the original sets. The radio also ran on DC,
through the original selenium forward resistance with no capacitors to
provide voltage up-kiting.
Attachment:
Zenith H500 ran on DC too.JPG
Zenith H500 ran on DC too.JPG [ 68.18 KiB | Viewed 293 times ]




There was little DC shore power about in the 60s, however a number
of our customers were sailors on ships that had only DC in crew quarters.

German sets avoided the issue totally by furnishing a single cell storage cell
(accumulator) to power the filaments, and especially the DK 96 tube.
Attachment:
deac.jpg
deac.jpg [ 144.67 KiB | Viewed 293 times ]

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Fri 15, 2019 10:16 pm 
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radiotechnician wrote:
One trick is to have a fresh 1.5 volt battery (I used D cell)
available to connect across the 1L6 filament pins with the correct polarity.

This is a good diagnostic trick. Also the radios came with a spare set of tubes
held in clips inside.

This always ensured that the tube filament got the correct voltage, no matter
what the rest of the radio, or the line voltage was doing.



I'm not sure I understand how you hooked up a D cell. Did you disconnect the 100 ohm resistor that is across the 1L6 filament and use that resistor to keep power to the rest of the tubes?

As far as running the radio on 117 VDC - I do have a homemade bridge rectifier/filter caps, etc. that I plug into my variac/isolation transformer setup to get an adjustable high voltage DC power supply. I could try that on this radio, but I suspect the result would be the same. Looks like I just have a weak 1L6, even though my tube tester doesn't think so.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Fri 15, 2019 10:41 pm 
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devilsmist wrote:
If your 1R5 works on the higher bands it may well be a 1L6 in disguise. Some 1L6's were marked as 1R5's, I think this was due to a shortage of 1R5's at one time.

DM


Now that's an interesting thought. But I'm usually not that lucky! I can see enough of their insides to see they are constructed a little differently. It looks like from what is left of the printing, they were mfg'd by two different companies. The only tube type markings for the 1R5 are on the AES box it came in. The original 1L6 is clearly marked.

The only reason I'm hung up on the 1L6 is because when I first got this radio it worked sort of mediocre on the high bands the first 5 minutes it was turned on. Then it would instantly come alive (louder) with all kinds of high band stations. Within a few days that great reception started coming and going even after 5 minutes. Finally it would never get better. I wasn't sure what the problem was. But since I had a spare 1R5 I stuck it in and the radio was consistently very good on the high bands. But I could tell it still wasn't quite as good as the original 1L6 was when it was really working. Even now the 1L6 seems to work just fine on the low bands.

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 Post subject: Re: Can a 1L6 tube be rejuvenated?
PostPosted: Nov Sat 16, 2019 2:22 am 
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I'm not sure I understand how you hooked up a D cell. Did you disconnect the 100 ohm resistor that is across the 1L6 filament and use that resistor to keep power to the rest of the tubes?


No, the diagnostic jig was just a D cell with two leads attached, with a couple
of slip proof sleeved alligator clips.

It was clipped across the filament pins in the set, 100 ohm resistor was left
as is. Then the set was operated on whatever current source chosen,
i.e. battery pack, 110 or 220 AC or DC, or on a Variac . Gave you the
knowledge that the 1L6 wasn't causing the set to fail because of filament
voltage.

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