Hi all,

This is probably going to be seen as a dumb question, but I am new to working with eye tubes. I have an Eico 950A with a 1629 eye indicator tube. The heater warms up like normal when the unit is powered up, but as far as I can see, there is ABSOLUTELY NO green fluorescence visible in the eye. Now, obviously I would assume this means it is bad. I know these tubes don't have a terribly long lifespan before the phosphor is worn out. But I am just confirming that is the case and that there isn't something else wrong in the tester that would cause NO glow. If it was simply a very very weak glow, I would definitely just assume it was a tired tube. But I can't distinguish any green whatsoever.

Again, apologies for what is probably an obvious question and answer, but before I spend on a tube, I wanted to confirm there isn't something else going on.

Side note: what is up with 1629 prices??? I can find them for about $20 from a few sellers around the internet, but on eBay they are far more than even that! Whenever I research them I find people talking about the 1629 being a good replacement (with an adapter) for the "harder to find and more expensive" tubes like 6E5 and 6U5. But from my brief searching, those tubes are actually far cheaper and more readily available than 1629. Am I missing something? What happened?

Thanks in advance all!

If you have B+ on the target and the cathode is ground or B-, and the filament is lit, it should glow. Try testing it in a tube tester just to verify, but they can get so dim you can barely make out any green color even in a darkened room.

At one time, there were what seemed like millions of military surplus 1629's available for next to nothing, which is probably why several test equipment manufacturers in the 50's and 60's built units having a 12 volt filament supply so they could use cheap readily available tubes.

In more recent years, plenty of them have been used up by people making adapters to replace the usual 6 volt types, which has reduced the supply and driven the price up.

Thanks Mr. Detrola!

That makes sense regarding the supply. That's too bad that they have ironically now gotten HARDER to find. I take it there's not a common replacement for it (besides the other mentioned tubes with an adapter to go the other way)?

I have tested it on my Superior TV-10 tester. It tested good, though not exactly pegging the needle. But still no visible glow from it. So I'm guessing it's a dud. Bummer. Guess I'll start looking for a deal on a replacement!

EICO used a short version of the 1629 in some models - the usual size won't fit...

Most of the round target eye tubes lasted around 1,000 to 2,000 hours before they got too dim to see in normal room light. Nobody ever uses a capacitor tester that long, so the tubes would ordinarily last the life of the tester. Sadly that has changed in recent years. People are taking the good tubes out of testers they find in yard and estate sales, sticking their duds back in, then dumping them in "the bay" as-is unable to test. Happens with a lot of now-costly tubes like 12AX7s and 6DJ8s in old test gear, not just eye tubes.

I would not conclude the tube is bad from a tube tester, unless it is one of the fancier Hickok, Weston, or other models that actually lights the target. Many generic tube testers do not have the power supply voltages to operate eye tubes or VR tubes for that matter. What the simpler testers do is test the triode sections of eye tubes for emission or gain; they do not actually light the tubes up.

To test, use a DC voltmeter (DMM, VOM, VTVM). The target voltage on a 1629 is on pin 4 and the cathode is on pin 8. You should be getting about 250 VDC between those two points. The plate of the triode section is on pin 3. It is fed through a high value resistor (typically 1 megohm) so the voltage will be lower depending on how much current the triode section is drawing and the meter you use to take the reading. The lower this voltage is with respect to the target, the wider the shadow that will be displayed. The control grid is on pin 5; this will have a mix of AC and DC on it also at high impedance. The more positive this voltage goes with respect to the cathode, the wider the shadow. But for the purposes of just getting the target to light up, it shouldn't matter what the triode grid and plate voltages are.

The only other thing you could look at is the heater voltage, which should be approximately 12.6 VAC. About the only way this could be off is if there's a bad solder joint or if somebody replaced the power transformer with a 6.3-volt one for some reason, but you never know until you check. If the heater voltage more than a volt or two low, the heater might glow but not have enough emission to light the target.

Chris, thanks for the additional info!

I ran home just now to do a little checking from the info some folks gave me, and to test the stuff you suggested.

I finally determined that it is lighting but VERY weak. I had been cupping my hand around the tube to try to see if there was anything but couldn't and I finally moved the whole apparatus to a dark shelf under the bench, and that, combined with cupping my hand, I could finally see a very weak green glow. I think the room light was getting into the tube from behind and washing it out before, since I have it out of the chassis.

So the tube is bad. I have ordered a new one.

Another mystery solved, thanks to you wizards on ARF. I am very new to this hobby so I have zillions of questions and it's so nice to have a friendly forum that is chock-full of so much knowledge and experience to help me out!

Thanks!

You are not looking enough for a good used one, here is one that sold for $4.25
https://www.ebay.com/itm/275214393274?h ... 7675.l2557

Bob Dobush sells 'em for $12 new and $6 used-tests-good. Go to http://findatube.com/ and bookmark it. In my experience over the years, he has proven to be the best supplier of vacuum tubes. His prices tend to be better across the board than any other supplier.

Tom

Oh, that's a great resource Tom, thanks! I will definitely bookmark that into my other resources for vintage restoration...

Well, things continue to get more interesting...I got a NOS tube and installed it and got very similar results. Very dimly lit and the edges of the shadow are not well defined. My other eye tube gear have very sharp crisp shadows. So I'm wondering if perhaps something else is going on. I have replaced all the capacitors in the device, and reflowed most of the solder joints just to be sure.


I checked the heater voltage and it's actually a bit higher than 12.6 so that should not be the problem I don't think...


When I am back in the shop I'll check these other voltages and see if anything jumps out at me.

Any other things to check that could be causing this? Obviously even a NOS tube is very old at this point and so could be weak too, but I do find it interesting that it behaves almost identically to the old tube that came in the unit...

EDIT: I unfortunately don't have any other devices that use a 1629 tube that I could test either the original, or my NOS tube in to see if they behave differently. Everything else I have with eye tubes uses the common 6 volt variants. Is there a good way to test the actual glow on a cheaper tube tester like my Superior TV-10?

Thanks again everyone for your help!

Most of the Eico 950 and 950Bs sold were kits so it is possible something was miswired from day 1. You can never assume a kit worked perfectly before you got it. On the other hand, full documentation is available that tells you every step along the way of putting it together, so kit equipment is usually easier to fix and calibrate than anything else. This is why I suggested checking the voltages on the tube pins first.

If you want to test an eye tube just to see how bright the target is, all you need is four clip leads and a DC power supply that puts out 200-250 VDC. If you do not have a bench supply like that, you can "steal" the voltage from a transformer operated radio chassis. With a 1629 you'd need 12 volts or so of heater voltage which can come from a small filament transformer. It would be connected to pins 2 and 7. If it was a 6-volt tube you could probably "steal" the heater voltage from the radio chassis too. The target on pin 4 gets connected to the +200 to +250 volt B+ point in the radio, the cathode on pin 8 gets connected to the B-/negative/circuit common point. You will not see a shadow nor be able to test the triode operation this way but the target should illuminate. The triode section can be tested on your Superior tube tester; it it tests good it will probably produce a good shadow at the proper grid voltages in circuit.

Well, unless I'm just a complete idiot (which is definitely possible) something is definitely wrong here. I'm only reading about 44 VDC across those two.

I am working my way through checking all the connections and components to see if anything is wired wrong, but so far all looks good...

Very interesting...I'll keep you posted if I find anything. Any particular places to look for a cause of that voltage being so low?

I got it fixed! I traced that line back to the 6X5 tube, and found a resistor there that was supposed to be 100K and was reading 1.2M! That would sure do it...

After replacing that, both the original tube it came with, and the NOS tube are nice and strong! Guess I have a spare now, and that's never a bad thing.

Thanks everyone for your help!

I went looking for an 1629 Eye Tube last month for my 950. $5 and $9 on Ebay, but I really wasn't sure if there bright was like my bright. I checked Bob's tubes and he has them. Yup. $12 new. Ordered 2 along with other tubes I needed. They came in military Army/Navy box's. IMO a very fair price. Now I have an eye tube for my Signal Tracer also which was to dim. Looking forward to see how it works in that one. I've been using Bob's for years now and never had 1 problem with his products.

Freeman

This thread is a good example of why you should troubleshoot instead of just changing parts.

RRM

Right you are, Retired Radio Man! It was a learning experience for me as I am still new to this hobby.

But, to be honest, I don't mind having bought the tube and not needing it. With the eye tubes getting harder to find, I don't mind at all having a spare one on hand for testing future gear or replacing a tired tube in the future!