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Thread: My gas project

  1. #1
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    Default My gas project

    Hello to the people of this board.

    I recently received a lexel 88a head, what I knew from it was that it had been unused for several years but the tube itself should be new.

    After looking around it and checking vital parts I hooked it into my PSU, ramping idle voltage slowly upwards from 180V and trying to ignite the tube with it's own igniter. Igniter itself worked well but did not manage to get any kind of permanent glow into the tube even at 310V idle voltage.

    I increased filament voltage to about 4.3volts and tried again, no luck. Then I wanted to try giving some HV therapy for it, taking variac and microwave oven transformer. Removing anode wire from igniter, I began increasin the voltage across tube while filament was glowing, at around 800-900volts of rectified current the tube struck without any help. Voltage dropped around 250volts across tube and it draw 0.25 - 1.2amps depending on how much I let it have power via variac, voltage dropped some volts as I increased current.

    So far I have kept it about 3 hours on the ~0.4A load via MOT(any more current and the transformer will literally boil on longer runs), trying to get the pressure down, I have not used the magnet while doing this.

    As I don't have much experience on any larger ion lasers than alc60, is this safe to be continued more and hope it will take the pressure down eventually? From what I have read some some pages about reviving old tubes, really scares me, like ovening the tube(might eventually go that far, but not yet). Laserfaq suggests to using of oudin coil(?), but I believe my variac+mot delivers the same effect?

    Igniter nicely flashes across the tube, just not enough yet for glow.

    ps. The PSU I am using is not a lexel PSU, it's a home-made current regulated SMPS, this might sound the fault of failure to get the tube lasing, but it has actually ran the alc60 without effort and is made to handle lot more current. So atleast it should be not any type of interlock fault.

    - Anssi

  2. #2
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    stop with the "pseduo" gas pressure treatment, lexels DONT need that because of ballast tank. THEY DO NOT GO HIGH PRESSURE. every hour you run that cathode at .4 amps discharge is causing it to potentially sag. cathodes need a minimum discharge current in shortframe tubes like lexel forget what you know about ALCs, this is whole different operating regime, and the tube has a slightly positive I-V curve, unlike a ALC which is negitive. your supply may be under powered or unable to handle the positive IV curve.

    tube voltage is 165 V nominal, minimum current about 6 amps once lit, but low currents are hard on the tube, preferably about 10A minimum to light, maybe higher if it hasnt ran for a while. normal run can be 15A typical 18 max amps. cathode is 3.2 @ say 24-28 amps. your supply voltage or minimum current is probably too low. You need about 280V open circuit minimum to light the tube,. A ALC sized supply will not generate the 3-5 kilowatts needed to run a 88. are you using the boosted ignitor in the head? does the the ignitor get a direct feed to B+ and B- like it should ? if not it will be too weak to light the tube. use the lexel ignitor and hook it up per the 88 schematic in sam's laser faq. alc/omni ignitors are too weak to start the emission spot on a lexel cathode. If the caps on the lexel ignitor are not fully charged, it wont light either. magnet is powered and coil in magent is good? NO MAGNET = no ignite.

    ALC = 106 V tube times 10 A max = 1060 watts Lexel = 165 * 15 = 2475 watts
    btw, my credentials for this reply? I own two 88s and have access to the 1/3 or so of the lexel IP/ ware house that camb lasers didnt buy. I know what I'm saying!

    Steve Roberts
    Last edited by mixedgas; 08-29-2008 at 08:30.

  3. #3
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    Default

    Thanks for your words Steve, im sure you know more from these than I ever will.

    I'm little shamed to say that the fault indeed was in my PSU, minor thing, now being fixed up. I took it apart and used only one module witch I knew for sure is working. I got a stable glow inside tube around 6-7A @ 156V, laser light came out around at 9-10A @ 160V and I were only able to go up to 163V before the current limiter of one module kicked in.

    I'm using the original igniter in the head, it's drawing its power from the tube feed direcly, magnet has it's own supply.

    And just an little offtopic, my old alc60 is apparently so used up that it required much as 127V when current was at 10A, this certainly will not make it better, but for brief testing.

    I can later on post some pictures of the lexel when I have the psu running well.

  4. #4
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    It would appear that this one is single lined, but I can't say if it's either 488 or 514 line, not clearly blue or green, something between.
    I tried prism to separate possible wavelenghts but I only got the one itself.

    Though I have not yet ran the tube with much of current, 12-13amps tops with 167V.

    How precise the magnet current would have to be, does it matter if it has ripple voltage or should it be well regulated?

    http://koti.mbnet.fi/anssiw/l88.jpg
    Appears too blue on my screen atleast, could be my eyes aswell tho.

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    Cool

    Looks like 488 nm to me.

    I'm rather surprised you can fire that tube with your modified PSU. It doesn't sound like it's large enough for a Lexel 88, but from the picture you posted I guess it's working for now.

    For sure you'll want to pay attention to Steve's suggestions. He wrote the majority of the material in Sam's Laser FAQ under the Ion Laser heading.

    Adam

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    Yep. That's 488!

    Not very usefull as a blue in an RGB rig. It's way too close to green.

    James.

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    re 488 only or mostly 488, I can think of the following:

    either your optics are dirty, your resonator is mistuned, or you have a prism line selector as the rear mirror. A very low pressure old tube would lase only 488 as well. You need to run it up to the full tube current of 18 amps and graph me a graph of tube voltage vs current at every 5 amps. you need ~200 Vdc across the magnet, if you have that, you lase. Argon is not that field sensitive, and the tube voltage climbs dramatically with no or weak magnet, so you do not have a magnet fault.

    Steve

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    I'm quiet sure that it's just the mirrors that are made for 488nm, atleast I couldn't align mirrors any better, optics are clear, no prism, tube should be relatively new and magnet is running at 170V(measured ~54ohms across).

    I can't give yet very accurate graph about volt vs. current, I just yesterday ordered some analog meters from ebay. Took few more photos with current roughly 14-15amps to the laser and my current lashup of powersupply. I'm working to make a casing for it now as it's not safe to touch now at all.

    http://koti.mbnet.fi/anssiw/IMG_1290.jpg
    http://koti.mbnet.fi/anssiw/IMG_1291.jpg

  9. #9
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    whats the part numbers on the side of the optics, written in pencil?

    should be something like 540-xxx-xx

    need numbers off both Hr and OC

    Steve

  10. #10
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    sounds like you have the magnet at tube voltage, can you get it up to rectified line? ie at least 220V ? 200 is about minimum. you may need more filter caps and a seperate rectifier if your pulling your line that low.

    Steve

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