ALC68 strip down & clean - Help please

[stanwax]

Hi Jim

any probs with the PCAOM - you know my number

As for adding the red - go for it. The way you suggested will work you just need to check what orientation of the diode you will need to get the PCAOM to work best - remember they need a polarised beam but if that beam is not entering rotated correctly you will loose a lot of red. Work that out first then make up some means of mounting it.

Rob

[buffo]

Well done Jim! A fantastic job. Have you measured the output power yet? Also, what's the outlet air temperature?

I have an HGM5 with a 903 tube in it which is just slightly smaller than the 68B tube you have. I get about a watt (all lines) at 22-23 amps of tube current, and the outlet air temperature hovers around 145 degrees F. I can push the tube to a max of 29 amps, but I don't have a means to measure the power output at that level, and besides, I don't want to kill the tube!

Just curious what sort of numbers you're getting on that 68B... (In theory, you could get up to 3 watts at 32 amps of tube current for short periods, but you need to watch the temperature or you'll cook the thing.) I've been told that 170 degrees outlet temperature is a good number for redline.

With regard to mixing in the red... If you have a decent driver for the red that supports analog modulation, you may want to modulate the diode directly vs running it through the PCAOM. Reason being, you'll loose a good bit of power running the red through the PCAOM (10 to 15%), and you're probably going to want all the red you can get to balance your color palette.

Just a thought...

Adam

[James Lehman]

I bet you were glad to see that light blue beam!

That has to feel good!

You're in a select group of people now. You've done that!

James.

[smogthemog]

Thank you James, thank you Adam. And thank you Rob for your advice and telephone chats

I will get some figures the next time I lamp it up Adam.

---

One thing I have noticed when I do fire it up, with the top off, is that when the timed 'second' relay cuts in on the PSU and before the EHT igniter circuit has started to spark, there is purple plasma running from the Cathode tickler to somewhere in the middle of the tube, can't see where it goes too as it's under the mag coils, but it doesnt continue right the way through to the Anode tickler until the igniter fires it up properly and it then lases?
Is this usual?

Also, Is the spark gap critical? I have noticed if it is too close then it just won't ignite, too far away and it again struggles to ignite? somewhere in between and it ignites rather easily. Why is this?

Thanks again people for all your help on this - it's certainly been a learning curve

[smogthemog]

Oooh, just one other thing....

I measured the voltage output from the transformer last night, just to see what the volts drop was under load and was took aback by finding it was 215v under load and 230v off load (using 15m of 4mm cable at the moment, current was 34A full load)

I was expecting 110v not 230v with it being an American laser?
Does this mean I can remove the transformer totally? Why is it being used at all? I guess isolation is a good thing but the volts drop through it is a little worrying.

Strange

[QUAZAR]

He originally used ours with a 110v tranforrmer. This was mainly to protect PSU resevoir caps which were omly about 160 - 200v. These were changed for 300v replacements. The TX was weighed in for scrap. At current rates for copper, this would probably pay for new caps.

[stanwax]

I measured the voltage output from the transformer last night, just to see what the volts drop was under load and was took aback by finding it was 215v under load and 230v off load (using 15m of 4mm cable at the moment, current was 34A full load)

Jim I think you may have some volt drop in your cable. I used to run the 168 off 6mm cable but it really started to stall out at >30A tube. Now I have my 10mm feed it cranks up just fine. Get yerself some 6mm or better still 10mm then volt drop wont be an issue.

do some ohms law on it... you are loosing 15v @34A over 15m thats 15/34 = 0.44ohms for the 15metres. divide this by 15 and you are getting 0.029 ohms per metre. Not a lot but at that current.....

Rob

[smogthemog]

I think I am loosing lots through the transformer Rob to be honest. It's not very big at all. I am going to try it without it in line tonight. The caps are all 350's so it should be OK. It actually says on the plate it's 220v just not noticed it before.

[mixedgas]

Thats cleaner inside then a FACTORY 68, nice job! that one brewster stem is glowin a bit, may need another clean on that window. Welcome to the club, once you have a aligned a cavity, about the only real techie challenge left is building a tube from scratch or landing a airplane.
Therefore, be happy!. Plus it looks to me that you got it without the hene, which means you can see the little bit of amber light in the plasma, against the blue, which most of us mere mortals can't! I need to use a amber theatrical gel or stick in a white optic to do that! keep the current down, keep the cathode voltage correct, run it at least once a month, and keep the hot air from recircing back into the laser, and it will last a long , long, time.

Steve

[mixedgas]

begin quote
One thing I have noticed when I do fire it up, with the top off, is that when the timed 'second' relay cuts in on the PSU and before the EHT igniter circuit has started to spark, there is purple plasma running from the Cathode tickler to somewhere in the middle of the tube, can't see where it goes too as it's under the mag coils, but it doesnt continue right the way through to the Anode tickler until the igniter fires it up properly and it then lases?
Is this usual?

end quote

yeah, the tickler is at 800V and the plasma flows down the bore to the molyB wires in the gas return path, thats normal, the wires short out the possible plasma path in the return path, forcing the plasma down the main bore.

begin quote
Also, Is the spark gap critical? I have noticed if it is too close then it just won't ignite, too far away and it again struggles to ignite? somewhere in between and it ignites rather easily. Why is this?

end quote

The ignitor runs at resonace, too close and a small hot spark fires over and damps it, too far and its too weak to jump the air gap. They did that to avoid puncturing the metal ceramic seals. It did that on my model 5 too. Worse case, check the big lytic caps on the ignite board, the plastic ones had a habit of puncturing and breaking down at hv, yet showing good on a cap checker. If it gets a weak coil, you can always add a torroid and a blocking diode to the board you have and go series ignite.



Steve