Tubes in broken spotwelder & other questions

Well, I'm kinda shocked that it looks so modern and in good condition, yet with toobs.

I see your point, though -- some older toob radios have passed through my hands, and I review and revise the safety measures as necessary.

I've been following this thread for a time now and for what it is worth....

Solid state devices have it all over vacuum (and gas) tubes. I worked in a pharmaceutical plant for several years with antiquated packaging lines... Where ever we could (and that meant everywhere) we'd replace those 2D21's with Thyristors (SCR's) and the gas regulators with Zener diodes or other SS shunt regulators, and the 5U4's with simple 1N4000 series diodes.

It was often lots less work to do it that way then try to find working vacuum tubes from some antique parts store.

Don't get me wrong.. if you're doing it because you like cheerful glowing vacuum tubes, or convinced yourself that harmonic distortion sounds better than negligible distortion, by all means go for it.

I get it. I like varnished oak with brass corners and braces and large fancy gear trains turning potentiometers etc.. But if you actually want to use the silly thing, put in solid state components, or design it new from the ground up. (and you may never have to mess with it again - something I hate; doing it once is fun, repairing it sucks big time IMO)

And BTW we had one small bench top cap discharge spot welder that used a HV source (all new SS) to weld our thermocouples with... Piece of crap IMO, we would use a low voltage spot welder (cost ~$50 from Harbor Freight) to do the same job, safely and reliably.

I was thinking it looks rather "new" to be packed with tubes and the constuction is good. What company made it?

How much do you have to change? Can you just plug in semiconductor modules where the tubes used to be, or do you have to make other circuit modifications?

I was thinking the exact same thing. It really doesn't look terribly old. It has plastic stuff like cable ties and a small circuit breaker.

Are you set on making this work or just having a working spot welder?

Most of the changes were relatively minimal. Something like a 5U4 already has plug-in solid state replacements available off the shelf.

The thyristor behaves pretty much the same as a thyratron so replacement was a matter of checking out the firing circuit and maybe adding a diode to prevent the negative voltage the thyratron needs in the off state from reaching the gate ... on a case by case basis. Mounting and heatsinking were other considerations.

We were paying something like $50 for the 2D21's and sourcing and stocking them was a problem so it paid to spend a little time tinkering to make it work. AND there's nothing like having a packaging line go down at midnight to motivate people to find an alternative...

In the case of gas shunt regulators they were mainly relatively low current devices used in vacuum tube biasing supplies and back in the day we could often find zeners with similar specifications or by stringing several in series to dissipate the power at the necessary voltages. Something like 1-5 watts in the case of an 0A2 (if my memory serves me) And unlike the 0A2, ambient light doesn't affect the firing voltage...

PS

In the case of 5U4 and other rectifiers, I've often wondered what the warm-up characteristics might have done to prolong the life of other vacuum tubes in the circuits.

Back in the day, especially the early days, it was understood that you didn't want to apply plate voltage until the cathodes were up to temperature. The theory behind it was that while the cathodes were warming up from a cold start there would be uneven heating in the case of indirectly heated cathodes (and coated filaments) and the strong electrostatic field would cause bits of the emissive coatings to be ripped from the cathodes. Or so we were told...

This was especially true of large transmitting tubes because of the physical size, high voltages and powers present - and no doubt the cost of replacements. There'd often be a timed relay that would apply plate power after the tubes had a chance to heat.

Which makes me wonder about microwave ovens with their 2KV, 1000 watt vacuum tubes... Maybe coatings or cathode structures have improved?

The 0A2's would be the easiest. Even if you couldn't find a zener, it'd be pretty easy to make a semiconductor shunt regulator that would work fine. As long as someone wasn't using one in a Really Clever relaxation oscillator, you could just string together 6.3V zeners on a board until you had the right voltage.

I think that's mostly an issue with coated cathodes. Microwave oven tubes probably use thoriated tungsten filament cathodes -- they're structurally easy, cheap (like building a light bulb), and rugged as all hell. Especially if you design the tube for that service it should work just fine.

Unitek. They are still making spot welders but have been bought up by someone and no longer have any info available for their older machines. Eric