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Miller 330 A/BP build date

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  • Miller 330 A/BP build date

    Just acquired one of these monsters, and have a few questions. It is a
    Miller 330A /BP
    Serial R365015
    Checking the Miller site for build date did not turn up this serial number.
    Also, there do not seem to be any breakers or fuses on this machine, other than a 10A slow blow fuse on the left front panel. Everything seems to work as it should but the lack of breakers seems rather odd.
    It must be fairly old as the main power rectifiers and the smaller ones as well, are selenium.
    Last edited by 07wingnut; 04-23-2021, 12:10 AM.

  • #2
    Just going by this
    I'm guessing yours was made in 1966.
    I've got one from 1978. Great machines.


    • #3
      Mine is a 74. Should not be a “slow blow” fuse on the front panel. Might have been replaced by something from the hardware store somewhere along the line. The right fuses are hard to find, but I found a NOS box of them a couple years ago on eBay. It should be yellow and black. I’m not sure what can happen if you do blow that fuse, but I was advised to not use slow blow and get the correct one after I did some work on mine.


      • #4
        Here is a pic of the front panel. The fuse mentioned is on the left hand side.
        I know that in some of the older generators that have selenium rectifiers, the advice is to replace soonest, as they seem to die with age and use, and in the case of generators, usually take an unobtanium part with them. Is this also the case for these welders? Click image for larger version

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        • #5
          Just my two cents worth.....Selenium rectifiers should be replaced. Even if they are still working, it's just a matter of time. And as they age, they develop more internal resistance and become less efficient, delivering less welding current. Not to mention the smell of the rather copious amounts of smoke they make when they blow, which can linger for several days in a closed area--been there with industrial equipment--lab area stunk for days, and I hear tell it is NOT good to breath the smoke they make when they go. I replace them in any equipment I work on. In electronic gear, you often have to change the circuit design just a bit because the forward conducting resistance of the silicons is less, and you can generate too high a voltage from the power supply for some old vacuum tube circuits. I would suspect that is not a big deal for a welder, but can't say for sure. I can't think of any reason it would matter in that application. Without seeing a schematic diagram, it's hard to tell whether there are any other parts that they could take out. I suppose if they shorted, and stayed shorted, they could conceivably toast the transformer. But, I suspect with the robustness of the transformer in that machine, it would probably vaporize any shorted selenium in a rather exciting few-hundred-milliseconds-long light show if they fully shorted, following which they would be an open circuit. But, again, no direct experience, so I really can't say.

          Problem is, replacing them is going to take some effort, because you are going to have to either fabricate mounting and heat sinks for silicon diodes, or find a junk machine to cannibalize for the mounting parts. And you will need to be very careful about what mounting parts have to be insulated from other parts. I looked at the oldest manual I could find here on the website, and it had silicon diodes. The good news is they way they designed the mounting for the silicon diodes in the later machines, all four are the same polarity, so you can use four of the same part number.

          Overall though, the machine is probably well worth the effort. A fine, true classic. Nothing else like it.


          • #6
            That looks like the correct fuse to be honest, just by the markings and color. But I don’t even know what a selenium rectifier is but it sounds either expensive or dangerous.

            When you helped me with my helga a while back, Wayne, the Miller service tech I talked to on the phone when I was trying to track down those relays recommended I replace the diode bridge based strictly on its age. But alas, I did not because it’s working fine. I figured I’d cross that “bridge” when it fails....see what I did there?

            Looks like a clean machine man, I like the fancy panel meters. Custom.


            • #7
              Click image for larger version

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ID:	614688 I figured I’d cross that “bridge” when it fails....see what I did there? --Ahh-a literary master right here in the welding world! :-)

              Actually, selenium rectifiers weren't very expensive--- except for the fact that I would be surprised if anyone makes them any more. They were the first "solid state" step into electronics/electrical stuff, I think--and they were better than the old vacuum tube or mercury vapor rectifiers, but clearly something that had to be only a step on the path to a "better way". I can't think of a single reason to want or make one now. Here's a couple of pictures.

              Miller's recommendation to replace the bridge in your machine is interesting, since your main rectifiers are silicon diodes that are generally very long lived--I haven't replaced many over the years, and I've worked on a lot of old electronics stuff.

              I just took a look at a manual from Aug '73, thinking it's the one for your machine, Ryan, and while it has silicon for the main welding power, the bridges that supply DC for the control circuits, SR3 and SR4, are selenium--that's probably what he was talking about. Considering the transformer windings that supply AC to them are nowhere near as robust as the main welding windings, they may be worried about toasting them if the seleniums short out. Probably really would be a good idea to get a couple of modern bridge rectifiers and replace them. Only cost a couple of bucks. One mounting hole wherever you can find a nice big section of sheet metal as a heat sink, and some silicon heat-transfer grease, and you'll be in business. If only the control circuit windings in the transformer got damaged during a failure, you could just wire in another transformer and it would be no big deal. However, if those windings happen to get hot enough during the failure to affect insulation on other windings, you might end up having Helga's funeral. I rather doubt that would happen, but if it were mine, I'd get them out of there--would have mentioned that to you before if I had noticed them.

              Obviously, the same goes for the machine this thread is about--I would change them. I just did a quick look at's way overkill, but since they are all so cheap, I'd use a 600V 20 or 25 amp bridge.

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              • #8
                Its running and working?


                • #9
                  Everything on the machine seems to work fine, except for not being able to test the remote with foot pedal, since it came with no foot pedal. At the price these are selling, I may have to build my own. The major part, a 16 ohm 150 watt slide resistor is not that expensive.
                  Concerning the selenium rectifiers, yes the smaller ones are easily replaced at a very reasonable price. The main power rectifier however is also selenium, and where do you find a silicon bridge rectifier or individual power diodes that handle that kind of current and voltage and a reasonable price.


                  • #10
                    Actually, they're not that expensive--I was surprised. You would have to buy some really good heat sinks, which would cost more than the diodes, and perhaps an extra fan to blow on them, but not an insurmountable problem. This was the first one that popped up....might be able to find stud mount ones also.

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                    • #11
                      Following closely as I want to take good care of my 330 A/BP. Even though mine is not nearly as pretty outside she still welds beautifully!

                      Are these the rectifiers you are talking about, in the top of the second photo?
                      I am separated from my machine by about 1,500 miles but I have pictures (with all her clothes off ) and this was all I could find that resembles the posted pictures.
                      Attached Files


                      • #12
                        Having trouble getting pictures to upload!
                        Attached Files


                        • #13
                          Yep, those are the ones.


                          • #14


                            • #15
                              I will take a picture of mine and post them today