|I bought mine on a trip when the purchase of a dry one fell through. This was the only other one available, and this was the only other one available. If I knew then what I know now, I wouldn't have even considered the dry one.|
This elegant piece of kit started being produced back in 1961. Heathkit stopped making it in 1993 only because they ceased operations. It was then taken up, essentially unchanged, by both Vectronics and MFJ (now Vectronics' offering has been discontinued in favour of them carrying MFJ's).
An oil-filled paint can may seem chintzy to some, but it's actually a very elegant design. An internal sleeve around the large non-inductive resistor creates a convective flow that brings cool oil from the bottom of the can up. The resistor is held by clips made of silver plated brass so as to offer good conductivity without being magnetic. A very elegant design. Determining whether it is safe to continue transmitting is a simple matter just feeling how far down the hot area is. If just the top couple inches is hot, then you're good to keep going. When it gets close to the bottom, time to let it cool down. This convective flow makes it important, though, that the unit is never operated when the oil is low. The oil should be within .75" from the top of the can. It must never be below the top of the sleeve tube around the resistor or else the sleeve just becomes an insulator keeping the hot oil in place.
Other amateurs are daunted by the idea of trying to find proper transformer oil to fill it with. That was my issue. While real transformer oil is considered best, it's perfectly acceptable to fill it with mineral oil. I use medical grade light mineral oil, which I find is ideal for good convective flow. The derating curve suggests less transmit time is possible at high power when using mineral oil, but that has not been my experience. I have operated almost continuously for ten minutes at close to 900 watts. But then today's medical/food grade mineral oil is far more refined now than it was in 1961 when the derating curve was made. I first thought it would be difficult to source enough, but I found four litre jugs of medical grade mineral oil at a local farm animal supply store (Feeds & Needs in Nova Scotia) with little trouble. Veterinary supply outfits, or even veterinarians themselves that cater to larger animals also carry the jugs.
Make sure when you buy one used that you ensure the resistor is still ok. The fellow I bought mine from operated it 3/4 full of oil, which, as mentioned above, inhibited the convective flow and overheated the resistor. The resistance when I got it home read close to 80 ohms. While that was frustrating, I was able to find a replacement resistor with little trouble. Original Carborundum resistors are available on eBay, and new manufacture ones are available from MFJ and from Kanthal Globar.
I highly recommend a Cantenna. At the kilowatt level a lot of other dummy loads measure transmit time in the seconds. Either that or they cost a (not so) small fortune.