Just curious about these - how were they used? All that is known is they came from an audio lab. About 200 watt, 2.8 ohm vitreous resistors. 200watt estimated, by me, from surface area scaling of 2.5 and 6W ones, these are 210 mm long 35mm diameter. Some sort of multiple series and parallel for 2 ohm etc or with L and C for speaker simulation ?
To me 2.8 ohms is an odd value. I have DALE 8 ohm 250 watt, non inductive winding resistors. With four of them, I have 8 ohms at 250 watts, or 4 ohms at 500 watts. If I had felt the need I could have done some more complicated switching and also had 16 ohms at 500 watts.
They are very useful as you can hook them up as a dummy load and view on your scope, and run rather large amp right up to clipping, all while not having to listen to any of it. It also makes it easy to measure power at clipping, as long as you also monitor your mains voltage. (At clipping, you have to watch the voltage drop at your bench outlet, as you might be feeding the amp a lower mains voltage than it is specified for, which will reduce power output. I have a VARIAC to correct for this.)
I have seen one manufactured unit that used 7.87 ohm resistors, as they figured out the resistance of the wiring and switching. I put together my own, and did not feel the need for that level of accuracy.
The loads that I have are resistive, so not as difficult to drive as a speaker might be, but again, good enough for my needs.
You did not specify the make or model of the resistors, or how they were wired up.
Marked neatly on the curve 2R8 +/-5 % and then 87.31 perhaps 31/52 of 1987. I picked up the last 3 at a hamfest on Sunday, I did not think to ask how many he had originally. I'd not googled the Arcol name stamped on the brackets as they did not look original to the resistor section, thinking it was just a maker of steel brackets. Crinkle form type at top of this image
They'd have to be very cheap. Mr. Kook is a talented technician so I think that we can rule out that factor.
I use a pair of finned 300 watt 8 ohm resistors mounted on a giant power supply heat sink to raise their continuous rating. Got them from Yamaha way back in the 80's when I did warranty work for their pro-audio line. They aren't branded Yamaha. They sent them to our warranty station along with some other equipment they required to authorize us. If I recall the total startup cost was a couple thousand bucks but we also had a credit line with their service/parts dept.