| Second hand ones are OK if youi get one with a good condition carbon brush in the wiper, the one I replaced yesterday was a previously used one and it worked for all of a week. I replaced it with a newer one that has had hardly any use and have since found a spare brush for the one that failed
I had one of the shift electricians build me a control unit the other day to vary the heater power on our electron guns when testing them. All that is needed is a box, ideally a voltmeter (we also added a ammeter to the O/P side) an illuminated rocker switch for mains ON/OFF (we also added a switch to the O/P with a indicator lamp as the unit is interlocked to thw water cooling and vacuum) The mains in and O/P leads are by way of stuffing glands. So you should be able to build a 8 amp one for around £100.
If you can find a cheap one on ebay in good condition then go for it, do you know the sort of current you need.
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