CT-100 Dropping Power Resistor

[Penthode]

I have started this thread as a separate CT-100 discussion pertaining to the 315/800 ohm resistor mounted via an octal socket on the selenium rectifier frame. It is interesting the octal plug as the designers must have anticipated it would fail in service. That would not be surprising as each resistance is supposed to drop 115 VDC. This means that the 315 ohm section dissipates 41 watts and the 800 ohm section 16 watts.

I tested this evening a 330 ohm 50 watt chassis mount resistor with conveniently available 120vac from a power outlet to see how hot it would get. The resistor manufacturer suggests it should be derated for maximum long term reliability. The temperature curve shows that it can dissipate a full 50 watts up to 175 degrees celcius without derating. Upon applying power I measured the temperature of the resistor with my trusty Harbor Freight measuring gun and found it reached 175 degrees in about 150 seconds at which point I removed the power.

I was hoping I may be able to fit a couple of power resistors in the original can. Obviously the over power dissipation of the chassis mount resistors will prevent this. Another thought is a 100 watt 330 ohm resistor alone could fit the can with the lower dissipating 800 ohm mounted on the chassis near the rectifier.

I have stopped to think this through. I have not been able to find how others have coped with this and I am sure others must have had the same problem.

Any suggestions?

[Electronic M]

I didn't even try to install resistors in the old ballast. I used metal case chassis mount power resistors bolted to chassis (the upright that holds the seleniums)to improve heat dissipation.... It's been in service 4 years now (sometimes it's on all day long) and is doing fine.

[Penthode]

Originally Posted by Electronic M

I didn't even try to install resistors in the old ballast. I used metal case chassis mount power resistors bolted to chassis (the upright that holds the seleniums)to improve heat dissipation.... It's been in service 4 years now (sometimes it's on all day long) and is doing fine.

What power rating resistors did you use? Does the chassis upright holding the seleniums get noticeably warm?

[Penthode]

I tried clamping the 330 ohm 50 watt resistor to the selenium rectifier frame hooked up the 120 vac to it and waited. Ever 10 minutes she was really cookin'!

It touched 130 degrees Celsius which is 266 Fahrenheit. Getting better: better that the resistor with no heat sink and reaching 175 Celsius.

I am going to think about this. The answer is perhaps a different heatsink and fan which plugs in the octal socket or a minor redesign of the power supply to eliminate or at least minimize the huge voltage drop requiring this big resistor.

[ARC Tech-109]

Honestly I wouldn't even worry about it, either way its a 57 watt drop of energy and must be dissapated be it with a 50 watt or 100 watt resistor and that is going to cause a rise over the ambient. Unless it's causing a problem with the nearby selenium or other parts I wouldn't worry about the rise. A wirewound resistor can handle far more than its paper ratings without shifting its value.

[Electronic M]

Originally Posted by Penthode

What power rating resistors did you use? Does the chassis upright holding the seleniums get noticeably warm?

I don't remember, and I'm not sure if my mouser order history goes back that far. I think I rounded up (from my power calculations)to the nearest standard wattage and one of the parts was 50W.
I wanted to go double the wattage, but I think sticker shock on the parts I did buy and worse sticker shock on the 2X wattage stopped me (the 2X wattage may not have been available for one of the parts).

The chassis upright (especially where the resistors are) gets about as hot as a running tube (not something you want to touch). I didn't own a thermometer back then so no measurements were taken. I figure as long as the resistor is rated for higher wattage than it sees, then it should be fine. The chassis riser being hot shouldn't be a problem either...It's metal so it won't melt at those temps, and it would probably get that hot if it was next to a power tube anyway... there's really only wires, 2 diodes, 2 fuses and an octal socket on the riser so if you dress the wires to not touch the chassis there's really nothing that the heat can harm... Especially given the back is a very open metal mesh that lets the heat out well.
With how many tube heaters alone are in this set the ballast isn't that big of a heat producer in the grander scheme of things, and the original ballast put out the same amount of heat.