Thread Rating:
  • 1 Vote(s) - 5 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Potential fire from hot resistor?
#1

    I am done recapping my 53-1750 Radio/Phonograph console. There is a resistor (see photo) which connects between 2 power tubes and it gets HOT. I mean, really hot. I can touch it for about a second, anymore than that, I'm sure I would get a burn. This is the long green ceramic resistor you see in the photo - 270 ohms 7 Watts. What bothers me is that when re-installed in the console, this resistor will be only 1/4 inch from the wood on which the electronics box is mounted - and I'm concerned this is a fire hazard. Anyone come across this?? I was thinking of drilling a 2 inch diameter hole in the wood below where this resistor would be once the box is re attached? Any ideas on this?
Thanks,
Mike
#2

If you can touch it for a second, I guess it well below a combustible material burning temperature.
As long as the heat has a way of escaping it's OK.

As for the hole, remember - the hot air escapes up, not down.
#3

That looks like a filament dropping resistor. Those normally run quite hot, but no where near the ignition temperature of wood (or even paper, for that matter). It it were hot enough to even ignite paper, it would tend to melt its own solder connections. Icon_smile
#4

Well, all depends on whether the heat has a way of escaping. You do realize that even a low temp heater if well insulated can deliver enough heat to ignite just about anything.
A good example is when old lampshades were covered with newspapers and would catch fire.

However 7W (and a 7W resistor will not be made to dissipate 7W; it is probably 3-4W) is not 40-70W of an incandescent lightbulb, and thisck wood is not thin paper, and the heat has some way of escaping as the radios were designed with this in mind (well, most of them, I guess).
#5

Measure the voltage across the resistor and you can calculate the power (wattage) dissipated. You can use the nominal resistance for the calculation--it's close enough. If you want to be super accurate you can measure the actual resistance after desoldering one or the other lead.

E=IR, so I=E/R, and Power = I squared R. Power should be half of the rated value or less. If you are super concerned about the heat, you could buy a power resistor from Mouser or Allied that bolts to the side of the chassis and uses the chassis metal as a heat sink. It will run cooler than your power resistor, but the metal of the chassis will get a little warm. If you buy one of those, check out the derating specs of the resistor. They will often advise derating at 4 times the rated power or more, so if your resistor dissipates 3 watts, buy a 10 or 15 watt power resistor.

I think that's a better solution than drilling a large hole. But my guess is, Philco engineers did their homework and your resistor, assuming it is not defective or degraded, will be fine.

John Honeycutt




Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Jackson 715 not working
Mrfixr55, its working now. I cleaned the switches and sockets with spray cleaner. Thanks for helping on this one.daveone23 — 06:26 PM
Jackson 715 not working
Now that you posted the schematic, I don't know why that Sprague electrolytic cap is across the meter, as it is not indi...MrFixr55 — 05:51 PM
HiFi (Chifi) tube amp build - but my own design.
What may be lacking in the PP Tube amps may be the 2nd harmonics, which some, especially RCA back in the day called &quo...MrFixr55 — 05:32 PM
Jackson 715 not working
Usually in an emission tester, the tube under test is measured as if it were a diode. So, some testers connect all the g...RodB — 04:17 PM
Restoring Philco 37-604C
Yep. F5 is green, D5 is Red. Red is Bad. Green is Clean.morzh — 01:30 PM
Jackson 715 not working
I did start to do that but I stalled out because I could not figure out how the grid and plate get voltage. In this diag...daveone23 — 11:52 AM
Restoring Philco 37-604C
(Insert Homer Simpson "DOPF" Here.) When all fails, look at the can. Took the Ron Ramirez advice, red Caig D...MrFixr55 — 09:23 AM
Philco 91 Speaker Replacement
From your text I am not sure if you intend to use the existing speaker with a resistor instead of the field coil. It wo...morzh — 08:44 AM
Philco 91 Speaker Replacement
My field coil is bad. I am still hoping to find an original, but if I can't I will go with a fitting Philco speaker, 125...dconant — 08:34 AM
Philco 91 Speaker Replacement
As Rod said, it is OK to use a fitting speaker, and then look for an original one. If you buy a Hammond 125 output tr...morzh — 08:15 AM

[-]
Who's Online
There are currently no members online.

>