Burning smell...

[Colonel_Sanders]

I turned my build on for the first time today and noticed a very pronounced burning smell after about 40 seconds of 'on-time'. Its definitely coming from the power or bias supply but I can't see anything unusual.

Any ideas on how to track the culprit down? I was thinking you pro's might have a smell chart eg. 'Burning resistors' smell like 'burnt steak' etc. Or should I just let the amp run until I can see something obvious?

Oh, and its not the Transformers, I don't have any tubes in yet, fuses are good, and the voltages are showing up fine. I'm thinking an under-rated resistor?

Thanks,
C_S

[Colonel_Sanders]

Nevermind, found it! It was an under-rated resistor. Off to the store tomorrow for a wirewound

[Colonel_Sanders]

How hot is 7.6W

So I've installed a 10W wirewound. I calculated the heat dissapation to be around 7.6W. My question is, how hot is 7.6W? Because the resistor is still too hot to touch.

Thanks,
C_S

[Albert Kreuzer]

My question is, how hot is 7.6W? Because the resistor is still too hot to touch.

Hi C_S,

that's ok. Just leave enough space around the resistor so that it doesn't burn up something else.
A 20W resistor would be better and stay a little cooler.

Cheers,
Albert
www.albertkreuzer.com

[tubeswell]

Thinking about this the heat would be proportional (or inversely proportional?) to the ability of the resistor to dissipate energy, so that for example a resistor with say relatively less surface area per unit of mass that is dissipating 7.5W should theoretically get hotter faster than a resistor dissipating 7.5W which is of the same mass but which has relatively more surface area/ability to dissipate heat.

But anyhow if your 10W resistor is hot, its probably still going to be ok if it is dissipating 7.5W. It might cause the amp to warm up some, which might change the resistances a bit after an hour or two, which might alter the tone in interesting (not necessarily undesireable) ways. I built a 5F2A with a chunky wire-wound cathode resistor, and although the whole shebang gets quite hot after a couple of hours, by then the amp is really singing. If you're really concerned about it, you could do something else to enhance the resistor's ability to dissipate energy, like chucking in a fan, or cooling fins/heat sink (or oops I just saw Albert's post - a bigger resistor - serves me right for rambling on).

[Colonel_Sanders]

Sweet! The resistors at the back of a 3U rack chassis, nice and far away from the rest of the circuit. I'm going to shove a fan in for the tubes anyway, so I might point it the resistors direction.

Thanks for the info!
C_S