March 17th, 2022, 2:24 pm
Roscoe Primrose wrote:Carbon comp resistors are also quite tolerant of short-term overloads as well. They're the resistor of choice in antenna work where very brief high currents can occur due to nearby lightning...
Roscoe
March 17th, 2022, 2:30 pm
David McGown wrote:Suggestion...Use a NTC thermistor on the transformer secondary or after the rectifier. It will limit the current (and get plenty hot in the process), but will cut back to a few ohms of resistance once it warms up. You can use a time delay relay to bypass it if you want.
David
March 17th, 2022, 2:33 pm
SoundMods wrote:Stuart Polansky wrote:The current through the resistor is 125mA or 160mA depending on the bias setting. The pair of output tubes in each channel is connected to a current sink which is set at either number, so always Class A. It's set at 125mA since EL34s are in use. So, the power dissipated in those 5 watt resistors SHOULD be ~1.6 watts. EXCEPT when charging the cap at turn on. Mistakenly, I thought the heat build up for that very short time wouldn't be an issue. And it isn't, because the resistors that failed show no signs of heat. The only thing that seems possible is instantaneous failure of the wiring at the weakest point, so localized heat not dissipated into the resistor body.
Am I missing something?
Stuart
I believe so. The body of the resistor did not have a chance to dissipate the heat fast enough to protect the winding preventing failure. I'll bet that the winding was glowing bright red/white before melt-down when it "fused" out.
The fatter resistors in terms of power rating have thicker windings along with more overall mass to dissipate heat. You can get non-inductive sand-stone resistors that are more over-current tolerant.
Keep in mind that the average fuse uses a nichrome element not unlike wire-wound power resistors.
March 17th, 2022, 2:41 pm
Stuart Polansky wrote:Agree with everything above. Even Walt! (That's almost a first!LMAO)
Here is a visual of the two size resistors. Keep in mind that each 5 watt Mills is being replaced by a parallel pair of 200R Dale resistors!
Not taking another chance! PITA to remove the power supply board!
March 17th, 2022, 2:42 pm
March 19th, 2022, 11:14 am
March 19th, 2022, 11:24 am
dberning wrote:You should use "energy rated" resistors for surges. For example, see
https://www.digikey.com/en/products/det ... kAiIAugL5A
I use these in a variety of wattages without problems. At one time I used the Mills resistors in my 845 amps under non-surge conditions and there were several failures. My opinion is the Mills resistors are junk.
March 19th, 2022, 10:13 pm
dberning wrote:You should use "energy rated" resistors for surges. For example, see
https://www.digikey.com/en/products/det ... kAiIAugL5A
I use these in a variety of wattages without problems. At one time I used the Mills resistors in my 845 amps under non-surge conditions and there were several failures. My opinion is the Mills resistors are junk.
March 20th, 2022, 11:12 pm