Lesson Learned

[Stuart Polansky]

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

I hate noise. That may not be an issue given that it's a Class A PP stage.

I also hate fire. Carbon comps make me queasy, except in virtually zero current cases like grid or gate stoppers.

Actually, I'm surprised that carbon comp is even made in sizes about 2 watts or so!

[Stuart Polansky]

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

There is a relay in the DC part of the secondary circuit (yes with a diode across the terminals in the reverse direction). There is a 100K 3 watt resistor across it. I wrongly figured that would make the load on resistors and diodes in the circuit safe. There is 30-40 volts on the B+ circuit, after the relay, when the contacts close. The resistors fail after the contacts close.

My B+ is marginally low as it is. There would definitely need to be a relay across the thermistor to keep voltage up.

The other amp, identical to this one, has been operating without issue for many, many hours at this point.

However, starting from scratch on the next amp, the thermistor route may be the one followed!

Thanks!

Stuart

[tomp]

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.

I thought the wire in a standard fuse was tin because of a lower melting point.

Tom

[SoundMods]

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!

Resistors.jpg

Yes -- and the Dales are rated at 10-watts each for a total dissipation of 20 watts.

[tomp]

Tin oxide resistors.pdf

(11.47 KiB) Downloaded 197 times

Instead of carbon composition, you might want to try tin oxide resistors. They can handle power surges better.

[dberning]

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.

[SoundMods]

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.

Check out the fine print: " Coating Conformal lead free vitreous enamel Core Ceramic. Terminals Solder-coated axial. RoHS solder composition is 96% Sn, 3.5% Ag, 0.5% Cu
Derating Linearly from 100% @ +25°C to 0% @ +350°C Tolerance ±5% standard; other tolerances available. Power rating Based on 25°C free air rating (other wattage's available)
Overload Under 7 watts: 5 times rated wattage for 5 seconds; 7 watts and over: 10 times rated wattage for 5 seconds"

NICE!!

[Grover Gardner]

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.

That's very helpful, David, thank you!

[Stuart Polansky]

Thank you for that excellent recommendation David!