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 Post subject: Lesson Learned
PostPosted: March 17th, 2022, 9:27 am 
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Learned, forgotten, then learned again.

Assembling a stereo push-pull amplifier with significant output stage (center tap) capacitance. Power supply to the output stages is a voltage doubler (Fisher 500C power transformer) using Hexfred diodes, 600uF & photoflash caps then a 420uF cap. This is followed by a time-delay relay, shunted with a 3 watt 100k resistor. After that is a pi network for each channel. The pi network is made up of a Mills 5 watt 100R resistor and a parallel pair of caps: 420uF and 10uF film.

The first time one was built like this, the Mills resistors failed. Not fried, on fire, exploded or anything like that. They look fine, like new. They just went open. Thinking something else went wrong, I double checked all, slowed down relay timing, etc., then replaced the resistors. Once again, after testing, installed in the system, they just opened.

When building the second stereo amp, you'd think that would be a memory not forgotten. Wrong. Did it again!

The answer is a parallel pair of 200R 10 watt Dale wirewound resistors (not the ceramic kind, mind you; decent devices). These have been just fine.

It would seem that the Mills resistors use too fine of a wire to allow the inrush current to charge the caps; the cathode current in the output tubes still ramps up relatively slowly.

There suckers ain't cheap, so beware!

Stuart


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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 10:18 am 
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Stuart Polansky wrote:
Learned, forgotten, then learned again.

Assembling a stereo push-pull amplifier with significant output stage (center tap) capacitance. Power supply to the output stages is a voltage doubler (Fisher 500C power transformer) using Hexfred diodes, 600uF & photoflash caps then a 420uF cap. This is followed by a time-delay relay, shunted with a 3 watt 100k resistor. After that is a pi network for each channel. The pi network is made up of a Mills 5 watt 100R resistor and a parallel pair of caps: 420uF and 10uF film.

The first time one was built like this, the Mills resistors failed. Not fried, on fire, exploded or anything like that. They look fine, like new. They just went open. Thinking something else went wrong, I double checked all, slowed down relay timing, etc., then replaced the resistors. Once again, after testing, installed in the system, they just opened.

When building the second stereo amp, you'd think that would be a memory not forgotten. Wrong. Did it again!

The answer is a parallel pair of 200R 10 watt Dale wirewound resistors (not the ceramic kind, mind you; decent devices). These have been just fine.

It would seem that the Mills resistors use too fine of a wire to allow the inrush current to charge the caps; the cathode current in the output tubes still ramps up relatively slowly.

There suckers ain't cheap, so beware!

Stuart

I have a rule-of-thumb regarding the use of resistors in power circuits or even cathode bias -- Required current X 5 = power rating needed

In power circuits you have "wiggle room" for surges and in cathode circuits you limit hysteresis effects.

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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 10:22 am 
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Joined: April 22nd, 2013, 12:58 pm
Posts: 285
It's hard to imagine that much power being developed across 100R resistance, but I try not to argue with reality......


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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 10:26 am 
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FerdinandII wrote:
It's hard to imagine that much power being developed across 100R resistance, but I try not to argue with reality......

Hindsight is always 20/20. Stu is not the only one that has learned that lesson. The lesson learned also applies to choke coils in a Pi-filter network.

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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 10:40 am 
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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


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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 11:03 am 
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"......instantaneous failure of the wiring at the weakest point, so localized heat not dissipated into the resistor body."
You got it right, IMHO

The spike in current when charging a cap is wicked.
Image


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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 11:17 am 
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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.

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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 1:05 pm 
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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

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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 2:00 pm 
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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


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 Post subject: Re: Lesson Learned
PostPosted: March 17th, 2022, 2:19 pm 
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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!

Attachment:
Resistors.jpg
Resistors.jpg [ 215.23 KiB | Viewed 6742 times ]


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