The ultimate dual 500 "rising from the ashes" build

Bias increase with heat right

So the bias voltage increases with heat. Not sure how that reduces heat in an amp. I thought the bias voltage would dropped as thing get hotter
 
grapplesaw said:
So the bias voltage increases with heat. Not sure how that reduces heat in an amp. I thought the bias voltage would dropped as thing get hotter
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Bias in a solid state amp should decrease when the output stages increase in heat.

The base-emitter junction turn on voltage of NPN devices decreases as they get hotter so it is necessary to DROP the bias as the output stage heats up. If this does not occur then runaway occurs and blows fuses as you have seen.

Also bias in the .46 range is in the danger zone. That is dangerously close to the turn on voltage of a hot output semiconductor. You need to bring the maximum bias down to the 0.35-0.38V range.
 
grapplesaw said:
Eyeh .9 volts

is it worth testing bias transistor or just change it and the 180 ohm resistor on the first driver?.
Click to expand...

I moved to 150 ohms in the first predriver position long ago. That is not your problem but that is my recommendation regardless.
 
So.......with no load and no input, if you heat the bias transistor the bias voltage will drop??
 
laatsch55 said:
So.......with no load and no input, if you heat the bias transistor the bias voltage will drop??
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Yes, that is the way it is supposed to work. The bias transistor servos to the temperature of the output devices. The output devices lead since they are the source of the heat but the bias transistor is supposed to pick up that heat and dial down the bias as the outputs get hotter.
 
Hard to tell that when the amp is loaded cause the load drives the bias up correct??
 
Gepetto said:
Bias in a solid state amp should decrease when the output stages increase in heat.

The base-emitter junction turn on voltage of NPN devices decreases as they get hotter so it is necessary to DROP the bias as the output stage heats up. If this does not occur then runaway occurs and blows fuses as you have seen.

Also bias in the .46 range is in the danger zone. That is dangerously close to the turn on voltage of a hot output semiconductor. You need to bring the maximum bias down to the 0.35-0.38V range.
Click to expand...
So we set bias with not load on input and no output to say .350volts after heat up time is stable.

Now after connecting light input voltage and 8 ohm dummy the biass instantly jumps up to say .4 volts and as time goes on it continues to rise. Should this happen? Is there a threshold where it will actually start to go down?
 
grapplesaw said:
So we set bias with not load on input and no output to say .350volts after heat up time is stable.

Now after connecting light input voltage and 8 ohm dummy the biass instantly jumps up to say .4 volts and as time goes on it continues to rise. Should this happen? Is there a threshold where it will actually start to go down?
Click to expand...

That should not happen Glenn.
 
grapplesaw said:
Ok that is what I thought. So now to find what is causing this. First thing I will replace the bias transistors but that is not it. It's strange isn't it
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Does your D500 have the bias transistor thermally coupled to the heatsinks?
 
Joe, every amp I've ever tested , the bias jumps as soon as I turn on the generators, what I haven't done is wait and see when the bias starts dialing down...
 
laatsch55 said:
Joe, every amp I've ever tested , the bias jumps as soon as I turn on the generators, what I haven't done is wait and see when the bias starts dialing down...
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As I stated Lee, the bias servo lags the heat in the outputs as the heat has to conduct from the outputs and into the bias transistor.
 
Gepetto said:
As I stated Lee, the bias servo lags the heat in the outputs as the heat has to conduct from the outputs and into the bias transistor.
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Yes Joe
they are attached to the heat sink. Please see photo
 

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Gepetto said:
Bias in a solid state amp should decrease when the output stages increase in heat.

The base-emitter junction turn on voltage of NPN devices decreases as they get hotter so it is necessary to DROP the bias as the output stage heats up. If this does not occur then runaway occurs and blows fuses as you have seen.

Also bias in the .46 range is in the danger zone. That is dangerously close to the turn on voltage of a hot output semiconductor. You need to bring the maximum bias down to the 0.35-0.38V range.
Click to expand...
I agree with you on max bias voltage. The manual goes as far as .6 volts (. Scary) see attached
 

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RIP runaway bias dual 500

Thanks for your ideas along the way on this problem
it has got to a point to which l as sorry to say but the project has become to
much for me so I have gone ahead and crushed the beast in my new 150 ton press

Off to the scrap yard with it then

Not!!!

here is the latest
first the thing goes critical as it heats up
it crashes more now than when I got it because I am messing with it

last night I changed out the Bias transistors and the 10 ohm and 180 ohm on th the backplate

guess what I found

one 10 ohm on the left channel was a cold solder and as I wiggled it to remove saw it moved.

So do I have replaced both channel 10 and 180 ohm resistors along with new 5088 bias transistors and am heating it at this time

fingers crossed and with suspicion of some sort of heat up problem this may be it

i will update soon
 
I guess the "bias resistor" might affect the bias circuit eh??
 
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