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wattsabundant

Chief Journeyman
Joined
Apr 26, 2011
Messages
616
Location
Central Ohio
The concern is thermal run away. As bipolar transistors heat, up they tend to want to conduct harder which makes them hotter, which makes them conduct harder...

The emitter resistors force the outputs to share the load equally. That's why P/L manual calls out a load sharing test. The bias transistor has 2 functions. The first function is to set the bias. The second function takes advantage of the bipolar bias transistors' desire to conduct harder when it heats up. When the outputs heat up and the heatsinks therefore heats up, the bias transistor does it's thing (conducts harder) and reduces the overall bias.

On the other hand, if the bias transistor opens, due to a broken transistor lead, or broken connection to the driver board, the outputs turn full on and the effect is they go to war with the power supply. The power supply wins, the outputs fail. The fuses are of no help as they are too slow and the fuses only prevent a bigger fire.

I had this happen on a 700B about 35 years ago. I had to eat 12 new 2SD555's. You don't forget that lesson soon.

Joe's technique of soldering the bias transistor directly to the backplane board eliminates the stranded lead wire in the factory configuration. As he points out in the instructions, a good thermal contact to the chassis is critical. I have seen several photos where the case of the bias transistor rotated when the mounting nut was tightened down, and added some stress to the soldered leads. This opens up the possibility of a broken transistor lead, if not today, down the road.
 

George S.

Veteran and General Yakker
Joined
Feb 18, 2020
Messages
2,670
The concern is thermal run away. As bipolar transistors heat, up they tend to want to conduct harder which makes them hotter, which makes them conduct harder...

The emitter resistors force the outputs to share the load equally. That's why P/L manual calls out a load sharing test. The bias transistor has 2 functions. The first function is to set the bias. The second function takes advantage of the bipolar bias transistors' desire to conduct harder when it heats up. When the outputs heat up and the heatsinks therefore heats up, the bias transistor does it's thing (conducts harder) and reduces the overall bias.

On the other hand, if the bias transistor opens, due to a broken transistor lead, or broken connection to the driver board, the outputs turn full on and the effect is they go to war with the power supply. The power supply wins, the outputs fail. The fuses are of no help as they are too slow and the fuses only prevent a bigger fire.

I had this happen on a 700B about 35 years ago. I had to eat 12 new 2SD555's. You don't forget that lesson soon.

Joe's technique of soldering the bias transistor directly to the backplane board eliminates the stranded lead wire in the factory configuration. As he points out in the instructions, a good thermal contact to the chassis is critical. I have seen several photos where the case of the bias transistor rotated when the mounting nut was tightened down, and added some stress to the soldered leads. This opens up the possibility of a broken transistor lead, if not today, down the road.
Thanks Don, nicely explained so a hobbyist like myself understands.
 
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Always Thinking Outside The Box.
The concern is thermal run away. As bipolar transistors heat, up they tend to want to conduct harder which makes them hotter, which makes them conduct harder...
The emitter resistors force the outputs to share the load equally. That's why P/L manual calls out a load sharing test. The bias transistor has 2 functions. The first function is to set the bias. The second function takes advantage of the bipolar bias transistors' desire to conduct harder when it heats up. When the outputs heat up and the heatsinks therefore heats up, the bias transistor does it's thing (conducts harder) and reduces the overall bias.
On the other hand, if the bias transistor opens, due to a broken transistor lead, or broken connection to the driver board, the outputs turn full on and the effect is they go to war with the power supply. The power supply wins, the outputs fail. The fuses are of no help as they are too slow and the fuses only prevent a bigger fire.
I had this happen on a 700B about 35 years ago. I had to eat 12 new 2SD555's. You don't forget that lesson soon.
Joe's technique of soldering the bias transistor directly to the backplane board eliminates the stranded lead wire in the factory configuration. As he points out in the instructions, a good thermal contact to the chassis is critical. I have seen several photos where the case of the bias transistor rotated when the mounting nut was tightened down, and added some stress to the soldered leads. This opens up the possibility of a broken transistor lead, if not today, down the road.
Thanks
for taking the time to write this and explained it. :)
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
2,851
Location
Jacksonville, FL
On the other hand, if the bias transistor opens, due to a broken transistor lead, or broken connection to the driver board
The first thing I do in any amp that is going to stay quasi-complementary (no new WOA Backplane Boards) is to tie wrap the three Bias Transistor leads to one of the bus wires or to an emitter resistor, to prevent a lead from breaking off at the base of the transistor. If done to both channels, it works well. I will also do this if I replace the original 2N3403 Bias Transistors with the venerable 2N5088 Bias Transistors
 
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