When browsing on the TIP147, i stumled onto thsi Fnder tech note which seems to confirm your suspicion on the bias! http://natcade.tripod.com/pages/tech_f2.html
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March, 1993
Overheating Solid State Guitar Amps
In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar.
Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible.
A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC.
Modification procedure:
Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators).
Install Mica Pads using a liberal amount of thermal grease to both side.
Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads.
Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC
The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).
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March, 1993
Overheating Solid State Guitar Amps
In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar.
Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible.
A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC.
Modification procedure:
Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators).
Install Mica Pads using a liberal amount of thermal grease to both side.
Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads.
Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC
The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).
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