Question John Shearne Phase 2 missing transistors

Skisocks

Novice Member
I recently acquired a John Shearne Phase 2 that appears to be missing four transistors. I have searched for images of the amplifier online but have been unable to work out what the transistors are. I have been able to identify the transistors that feature in the Phase 3.5 as Motorola MJ11015's and MJ11016's (see image).

Would anyone be able to tell me whether these are the correct transistors for the Phase 2? Any additional information would be very useful.
 

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noiseboy72

Well-known Member
So has someone removed them?

There's a large number of variations of this amp - all still called Phase 2. There's the pre '94 handbuilt versions, the post '94 production line, Rapport, Reference...

Is it possible that the board supported multiple output devices, but to reduce distortion only a pair was fitted to each channel. Without seeing the board, it's impossible to tell.

Those Motorola devices would most probably work, as they are used in the later models. You may need to adjust the crossover bias to reduce distortion, as a purchased pair will not match up as well as new.
 

Skisocks

Novice Member
Yes. It is my understanding that a number of years ago the amp over heated or somehow broke. I'm guessing that whoever first looked at it noticed the transistors were cooked, removed them, and then recommended a power amp be used to avoid it breaking again - hence why they were never replaced. It has been used as a preamp every since.

Is there an easy way to identify each variation? I would prefer to use the original transistors if at all possible.

Thanks for the response.
 

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noiseboy72

Well-known Member
I'm not an expert on these amps, but it has the Rapport branding. The board also says Phase 2/3 amp, so it's safe to say that the Motorola transistors will be fine.

Different MOSFET transistors will sound the same, as it's the supporting circuitry that affects the tonal balance. The important bit will be minimising crossover distortion, but you want an oscilloscope to set this up properly. There's also the possibility that other transistors have blown in the driver stage, as the output devices could have failed short circuit. It's worth checking if the devices need isolating from the heatsink or just want some heat transfer paste.

Just refitting new transistors is unlikely to restore the amplifier to peak performance, so consider getting a repair shop to fix this.
 
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