Monday, August 1, 2016

Music Man Driver Transistors

While I was away, a Music Man amp was sitting on the work bench waiting until I got home. After rehabilitating the bias network and power supply electrolytics and installing a pair of  2N6292 driver transistors the results were discouraging. Measuring across the 3.9 ohm emitter resistors revealed a significant split in voltages, no improvement from what was inside which was a mismatched pair where one had been replaced.

Then, I uncovered the updated service bulletin number 7 which indicates that the 2N6488 is the best replacement around for the 2N6292. And so it is, being matched within 2 mv. An adjustment of the bias pot finishes the job.

Note that with the use of the two stepped plastic washers and the mica insulator wafer in the 4724 installation kit, the driver transistor is insulated from ground and uses the chassis as a heat sink. Make sure that you use a good grade of thermal grease. also known as heat sink compound, bird poo and other unflattering names.

The 534-4724 Keystone mounting kits are available from Mouser Electronics as are the improved driver transistors.

The complete file of MusicMan service bulletins are available here including biasing instructions for all models including the earlier amps that used a 12AX7 driver tube.

Here's SB7 with a smallish shop drawing showing the positioning of the insulator wafer and stepped plastic or nylon insulating washers.

Thanks to the folks at Ernie Ball for maintaining this resource for the user and repair community.

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