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Thread: Warm 147 amp and PT

  1. #11
    ff Fortissimo muckelroy's Avatar
    Join Date
    Jun 2011
    Austin, TX

    I had to go back and recount something I worked on about 4 years ago. The ONLY time I have ever replaced a Leslie PT was one in which the screen / plate voltages were measured 30 or 40 VDC higher than normal. The amp was distorting badly prematurely at 3 or 4 gain level, and it was not the organ's fault.

    I could not put a finger on WHY the operating voltages were so much higher. I decided to check the power transformer alone. When I disconnected the HV winding and powered the amp up, measuring the HV winding by itself (not connected to the diodes), it measured 394 VAC. Another Leslie amp on the bench with the same test done measured 374 VAC.

    This PT had a bit of waxy buildup on its underside and the wires. I replaced the PT, and all was well after that. I've shared this scenario with some other technicians, and they said they have never seen this happen before. I've been told that PTs only fail in a couple different ways (shorting causing fuses to blow, or opening up), but this failure mode of voltages being too high was not one of those, and probably rare. I did not measure the physical heat of this PT during the initial tests.

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  2. #12
    pp Pianissimo tpappano's Avatar
    Join Date
    Apr 2018
    Tulsa, Oklahoma

    One scenario I can imagine that would cause that symptom might be shorted primary turns. This would change the primary:secondary turns ratio causing the output voltage to increase, and the shorted turns would also cause the transformer to run abnormally hot. Do people still use the "5 second rule"? If you can't keep a finger on it for at least 5 seconds, its too hot! (tubes don't count!)

  3. #13
    p Piano Drawbar Dave's Avatar
    Join Date
    Jul 2017
    West Sussex, United Kingdom

    I let it run for 3 hours tonight on chorale and although it got hot the fuse never blew. It passed the 5 second test. I think Iíll leave it alone all the time that the voltages are ok and keep the new transformer for backup.

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