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Fender Dual Showman Diagnosis and Repair
First Pass -- 2015

Note: this web page is currently being created - as I tackle this project.

Fender Dual Showman silver face Model AB763 (1968)

This is a 'blog' of my efforts to repair and rejuvinate the guitar amp that I bought new in 1968 and has sat idle in my basement family room for 30 years.

Diagnosis and testing

  • Left channel works but volume is not good (maybe 1/3).
  • The right channel (with Vibrato) doesn't work at all.
  • All tubes light and there's no hum or noise.
  • All tubes test Okay and No Shorts. Tested with Seco Model 78 Tube Tester (see references)
  • All Caps are orriginal except for one electolytic (Cathode bias on pin8 12AX7 - vibrato).
  • No physical sign of any other work.

Some test voltages (yes I am very careful using current limiter and one hand in my pocket):

  • I noticed that the pilot light dims when switching off standby.
  • Transformer output is 337VAC (spec 343V) on each side of center tap. .
  • Output of Rectifier 476 VDC on standby and 341VDC switch on. (Spec 450VDC).
  • Measured the following Plate Dissipation with new 6L6 power tubes installed:
    Tube
    Pair    		 Xformer
    Resistance    		 Xformer
    Voltage Drop    		 Plate
    Current    		 Plate
    Voltage    		 Plate
    Dissipation
    Outer 34.7 Ohm 1.14 Volt 0.0304 Amp 343.0 Volt 10.4 Watt
    Inner 36.8 Ohm 2.20 Volt 0.0167 Amp 343.0 Volt 5.48 Watt
    Notes:
    1. Maximum rated Plate Dissipation for 6L6GC power tube is 30 Watts.
    2. Recommended operating power is 75% (21.0 Watts).
    3. These values are very low. I wonder if this is because the plate voltage is low.

References: