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.
- History of this Amp and Project Background:
- Research on Tube Amp Design and Truobleshooting:
- Second (final) pass at fixing this amp...:
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
PairXformer
ResistanceXformer
Voltage DropPlate
CurrentPlate
VoltagePlate
DissipationOuter 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 - Maximum rated Plate Dissipation for 6L6GC power tube is 30 Watts.
- Recommended operating power is 75% (21.0 Watts).
- These values are very low. I wonder if this is because the plate voltage is low.
References:
- Cap Set (Sprague Atoms) From Tubes & More (now "Antique Electronic Supply") This supplier is far cheaper than Vintage Fender Repair.
- Schematics for Model AB763 (PDF)
- Layout for Model AB763 (PDF)
- Seco Model 78 Tube Tester Manual (Cape Old Radio Web site)
- Tube Tester Instruction book (PDF)
- How Tube Amplifiers Work (an article on PremierGuartar.com ) (PDF)
- YouTube video repair Fender deluxe (fixing repair)(link)
- Great video on how to make a transformer (youtube link)
- Vintage Fender Amp Repair [web site] Offer advice along with parts (re-cap kits, etc)