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Need help getting schematic for my 68 Dual Showman

Messages
108
Hi.
I jsut got a 68 silver face dual showman head off ebay from a seller with a good reputation. However, the amp seems to have some serious problems. All the caps were bad and some even popped.

The biggest problem is that my tech took a look at it for 10 seconds and pointed out that someone modified the power section into a class A1 circuit .So his first recommendation was to replace all the caps and return the amp back to class A/B

I'm having trouble finding the right schematic. And my tech is a competent man but he simply is new to full time repair and hasn't acquired the volumes of schematics that many other guys already have.

So any help determining/finding the right schematic is greatly appreciated.Thanks for your time.

P.S. My tech does have the groove tubes book with two fender schematics that are listed as "Showman" nothing more than that. Any thoughts on how accurate that book is ? Thanks
 

rooster

Member
Messages
2,119
The 3-digit part of the code is the month/year that the circuit was released. I would go for the AB763, were I you, as that is the "classic" Blackface circuit. Many people like the "silverface" circuits as much or more, but to me, the 763 was THE blackface sound. Take your time, and make sure you get good coupling caps if any need replacing. I'd recommend Mallory 150's for a good cost/performance ratio. As well, make sure all the electrolytic caps under the ashtray and on the cathodes of the tubes are up to snuff, Sprague Atoms are great for them. Another thing I like to do is replace the diode and cap in the bias supply, as the selenium diode gives off a highly poisonous gas if it shorts. Enough to hurt you in a closed-in room. I'd recommend a FRED for all silicon diode replacement, as they only cost about 1 cent more than a normal silicon diode when you buy them by the 500 (which I assume your tech does).

Oh, and make sure the bias circuit is converted over from the "hum-balance" to the adjustable-fixed bias as in the 763.

Good Luck, Mr. Phelps.

rooster.
 
Messages
108
Thanks a bunch.I appreciate the help alot.I probably should have mentioned that my dual showman is a reverb. Still has the original accutro nix tank and fender footswitch. Does the amp being a "reverb" necessarily change which schematic is most appropriate ? And here's a couple of pics of the guts of the amp




 

JJman

Member
Messages
994
Yes those filters look original and should be changed. The giant ceramic resistors on the power tubes (to the chassis) are the culprits rendering the amp "one of those wacky cathode and fixed bias" Fenders from 1968.

The AA768 schem shows them. The AA769 shows them removed so that's your "most vintage" scheme that's not "wacky."

If it were my amp, I would jumper the 150 ohmers with wires leaving them in the amp, but out of the circuit. This would preserve some originality and keep those conversation pieces intact.
 






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