Shallow Vibrato on '70 SFDR

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Stringrazor, Oct 1, 2004.


  1. Stringrazor

    Stringrazor Member

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    I just received a ~69/70 silverface Deluxe Reverb that I bought from another forum member. It's in amazing cosmetic shape and sounds great with the exception of the Vibrato. The Vib works but I don't hear any effect until the Intensity is at about 4.5. At 10, it doesn't have the depth I expected. I don't have much experience with Fender amps but I don't think this is how it should be. What components affect the Vibrato depth?


    Also, from the quick dating I did from speaker codes and tranny stamps, the parts were made in late '69. It has "AB763" on the tube chart but I realize that's probably incorrect. How can I tell for sure?
     
  2. John Phillips

    John Phillips Member

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    First try changing the vibrato tube (second preamp tube from the power tubes). If it's not that, it could be either one of the components in the oscillator circuit (caps and resistors) or more likely the opto-coupler. Luckily these are available from Fender since they're a non-standard part made from two other components.

    It could be an AB763 even as late as '69. AFAIK the circuit did not change until the AB868 (August '68) and even then the production change did not necessarily occur immediately. What type of rectifier tube does it have? The AB763 uses a GZ34 and the AB868 a 5U4. It's possible the wrong type is fitted though, so check the filter caps (under the pan on the chassis) - an AB763 with have a single 16uF cap as the first stage (four caps in total), an AB868 with have two in parallel (five in total). It's possible - in fact preferable - that they've been changed though, so it still might not be conclusive. The bias cap should also be a different value (25u/50V on the AB763, 50u/70V on the AB868, but again may have already been changed. The AB868 also has the 'tone killer' caps from grid to ground on the power tube sockets, but it's very common to find these removed...
     
  3. Stringrazor

    Stringrazor Member

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    Thanks, John.

    I tried a few used 12AX7 tubes but there was no change in the vibrato depth at all. Where is the opto?

    From what you said and what I saw when I pulled the chassis, it seesm there's no doubt my newly acquired SFDR has the AB868 circuit. It looks completely original except for a grounded power cord.

    What is the effect of the "tone killer" caps? Can they simply be removed?
     
  4. John Phillips

    John Phillips Member

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    Often, yes. They were fitted to cure self-oscillation problems caused by poor wiring layout consistency - this was one of the first symptoms of CBS-related quality drop. But not all amps suffered from it anyway, it was just a way of guaranteeing they wouldn't have to check for this and rework badly wired amps :rolleyes:.

    If you remove the caps (you'll get noticeably better clarity and sparkle) and it does start to ocillate, all you need do is rewire the area around the power tubes neatly and tidily and it will go away again...

    The opto-coupler is the black 'roach'-like component with four wires coming from it, almost directly in line with the Ext. Speaker jack (you can usually see the neon pulsing by looking in through the jack, which is a good way to check if it's working!). It's usually the LDR part of it which fails, not the neon though. If it's not that, it's most likely to be one of the electrolytic cathode caps for the vibrato tube. Like all the electrolytic caps in the amp they really should be changed! They're 35 years old now which is getting on for double the commonly accepted life expectancy.

    If it really is that original, I would look into getting the caps done.
     
  5. Stringrazor

    Stringrazor Member

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    I'm trying to decide between replacing the caps myself (I'm also considering building a kit in the future and would like the experience) and having it serviced. Are there any resources you can point me to for details of how to do this work?
     

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