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Tremelo not working on a Fender bandmaster reverb

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  • #1

    Tremelo not working on a Fender bandmaster reverb

    Hey all,

    Well I haven't had any tube amps for the last two to three years and just got a Fender bandmaster reverb. One problem I have is the tremelo does not work. I see the light flashing but don't hear the tremelo doing anything. Maybe the variable resistor isn't working.

    What do most of you find the usual culprit for this issue? This amp is a 1968 and doesn't look like anyone has modded it other than new filter caps.

    Also this amp is the AA768 schematic. Any suggestions to sweeten up the sound? Louder, cleaner? I already put in a new SS rectifier.

    Thanks
    Tags: None
  • #2
    Do you have the footswitch?The tremolo switch grounds the circuit to engage the effect.If you don't have the switch you can engage the circuit by plugging a shorted RCA plug into the tremolo jack on the back of the head.If grounded, the circuit should work.If not, there are other problems. SG

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    • #3
      well the easiest thing to do would be to swap in a known good 12AX7. The trem tube is V5. Also put a dummy shorting plug into the footswitch jack if you haven't yet.

      That schematic is notoriously unreadable but the one for the Pro Reverb, AB668 is very close.
      On it you will see the plate and cathode voltages for the trem tube. Checking these voltages could reveal, say, an open plate or cathode resistor. Notice, too that both of those cathodes are bypassed with 25uF caps which mostly likely could stand to be refreshed.

      Last thing I'll mention for now is that the three caps that form the oscillator, .01, .01 and.02uF between the plate and grid of the left triode, wind up being the culprits in many a tremolo circuit.

      RWood

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      • #4
        Many people do not like the 1968 circuit changes that Fender implemented.Among these were the combination fixed/cathode bias and the bias balance circuit.
        The plate resistors in the PI were decreased to 47k with the object of giving the amp a cleaner tone with more headroom .Some think this hurt some of the desirable tonal characteristics of the amp.There were of other changes too.These have been thoroughly covered in the "blackfacing your silverface" threads

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        • #5
          It's worth ordering and keeping a roach for test purposes, methinks.

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          • #6
            He's not having oscillator problems, he can see the bulb flashing. That means the trem tube is working, the three feeedback caps are working, and his FS jack has been handled. SO that leaves the photocell or the depth pot, or of course a broken off wire. Most likely is a bad optocouple photocell. If the depth pot is open or disconnected, that can also kill the trem function.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Thanks for the posts everyone,

              Enzo was right. After I posted last night I unsoldered the roach and replaced it with an old known working one and the trem works well now. I peeled back the heatshrink on the broken one to find the photo cell bad.

              I guess the second part now is to change some of the Fender fixed/cathode bias and the bias balance circuit mods from 68 circuit and to change the bias back to an ajustable bias instead of a balance.

              This bandmaster sounds good but some of the odd things they did to the output tube circuit are kinda not so good. I really want to get this amp loud clean with little breakup as I use pedals for the distortion and not really into the amp breakup vibe. Marshalls are much better for that.

              Right now using the rectifier tube I Get about 440 vdc but using a SS rectifier I get about 478vdc so Im gussing that that will crank up the headroom.

              Maybe I'll but in a bassman ot too for a louder clean tone. Any suggestions on a better OT or loud clean 6L6 tubes???


              SLO

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              • #8
                Originally posted by Slobrain View Post
                Any suggestions on a better OT...
                twin reverb or dual showman OT for louder/cleaner

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                • #9
                  Originally posted by Slobrain View Post
                  Maybe I'll but in a bassman ot too for a louder clean tone. Any suggestions on a better OT or loud clean 6L6 tubes???


                  SLO
                  Allen TO40MT.

                  Allen Amplification - Parts Order Page

                  (Scroll down the page.) Fits Bandmasters, but is closer to a Super Reverb/Bassman OT.
                  "In theory, there is no difference between theory and practice. In practice there is."
                  - Yogi Berra

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                  • #10
                    Thanks for the post guys, Hey Joe,

                    How do those Allen OTs sound? The original OT is a bit lacking in the lows on this amp but not too bad. I may try the SS rectifier in this to see if that helps the headroom. I think it should but I might just try the Allen OT, if its Heyboer built it should be a good one.

                    Thanks

                    Slo

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