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1972 Fender Bassman sonic issue

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  • #1

    1972 Fender Bassman sonic issue

    Hi guys,
    Just feeling a little brain numb here... not sure what's going on...
    I got this 1972 Bassman 50 with an AA371 layout. I've modified it to an AA864 output section, got rid of all the treble reducing bypass caps, and then just mixed the 2 existing preamp stages with 220K resistors, wired directly to the coupling cap of the long-tail pair driver tube 12AT7. I've changed all the coupling caps to polyester orange drops and the power supply caps to newer F&T's. . the 12AX7 tube V2 is not used at all. 820 and 100 ohm R's used for feedback loop. New JJ 6L6 output tubes. I've done these mods to my other Fender amps and they sounded great.
    Thing is, this amp did not sound very good when I got it... sounded sorta mid-rangy and spiky. After all these changes, it still has that similar sound to it.... it's not open and clean and full of great sparkle and bottom end. Still sorta sounds like when I first got it. On the scope, it couldn't be better... rock solid waveforms pretty much from 60HZ and up to 6KHZ and higher. WTF?
    Could the OT be causing this problem ? Any other ideas ?
    Tags: None
  • #2
    Have you tried different speakers, or any chance it has speakers out of phase?
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Are you scoping it on a dummy load or across the speakers?

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      • #4
        I've read where the AB165 & later actually have the OT leads switched compared to the AA864 & earlier... they changed where the negative feedback was inserted, so they just swapped the OT leads to compensate. Positive feedback would result, which can do all kinds of crazy stuff, not just uncontrollable screeching. I never noticed til I read Machine Gun Amps's writeup about the AB165 & later. It's only 2 wires to swap... sure enough, my AB165 & a 74 Bassman 50 both had the OT secondaries swapped... check the layouts for AB165 & AA864 - black & green switched at the speaker jacks.

        Just a theory...

        Either way, let us know!
        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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        • #5
          Originally posted by g-one View Post
          Have you tried different speakers, or any chance it has speakers out of phase?
          speakers, tried 2 different sets, typically use a single Celestion G12-65.

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          • #6
            Originally posted by Mick Bailey View Post
            Are you scoping it on a dummy load or across the speakers?
            Scoping dummy load, both 8 and 4 ohms. Amp puts out 60 watts into 4 ohms.

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            • #7
              Originally posted by Justin Thomas View Post
              I've read where the AB165 & later actually have the OT leads switched compared to the AA864 & earlier... they changed where the negative feedback was inserted, so they just swapped the OT leads to compensate. Positive feedback would result, which can do all kinds of crazy stuff, not just uncontrollable screeching. I never noticed til I read Machine Gun Amps's writeup about the AB165 & later. It's only 2 wires to swap... sure enough, my AB165 & a 74 Bassman 50 both had the OT secondaries swapped... check the layouts for AB165 & AA864 - black & green switched at the speaker jacks.

              Just a theory...

              Either way, let us know!
              Justin
              Good thought... and forgot to mention... I'd actually switched the output leads myself when doing the mods, and there wasn't any oscillation or feedback or screeching afterwards, so I thought it was correct as far as that went. I don't see how I could run the amp at 60 watts thru it's entire audio spectrum without any feedback/oscillation issues if things were cross-wired feedback-wise... but, perhaps I need to disconnect the feedback loop and see how it sounds, see if there is something going on there...

              Comment

              • #8
                Have you checked the cathode caps in the pre-amp?

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                • #9
                  Originally posted by tubeamptech View Post
                  Have you checked the cathode caps in the pre-amp?
                  The dc bias voltages are good, so I didn't do anything else there.

                  Comment

                  • #10
                    FYI, I crossed the xformer output leads back to original, and sure enough, the amp started howling like a banshee (whatever that is..). So the polarity of the feedback is correct, having crossed them back yet again so it's not howling now. I re-tested it across the 60HZ to 6KHZ range again, 4 ohm dummy load, and the amp is impressivley flat all across that band, putting out 60 watts pretty much through all the range. I can't find anything wrong with any wiring at all. Not sure why it doesn't sound fantastic to my ears, will have to try some other output tubes when I can afford them. Using a NOS JAN 12AT7, an old RCA 12AX7, a new stock JJ 12AX7, and JJ 6l6GC's.

                    I've attached an image of the wiring for anyone curious. Note the new "DEEP" switch layout... this really works well, as the original for some reason didn't seem to work. bassman_AA371-AA864.pdf
                    Attached Files
                    Last edited by Chevy; 03-03-2014, 07:32 PM.

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                    • #11
                      You get 60 solid watts? WHy would we suspect the OT then?

                      The question is this: what do you have to put into the amp to GET the 60 watts out? It is one thing to have a guitar drive the thing to full out, it is something else altogether to put 10 volts of signal into the front to get 60 watts out, regardless of how clean and wideband the result.

                      You were asked about cathode bypass caps, and you said:
                      The dc bias voltages are good, so I didn't do anything else there.
                      The bypass caps won;t alter the DC voltage, they will affect the gain and freq response of the stage, so if you are operating stages without cathode bypass caps, that is certainly sapping gain and tone.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment

                      • #12
                        Originally posted by Enzo View Post
                        You get 60 solid watts? WHy would we suspect the OT then?

                        The question is this: what do you have to put into the amp to GET the 60 watts out? It is one thing to have a guitar drive the thing to full out, it is something else altogether to put 10 volts of signal into the front to get 60 watts out, regardless of how clean and wideband the result.

                        You were asked about cathode bypass caps, and you said:


                        The bypass caps won;t alter the DC voltage, they will affect the gain and freq response of the stage, so if you are operating stages without cathode bypass caps, that is certainly sapping gain and tone.
                        Had no idea what the problem was, that's why I was asking the guys for suggestions... Input was about 150mV to get full power out; if it was anything very unusual, I would've specified.
                        Cathode caps check out fine... except they migrated over the years to much higher values than stock spec, so I replaced them.
                        Amp still doesn't sound great to my ears; will have to try other output tubes when I can afford them. Tried several different 12AX7's as well.

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                        • #13
                          Originally posted by Chevy View Post
                          Amp still doesn't sound great to my ears; will have to try other output tubes when I can afford them.
                          How did you set the bias for the output stage?

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                          • #14
                            On the bass input, the cap to bypass cathode on pin 3, has to be 10uF -25 uF.
                            Otherwise the low frequencies won't pass that stage.
                            It looks like it says 1 uF on the schematic for the cathode bypass cap.

                            If that stage won't pass low frequencies, the deep switch won't work either.

                            Next V1B, same thing. It can't really be 5uF cause the lows will be lost...too much.

                            http://www.ampbooks.com/home/amplifi...ode-capacitor/

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                            • #15
                              Originally posted by 52 Bill View Post
                              How did you set the bias for the output stage?
                              1 ohm precision resistors on pin 8 of the output tubes... approx. 35mA idle current.

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