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Blackfaced AA270 Twin Reverb Weak Vibrato Channel

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  • Blackfaced AA270 Twin Reverb Weak Vibrato Channel

    Hello,

    I'm having a hard time tracking down why my Vibrato channel isn't as powerful/loud as it used to be. My Normal channel seems to be functioning normally. I blackfaced this amp years ago - including power supply resistors. It served me well on the road for many years. I had left the amp alone for a while and went to use it again recently, and that's when I noticed the weak output. So whatever is causing the bad channel is something that should not be because of the mods I'd performed on it previously.

    Troubleshooting steps I've taken so far:
    - Swapped preamp tubes for V2-5
    - Checked for shorted cathode caps or resistors, there was no significant drift from published value
    - Checked all grounds
    - Checked plate resistor values (these were changed to metal film during the blackfacing, but I checked them anyway)
    - Checked coupling caps value, but don't know how to check for leaks with just a DMM
    - Cleaned and retensioned all sockets
    - Cleaned switched input jack
    - Replaced resistors at input jack
    - Measured all voltages at preamp tubes and the voltage at the output of the rectifier and pin 6 of the power tubes. My voltages did not match the schematic (either AA270 or AB763) in a few places.

    Voltages are as follows:
    V1
    Pin 1 - 186 V
    Pin 3 - 2.8 V
    Pin 6 - 177 V
    Pin 8 - 2.45 V

    V2
    Pin 1 - 238 V
    Pin 3 - 1.95 V
    Pin 6 - 267 V (and a very loud POP when probed, much louder than any other pop while probing)
    Pin 8 - 2.45 V

    V3
    Pin 1 - 409 V
    Pin 3 - 7.33 V
    Pin 6 - (did not measure since no comparison value was on the layout document)

    V4
    Pin 1 - 248 V
    Pin 3 - 1.99 V
    Pin 6 - 245 V


    V5 (note: I have my intensity pot switchable to take the trem out of the circuit, these measurement were with the trem IN the circuit)
    Pin 1 - 407 V (very high compared to layout doc)
    Pin 3 - 0 V (checked this a few times)
    Pin 6 - 355 V
    Pin 8 - did not record

    V6
    Pin 1 - 212 V
    Pin 3 - 90 V
    Pin 6 - 206 V

    Voltage at rectifier output: 417 V
    Voltage at pin 6 on power tubes: 415 V

    Voltage at center lug of bias pot: -43 V (note: bias circuit was modded to adjustable bias)

    Any help would be greatly appreciated.

    Thanks in advance.

  • #2
    Can you post a schematic of what you "built" with mods? I realize AA270, but you also said blackfaced, switchable vibrato, etc.

    Edit: I'm also not certain by the description- Is just the vibrato effect week, the entire level of the vibrato channel, or both?
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      Check the 3.3Meg 10pF parallel combo at the Reverb send-return point.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        Originally posted by The Dude View Post
        Can you post a schematic of what you "built" with mods? I realize AA270, but you also said blackfaced, switchable vibrato, etc.

        Edit: I'm also not certain by the description- Is just the vibrato effect week, the entire level of the vibrato channel, or both?
        Thanks for your response.

        The whole channel is weak. I haven't tested the vibrato effect since I don't use it much, and I was more concerned about the channel level.

        The amp is basically the AA270 but with the resistor, PI cap value, and bias circuit of the AB763. The switched Intensity pot is the only real deviation from established norms for vintage Fender Black/Silverface amps. Since the channel worked well post-modification I'm hesitant to point at the mods as the source of the problem, but if it'll help I can annotate a schematic with all the changes. It just may take a day or so to get that info up.

        Comment


        • #5
          The tremolo bug could be a source of your problem even if you have the vibrato turned off. You can try removing the wire from the wiper of the intensity pot.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

          Comment


          • #6
            Originally posted by loudthud View Post
            Check the 3.3Meg 10pF parallel combo at the Reverb send-return point.
            Thanks. This is one area that I've made a modification to Mesa Mark I values: 2.2M and 20pF. Both parts have tested fine.

            Comment


            • #7
              Originally posted by g1 View Post
              The tremolo bug could be a source of your problem even if you have the vibrato turned off. You can try removing the wire from the wiper of the intensity pot.
              The tremolo mod that I mentioned does this exact thing. When switching it out of the circuit I'm unable to hear any significant change with the tremolo circuit in or out because of the low output.

              Comment


              • #8
                Here is the schematic of all that was done. I highlighted the symbols of all the components that were changed over the course of repairing, modifying, and Blackfacing the amp. If the value was changed I wrote it in blue ink near the highlighted symbol.

                Click image for larger version

Name:	AB763-AA270.JPG
Views:	1
Size:	1.76 MB
ID:	850008

                Quick Rundown:
                Blackfaced most of the relevant components including bias circuit
                I added MOV's per RG Keen's Immortal Guitar Amp mods
                Switchable tremolo
                Switched mid cap on push pull volume pot on Vibrato channel
                Replaced all electrolytic caps
                Replaced a few components with tighter tolerance modern equivalents with higher power ratings (film caps and metal film resistors)
                Replaced rectifier diodes with two large UF5408s

                Here's a gutshot if it helps at all.

                Click image for larger version

Name:	twin Rev gut.jpg
Views:	1
Size:	1.32 MB
ID:	850009

                Thanks

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