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  • ab763 pi saturation

    I just finished a Vibroverb build. The amp sounds good up to about 4-5 on the volume controls of both channels, then clipping begins. The voltages on the phase inverter are Pin1-291 volts, Pin 2-44 volts, Pins 3/8 70 volts, Pin 6 -283 volts, and pin 7 -44 volts. I checked all the component values in the phase inverter and measured the circuit resistances. Pin 3/8 to ground = 22,400 ohms, Pins 2 and 7 to pinn 3/8 = 1 meg, and pin 1 to pin 6 = 182Kohms. Could the low cathode bias be the cause of the clipping and what can cause the low reading?

  • #2
    Probably nothing wrong with the amp.
    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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    • #3
      See http://schems.com/manu/fender/vibroverb_ab763_schem.pdf and consider Notice #1.
      What's the VB+ at the 'C' node?
      Have you tried different tubes?
      What power is the amp putting out, max clean and max square wave (if you've got a true rms meter)?
      Be aware that the circuit driving the power tube control grids in a class AB1 amp won't be intended or able to pull them above Vk (0V in this case), so the wave will flat top at that point, due to the positively biased grid to cathode diode.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Originally posted by guitman321 View Post
        The amp sounds good up to about 4-5 on the volume controls of both channels, then clipping begins.
        At what point on the volume knob are you expecting (or wanting) clipping to occur?
        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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        • #5
          B+ at c node is 438v. I did try a new 12at7. I measured the output with an AC voltmeter and got 18 volts sine wave up to 26.6 volts almost square. The output is biased at 34ma on each side.

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          • #6
            Originally posted by g-one View Post
            At what point on the volume knob are you expecting (or wanting) clipping to occur?
            I only have one other ab763 amp, a deluxe reverb, and it starts to clip at about 8 on the volume knob.

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            • #7
              'I only have one other ab763 amp, a deluxe reverb, and it starts to clip at about 8 on the volume knob.'

              That seems unusual; maybe the volume controls have been replaced with ones of a different taper, eg 10% audio rather than 30%, or there's some other reason for its gain being low.

              18Vac across 8 ohms is 40 watts, which seems good for 2x6L6GC fed by ~450V via a GZ34, and agrees with 1963-1964 "Black Face" Vibroverb

              So, as the amp is able to achieve good power output, it seems likely that the LTP outputs are providing sufficient signal swing and are so at full output are perhaps being clipped by the power tube grids, rather than being unable to drive the power tubes fully.

              Are the 12AT7 you've tried recent production types? It may be beneficial to try some 'known good' old production ones.
              Do they pass expected currents in other circuits?
              Have you checked the 470 ohm bias resistor value?

              'B+ at c node is 438v'
              That's a little lower than the schematic value, which will leads to other LTP voltages being proportionally lower.

              It's a technical point only, as the amp is putting out full power.
              How about seeing how some BF RIs in a friendly store compare to your amps?
              Last edited by pdf64; 09-27-2014, 10:49 AM.
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                Originally posted by loudthud View Post
                Probably nothing wrong with the amp.
                I guess you are right. I went back over everything, and both amps start to clip at about 6 or so, and only when I hit the strings pretty hard. Thanks!

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                • #9
                  [QUOTE=pdf64;361551]'I only have one other ab763 amp, a deluxe reverb, and it starts to clip at about 8 on the volume knob.'

                  That seems unusual; maybe the volume controls have been replaced with ones of a different taper, eg 10% audio rather than 30%, or there's some other reason for its gain being low.

                  You're absolutely right. I was testing with a 100mv sine wave. When I plug in the guitar, and really hit a chord hard it starts at 5 to 6.

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                  • #10
                    I did notice that nothing was said about where the tone controls were set.

                    Nor what frequencies where used for test purposes.

                    Fender amps will typically clip the bass tones first. (tone controls at 5)

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                    • #11
                      Yes; on those with a mid control, turn all 3 tone controls to minimum and the amp will not clip (or put out any signal)!

                      See some of the RI schematics, which have typical test signal voltages along the circuit path, in the rounded boxes http://support.fender.com/schematics..._schematic.pdf
                      The DRRI, having an 8 ohm secondary, should have signal levels in the same ballpark.
                      My band:- http://www.youtube.com/user/RedwingBand

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                      • #12
                        Originally posted by guitman321 View Post
                        I just finished a Vibroverb build. The amp sounds good up to about 4-5 on the volume controls of both channels, then clipping begins.
                        Interesting statement. Is that to say that the amp doesn't sound good clipping? Or that you don't like clipped tones?

                        Maybe off topic, but I want to cover an issue that use to come up here a lot. AB763 amps have a tone stack that is bass heavy and digs pretty low into the LF. This is great for clean work, but generally bad for clipping. Also, the designs are often in shallow, open back cabinets that don't have good LF reproduction. For this sort of cabinet the bass heavy tone stack compensates, but only when playing clean. Many players wanting more bottom end in their overdriven tone turn up the bass knob only to find that the tone gets flabby or farty with very little improvement in the actual amount of bottom end in the final EQ. This is normal. The amp can only produce so much bass because of it's limited design and the preamp is capable of far more bass than is good for clipping. This is especially true because the tone stack is placed very early in the AB763 design. If you want a clipped tone you should turn the treble up to at least 7. Now adjust the volume for the amount of dirt you want. Then turn up the bass a little at a time until it just starts to sound flabby or farty, and then back it back down a little. There. That's as much good bass response as you're going to get. If you need more bottom end you'll need to do something like build a closed back extension cabinet or install a very bass heavy speaker in the combo. Turning up the bass knob won't get you anywhere.
                        "Take two placebos, works twice as well." Enzo

                        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                        "If you're not interested in opinions and the experience of others, why even start a thread?
                        You can't just expect consent." Helmholtz

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                        • #13
                          Given the posts above the out put wattage seems to be spot on. Could it be that you just don't like the kind of break up you get. If so, it can be as simple as just getting the "right" tubes. Try messing around with different power tubes and try biasing by ear. If you get a break up you like, control the idle current/voltage so you don't fry the power tubes.

                          Edit, from my experience vintage Fender the 'bass break up' can some horrible, if the power tubes are sort of mismatched with the amplifier.
                          In this forum everyone is entitled to my opinion.

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