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AO67 negative bias voltage

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  • AO67 negative bias voltage

    Hello
    i have a AO67 amp with a 66 reverb amp
    The power tubes are red plating
    B+ plate volts DC about 315vdc
    Screen volts about 305vdc
    Output transformer center tap about 325vdc
    Bias voltage -14.5vdc supposed to be -18vdc

  • #2
    The plate and screen voltages are well within the 20% tolerance
    the amp seems to sound ok, maybe a bit overdriven

    Comment


    • #3
      Would refreshing the power supply and bias capacitor's solve this?

      Comment


      • #4
        They should certainly be checked at this point in time given their age.
        Hammonds; BV, M101, T202, T402, R100
        Lowreys; Heritage DSA, Berkshire Deluxe TBO-1
        Farfisa Compact Duo MK2 CD/30
        Analog Korg CX3
        Leslies; 147, 145, HL822 ​

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        • #5
          There are a number of things to check. This is a "back-bias" amp, a variation of cathode bias, not fixed-bias with an independent negative grid supply.

          Check the value of resistors R102 and R103 since these determine the -18V value, acting as voltage dividers for the -21V potential you should find at the PT B+ winding center tap. Also check the values of R101 and R101A since these determine the -21V potential. The AO67 seems to have a slightly unusual feature of using overall B+ current to power the V4 12AX7 heaters in conjunction with R101 and R101A. Check or replace capacitor C64. And check C24 and C25 for DC leakage.
          I'm David. 'Dave' is someone else's name.

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          • #6
            Ok thank you
            I was planning on recapping the electrolictics and replacing the bias resistors
            i will have to recheck the -21vdc can't remember if it was ok or not
            So my big question is with -14.5vdc is that hot or cold

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            • #7
              With -14V, your output stage will be biased too hot. If it were cold, your tubes wouldn't be red-plating.
              I'm David. 'Dave' is someone else's name.

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              • #8
                Ok
                thank you
                kinda what I thought
                Kinda like a throttle
                Thank you for clarifying

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                • #9
                  I replaced the two resistors 10k and 56k and replaced the capacitor 25uF/25vdc
                  the voltage is now at -16.5vdc sometimes it goes to -22vd
                  It's strange slowly goes to -22vdc then starts going to -16.5vdc
                  I think the power supply needs to be redone
                  I'm thinking that there is some DC ripple voltage passing through the coupling capacitor's.

                  ​​​​​

                  ​​​​​

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                  • #10
                    Originally posted by Baseman norm View Post
                    I'm thinking that there is some DC ripple voltage passing through the coupling capacitor's​​​​​
                    That's not how this works. You might have direct current leakage through the coupling capacitors, though this would tend to affect one output tube more than the other, especially since the two coupling capacitors are at different voltages. The one at higher voltage would be more likely to fail. I mentioned checking them to cover all the bases.

                    Power supply ripple at 120Hz can get into audio stages, but that manifests itself as hum, not red-plating.

                    The power supply might need to be recapped, but it's unlikely to cause this problem.

                    I'm David. 'Dave' is someone else's name.

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                    • #11
                      Oh okay thank you
                      no humm going on
                      One tube is slightly more red plating actually it's more noteable than the other
                      One thing that I haven't mentioned yet is the reverb amp is not being used, I disconnected it completely.
                      Would it be the reverb amp not there loading the power supply have any effect on the power supply voltages? B+300vdc
                      I'm pretty new at this stuff
                      the plate voltage is 328vdc a lot higher than spec of 308vdc
                      Grid 324vdc spec is 300vdc
                      After redoing the capacitor 25uF/25vdc C64 and the two resistors 10k and 56k
                      The negative bias went to -16.5vdc from -14.5vdc spec is -18vdc
                      how big a deal is a small change in bias voltage?
                      Meaning how critical is it?

                      Thanks for your time and guidence

                      Comment


                      • #12
                        Good evening,
                        > The diagram is this:
                        Click image for larger version  Name:	AO-67.jpg Views:	0 Size:	26.4 KB ID:	735637
                        > As mentioned by David there may be a problem with C24-R44 C25-R45.
                        - PowerOff, Check these components.
                        - PowerOn no signal, Verify that the voltages across R44 and R45 are indeed zero when the amplifier is energized. This test can also be done without the 6L6.
                        - Swap the 6V6 and observe the plate color in the dark.

                        JP

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                        • #13
                          It's tricky to test C24 and C25 even with the output tubes removed in this amp. Because it's a back-bias amp, all the B+ current flows through the resistors that produce the negative grid control voltage, even the preamp tubes. You could disconnect C24 and C25 at one end, which would remove them from the circuit. Then you could measure the voltages with no possibility of leakage.

                          JP's idea to swap the 6V6s is a good idea. If C24 is leaking, it will pull the -18V control voltage higher. It will affect both tubes, but will affect V6 more than V5.

                          Also, if tubes have red-plated too much, they can be damaged and will no longer hold a stable bias even if the amp is fixed.

                          As for the measured voltages, wall voltages are a good bit higher these days, and that means that B+ voltages in tube amps are almost always higher than the schematics show. They varied a lot even in the old days. It's extremely rare to find a tube amp where plate and cathode voltages all match the schematic perfectly.

                          AFAIK, the reverb amp has a completely separate power supply.
                          I'm David. 'Dave' is someone else's name.

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                          • #14
                            Tube swap is about the same no noteable change.
                            i have some fresh mylar .047 caps to change the c24 c25.
                            I will check r44 r45.
                            Not really sure on how to check for DC leakage through c24 c25.
                            Voltage across r44 r45 is that from pin 5 to pin 5 ?
                            Thank you for your time and guidance.

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                            • #15
                              The power for the AO66 reverb amp gets B+300vdc from the AO67 power supply

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