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AO67 negative bias voltage

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  • #16
    Yes I figured the tubes would get damaged but I don't want install freash ones yet I have a pair of matched groove tube 6v6gc


    • #17
      Ok that's good to know that the wall voltages are ok


      • #18

        The direct measurement of the voltages between GND and the grids of the 6V6 is not possible because the meter itself will disturb the reading.
        Responsible are 330k resistor R44 and R45.
        This is why we prefer a zero method which consists in measuring the voltage across this resistor. With or without the 6V6, you must find zero which indicates that C24 and C25 are not leaking. If not zéro, there is something wrong in the amplifier.

        Sometimes we regret not having a tube tester.......



        • #19
          Something isn't making sense to me here. When tubes are red-plating, it means they are drawing too much current, but if they are drawing more current, the -21V reading at the B+ winding center tap should be more negative.

          The potential downside to a back-bias amp like the AO-67 is that it sums ALL the current drawn by all the tubes to produce that negative voltage. If other parts of the circuit are not drawing the expected current, the bias voltage will not be adequate to control the 6V6s.

          If you look at the AO-29, it specifies ~140mA being drawn from the B+ supply at idle. For the AO-67, the figure is higher.

          Ah, but I did just realize something. I've never dealt with a Hammond with the AO-66 reverb amp, and the AO-66 draws about 90mA, which is returned to the AO-67 ground. Thus, all current drawn by the AO-66 runs through the bias resistor network (R101, R101A, and the V4 heater filament) to produce the -21V control voltage. By removing the AO-66, you have reduced the overall B+ current, changing the bias voltage and red-plating your 6V6s.

          To operate the AO-67 like the AO-29 -- without the AO-66 -- you'll need to reconfigure the AO-67's power supply to match the AO-29 with its 180Ω resistor and a few other small changes. (If you do the math on R101, R101A, and the V4 filament, it works out to around 100Ω where you have 180Ω in the AO-29.)

          Removing the AO-66 didn't bump the B+ voltage up that much, but it reduced the overall current draw of the B+ supply significantly because it's a parallel load to all the other circuits. That's what's causing your problem. Sorry I didn't see this earlier -- had a very stressful last few days.

          In the end, all this is the result of disconnecting the reverb amp.
          I'm David. 'Dave' is someone else's name.


        • #20
          Ok thank you
          makes sense
          I did some rework on the power supply
          I am biased at about -20vdc now

          One thing that I am also doing is
          This amp AO67 to power the 15 woofer in the frankenleslie 600 and using the AO29 for the rotosonic and horn, getting treble signal from the PR40 treble speaker and going to use the PR40 bass speaker for the bass signal to to the AO67

          Being that I only need the power section, I have removed all the preamp tubes ,sockets,power supply,swell volume box, voltage divider board etc

          I added up all the heater currents and plate currents I have more than enough current available for switch up to 6L6gc biased at about -20.5vdc
          I was thinking about adding trim pots to fine tune and balance the -bias.

          In the dark there might be a very very faint glow of possible red plating.

          Tested it with headphone level signal from a pioneer receiver to the 12au7 v3 driver tube at pin 7, sounds pretty good.can turn the volume up to full remains clear, alias Leslie 251 type input without a volume control,
          Which still needs to be added.

          I would like to have additional input stage stages for guitar (bass) with volume control and a simple tone control.

          The cabinet is completed and functioning two speed motors cleaned and serviced,
          recapped the crossover but something is odd tho, one cap only has one leg connected to the positive terminal of the woofer the other leg is not connected to anything.

          i have been trying to source a schematic for a Leslie 600
          the one at captain foldback.com is very blurry.
          If anyone has a clear copy that would be great.

          The original caps are 8uF/50vdc for the horn,
          with an inductor for the rotosonic.
          The woofer is a 20uF/50vdc

          i am very pleased at the results so far the rotosonic with the treble signal from the PR40 powered by the AO29 rca jack has a real nice growl even better when playing on the great

          A105 with PR40 and Leslie 51c changed up to a 251 with the Frankenleslie 600 (no space generator) still got to sort out the signal switching and teach the wife to use her heal more on the throttle. Lol
          Got lots of power no competition with the guitars and drums
          She likes it a lot more than the T211 or the M103 or the M3

          The M3 was playing flat solved that and sold it.

          She liked M103 but it didn't have enough power and the swell box slipped on the shaft and the line out box didn't work out very well as it caused the amp AO67 (same amp) to stop working.
          She didn't really care for the T211 kinda sounded pale and thin compared to the M103 and the M3
          But she sure does like the A105

          You still here?
          Anyways that's my story and I'm sticking to it.


          • #21
            pictures to be posted soon


            • #22
              Oh, I didn't realize that you were simply using it as part of a Franken-Leslie and doing away with all the other AO-67/AO-29 circuitry. In that case, I would probably do away with the back-bias circuitry altogether and just cathode-bias the 6V6s with a fairly standard 250Ω to 300Ω 5W cathode resistor (like any of dozens of 1950s tube amps), bypassed with a capacitor. And, of course, rewire V4 to the 6.3V heater winding.

              I thought we were talking about an M-100.
              I'm David. 'Dave' is someone else's name.


              • #23
                I was thinking of changing v4 to 6.3acv heater current wise
                I have room for two more 12xx7 or maybe a 6au6 or 6c4 for guitar or bass with a volume control like in a 251 and a simple tone control.
                Lots of possible scenarios.

                M 103 is where the 67 come from got it for free cabinet was pretty rough
                It still worked pitch was spot on no hum every thing else worked not very load tho had to mic it which created new issues with the room.

                I needed a way to power the Franken600
                ​​​​​Didn't want to use the solid state stuff.

                I made some power supply changes and switched up to 6l6gc fender groove tube blues I still have lots of available plate current left over.

                Played music through it last night headphone signal to the 12au7
                Sounds pretty clear full range, I'm pleased. This has been a very good project for me keeping me busy and the mental health stimulation with the current situation with what's going on.

                It has been a real pleasure to me joining this forum
                It's been so nice to have the opportunity to chat with people.
                Thank you


                • #24
                  Keep in mind that 6L6GCs draw twice the heater current as 6V6s. It's not just B+. I have a friend who put 6L6s in a Fender Princeton and burned up the power transformer.
                  I'm David. 'Dave' is someone else's name.