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Help me understand power section of AC30

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  • Help me understand power section of AC30

    My bandmates have two 1979 AC30 Dallas/Rose Morris era Top Boosts. One of them was recently going really low in volume, like ballpark of AC5. I have recapped power section and found out also one bad coupling cap (that may be the reason of the low output).

    The tubes are slightly red-plating. They are JJ EL844 which are "lower wattage" versions of EL84. They test around 50% emission so they will be replaced soon...
    My plate voltage is 345V measured at the tobe socket with amp working, so seems about right.
    One of the 100ohm resistors on the power board was blown, I have replaced it for new 100ohm, but 5W (hope it's ok in this place to have that strong one(?) This is the big lime one on the shot below.
    I'm getting 7,3V across the 47ohm cathode resistor.

    What confuses me it's 5W 22k resistor (white on the photo below) which on schematics I found is 220k, so ten times more... I'm getting around 113V across it. Should I replace it for 220k? What is the function of this one in AC30s?

    Click image for larger version

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    Last edited by boroman; 02-17-2020, 04:11 PM.

  • #2
    I don't see it. I see in schematic 220k grid returns for the power tubes. I see a layout drawing that does not match your photo. The large resistor in the photo does not appear to connect to the 1.5k resistors. In the schematic I DO see a 22k power resistor in teh B+ string to the phase inverter.

    So just WHAT does your mystery resistor actually connect to in the amp?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Slightly red-plating? That's still probably quite bad.

      The 22k should certainly also be a 220k, matching the one on the other side of the phase inverter. (Assuming, of course, that's the 220k you're referring to.) Note that I'm looking at a stock AC30 schematic and not the one you provided. (AC30-60-02, which .... well I guess it might not match.)

      113V across it seems awfully high to me. Are you sure the coupling capacitors are both okay? That is, the 110nF caps that feed these 220k resistors and the 1.5k grid resistors. The only other place that resistor could be drawing voltage (if not through a leaky cap) would be the grids of the EL84's, and since you said the cathodes are sitting at 7.3V that puts the grids quite positive with respect the cathodes, and that's also not what you want to see.

      If you replaced one of the 100Ω screen resistors with a 5W, I'd recommend taking it out and going back to spec 1/2W. You want that screen resistor to blow before a failing tube can further damage something expensive like the output transformer.
      Last edited by BobK; 02-17-2020, 05:27 PM. Reason: clarity, I hope

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      • #4
        Originally posted by Enzo View Post
        In the schematic I DO see a 22k power resistor in teh B+ string to the phase inverter.
        Originally posted by BobK View Post
        113V across it seems awfully high to me.
        If the 22k resistor in question is in the power supply, 5ma to a couple 12AX7 tubes sounds about right. On the other hand, 5ma across a grid leak? It hurts to imagine.

        As an aside, my experience with EL844 tubes amounts to a verification that they do draw way less current, and bias on the cathode resistor to a much lower voltage, than EL84s. My recommendation is to put factory-approved tubes into the amp until you've fixed anything wrong. Then see what EL844s do in it.
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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        • #5
          I wouldn't replace a component with one of a different value just because it maybe looks wrong as the amp could have been running with this value for decades. If it's in a dropper string then 22k seems correct.

          Comment


          • #6
            Originally posted by Enzo View Post
            So just WHAT does your mystery resistor actually connect to in the amp?
            Enzo, it goes to the other board (brown wiring) and connect to terminal that goes to the 32uF positive.

            The 22k should certainly also be a 220k, matching the one on the other side of the phase inverter. (Assuming, of course, that's the 220k you're referring to.) Note that I'm looking at a stock AC30 schematic and not the one you provided. (AC30-60-02, which .... well I guess it might not match.) 113V across it seems awfully high to me. Are you sure the coupling capacitors are both okay? That is, the 110nF caps that feed these 220k resistors and the 1.5k grid resistors.
            The coupling mustard caps are 0,15uf (150nf) here. One of them was not connected properly, that's probably why the amp ran very low and distorted... Both caps measure okay.

            If you replaced one of the 100Ω screen resistors with a 5W, I'd recommend taking it out and going back to spec 1/2W. You want that screen resistor to blow before a failing tube can further damage something expensive like the output transformer.
            Yeah... that's what I've thought. I don't have them but will order some.

            As an aside, my experience with EL844 tubes amounts to a verification that they do draw way less current, and bias on the cathode resistor to a much lower voltage, than EL84s. My recommendation is to put factory-approved tubes into the amp until you've fixed anything wrong. Then see what EL844s do in it.
            My friend already ordered quarter of TAD's blackplate EL84s.

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            • #7
              So22k sounds OK.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                I have put new TungSol matched set of EL84s
                I get now 10.5VDC across the cathode resistor - and bit of redplating I think... 150% dissipation, should I try higher value cathode resistor? 60 ohm? 80ohm?

                See:Click image for larger version

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                Last edited by boroman; 02-20-2020, 10:37 AM.

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                • #9
                  I don't see your plate voltage in the screen shot. Try the Weber bias calculator and see if that ties in. I plugged in your original B+ and the rest of your values and it does look rather high at 150%. Did you measure the cathode resistor value?
                  Last edited by Mick Bailey; 02-20-2020, 11:39 AM.

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                  • #10
                    Original 60s AC30s ran the tubes at around 14W plate dissipation. Also used a cathode R of 47R, but at a plate voltage of 310V (B+ = 320V).

                    I would change the cathode resistor to 68R.
                    Last edited by Helmholtz; 02-20-2020, 12:40 PM.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by Mick Bailey View Post
                      I don't see your plate voltage in the screen shot. Try the Weber bias calculator and see if that ties in. I plugged in your original B+ and the rest of your values and it does look rather high at 150%. Did you measure the cathode resistor value?
                      PV is 350V (from the tube socket)
                      Resistor value on the resistor 47R, measured 48ohm.

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                      • #12
                        For me, I'd install a MOSFET B+ dropper and that would take care of the rest of the voltages at the same time and give the whole amp an easier life. You can tailor exactly the right B+ you want.

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                        • #13
                          Originally posted by boroman View Post
                          I have put new TungSol matched set of EL84s
                          I get now 10.5VDC across the cathode resistor - and bit of redplating I think... 150% dissipation, should I try higher value cathode resistor? 60 ohm? 80ohm?

                          See:[ATTACH=CONFIG]57024[/ATTACH]
                          Is this an online calculator? Do you have a link?

                          Edit: Found it, it's here: https://robrobinette.com/Tube_Bias_Calculator.htm
                          Last edited by Helmholtz; 02-20-2020, 01:19 PM.
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            Original 60s AC30s ran the tubes at around 14W plate dissipation. Also used a cathode R of 47R, but at a plate voltage of 310V (B+ = 320V).

                            I would change the cathode resistor to 68R.
                            I second this. I just had an AC30 here with new JJ EL84's that with the 50 ohm cathode resistor, new coupling caps, new cathode bypass cap and grid leaks that would slowly go into thermal runaway. I went with 68 ohms too.
                            Experience is something you get, just after you really needed it.

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                            • #15
                              Originally posted by nickb View Post
                              I second this. I just had an AC30 here with new JJ EL84's that with the 50 ohm cathode resistor, new coupling caps, new cathode bypass cap and grid leaks that would slowly go into thermal runaway. I went with 68 ohms too.
                              I'll try that!

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