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High dissipation on VOX AC30C2

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  • High dissipation on VOX AC30C2

    Hi,

    I recently sold a VOX AC30C2, after a week the new owner told me that the amp was making poping noises, so I have to take it back. Measured the tubes on a BK Precision 620 and to my surprise 4 of the 7 tubes tested bad.

    I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?

    First I measured resistance from the center tap of the OT to the plates (pin 7) of the power tubes:

    T7P7: 17.9
    T6P7: 17.2
    T5P7: 17.4
    T4P7: 17.2

    Then I measured the DCV on the same points to determine I.

    T7P7: -1.32V
    T6P7: -1.32V
    T5P7: -1.4V
    T4P7: -1.4V

    Plate voltages on the power tubes are:

    T7: 312.2V
    T6: 312.6V
    T5: 312.7V
    T4: 312.7V

    Those measurements gave me the next currents:

    T7: 78mA
    T6: 76mA
    T5: 80mA
    T4: 80mA

    For a dissipation of:

    T7: 24.35W
    T6: 23.75W
    T5: 25W
    T4: 25W

    Pin 4 and 5 (heaters) of every EL84 is getting 2.95 VAC.

    By the way, there is no red plating on the tubes.

  • #2
    Originally posted by caesparza View Post
    Hi,

    I recently sold a VOX AC30C2, after a week the new owner told me that the amp was making poping noises, so I have to take it back. Measured the tubes on a BK Precision 620 and to my surprise 4 of the 7 tubes tested bad.

    I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?

    First I measured resistance from the center tap of the OT to the plates (pin 7) of the power tubes:

    T7P7: 17.9
    T6P7: 17.2
    T5P7: 17.4
    T4P7: 17.2

    Then I measured the DCV on the same points to determine I.

    T7P7: -1.32V
    T6P7: -1.32V
    T5P7: -1.4V
    T4P7: -1.4V

    Plate voltages on the power tubes are:

    T7: 312.2V
    T6: 312.6V
    T5: 312.7V
    T4: 312.7V

    Those measurements gave me the next currents:

    T7: 78mA
    T6: 76mA
    T5: 80mA
    T4: 80mA

    For a dissipation of:

    T7: 24.35W
    T6: 23.75W
    T5: 25W
    T4: 25W

    Pin 4 and 5 (heaters) of every EL84 is getting 2.95 VAC.

    By the way, there is no red plating on the tubes.
    What is the cathode voltage on your power tubes? Are you set for 50 ohms or 82 ohms on the cathode resistor network? If it's set to 50 ohms, your current calculations suggest that cathode voltage is about 7.7VDC.

    Measure the voltage at both settings (50 ohms & 82 ohms), and see if the shared current is similar to what you've calculated. Plate currents as high as you're calculating seem much higher than I've seen in the large collection of AC30-CC2X I maintain here at CenterStaging, LLC in Burbank, CA.

    And, something is wrong with the heater voltage you're reading.

    I've attached the Service manual for the AC30CC2X

    AC30CC Service Manual.pdf

    They DO run quite hot, though I've not seen them THAT hot.

    Were all of your tubes 'bad' when you sold the amp? While I have a good Hickock 539A Tube Tester, I instead go by how the tubes behave in the equipment and how they measure in-circuit. Most of use here go by that, as that is the tube in the applied circuit.
    Logic is an organized way of going wrong with confidence

    Comment


    • #3
      Thanks for your answer nevetslab.

      This one is diode rectified AC30C2 not the one you got AC30CC2 which has a GZ34 rectifier. I'll take the measurements you mention and come back with the results. The cathode resistor is a 50 ohms.

      Here you have a link to the schematic: https://el34world.com/charts/Schemat...Vox_ac30c2.pdf

      Comment


      • #4
        Originally posted by caesparza View Post
        I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?
        You need to divide the calculated current by two as the primary current is shared between two tubes.

        Also if it hasn't already been done subtract the cathode voltage (about 12V) from the measured plate voltage to get the plate to cathode voltage to use in the power calculation.

        80mA/2 * (312-12) = 12W
        Last edited by Dave H; 04-23-2019, 07:00 PM.

        Comment


        • #5
          Hi Dave,

          Thanks for the information, that was the missing part!!.

          I got the following readings:

          Cathode Resistor Voltage Drop: 8.53 VDC

          So the plate to cathode voltage would be:

          T7: 303.67V
          T6: 304.07V
          T5: 304.17V
          T4: 304.17V

          Dividing the current in pairs I get a pretty good figure for dissipation:

          T7: 11.84W
          T6: 11.85W
          T5: 12.16W
          T4: 12.16W

          It's the first AC30 that I have to check, I missed completely the fact that the power tubes are connected by pairs. Dissipating a little over 100%, according to the facts I have gathered about the AC30, not bad number at all.

          Thanks a lot for the help everybody.

          Comment


          • #6
            While you are in there, check that the EL84s' grid stoppers are on the tube sockets, not on the PCB. JJs can have stability (oscillation) issues when the stoppers are on the board. I've seen this with JJ EL84s in (early) Vietnamese handwired AC30s, and with JJ 6L6Gs in a 2001 Fender Tone Master.

            Comment


            • #7
              Did you re-measure the heater voltage again to confirm if it was indeed 2.95Vac on the EL84 - that is still a possible problem. You can also measure the DC heater voltage going to the preamp tubes to see if they are nominal 6.3Vdc (if they are about 6.3Vdc, then you are not correctly measuring the EL84 heaters correctly).

              Your 17 ohm output transformer primary winding resistances also seem way off the 70 to 72 ohm resistance I measured on my AC30CC2 from B+ to either plate terminal. Do you have another meter for measuring resistance - some meters don't like measuring resistance when it is part of a large inductance.

              Comment

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