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32H Series 3 Two Speed Conversion and Blown Filter Can?

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  • 32H Series 3 Two Speed Conversion and Blown Filter Can?

    I’m trying to determine if there is something I did that caused the electrolytic can to fry. Before I launch into my blown can question, I’ll set it up with the order of events.

    I recently purchased a Tallboy that had all of the components but not wired up (speakers, motors, etc…not plugged into the amp). Previous owner had work done on the amp (a few caps from what I can see) and never got around to finishing the project. All caps measured fine, 30/30/10 can all within 20% tolerance. This can not recently replaced as some other components - perhaps original.

    I put the amp on the bench for the standard blow it out and inspect routine. I wired up AC to pins 3 and 4, and an audio input (phone thru a headphone amp) to pins 1 and 2, a 16 ohm speaker (not the field coil) and fired it up. To my relief, the amp worked and only had minor hum (which I attributed potentially to my input rig).

    2 Speed conversion: I removed the source A/C (orange wire - pin 4) from the “always on” outlet. The outlet was used to bus the fuse, the relay source, and the 0.1 uF cap from the switched outlets….so I removed all from the outlet and used a wire nut for now. The outlet at this point is open with only common still wired to pin 3. I ran the source side of the outlet to the brake socket pin 4 - a bus point for the (NO) side of the relay switch. I double checked by eye and also used a continuity meter to verify the wiring - manually pressing the relay to simulate switching. I made no other alterations other than cleaning up the relay contacts.

    Some days pass and the next step was to test the crossover and drivers. AC and input wired as above, I tuned it on and heard a loud hum and no audio. I removed the crossover and drivers, and wired the first speaker with the same results. The volume knob did not affect the hum level.

    Next step was to yank power tubes one-by one. I found that one 5881 socket seemed to be the culprit and noticed that while making sure it was seated and pushing gently, all noise stopped - then when I let go it would hum loudly again. I found you can do this for no more than 6 seconds, because at 7 seconds the filter can pops and smokes (just in case you want to try this at home). From underneath, the socket was found to be loose in the chassis but otherwise appeared intact.

    So can we first confirm that my 2-speed conversion is correct before we move on to the smoking can question? Then if we’re relatively confident that the conversion is not directly responsible, can someone give me trouble shooting guidance or post a previous topic thread?

  • #2
    There's an easier way to do 2-speed motors with a 32H amp -- or any with the 6-pin brake socket.

    Use a 6-pin plug with AC wires hooked to pins 3 and 4, plug it into the brake socket, and run the wires to the slow motors. Use the fast motor outlets as they are. Add another 0.1uF arc suppression capacitor to the slow motor side of the relay. No rewiring is required.

    The motor AC circuits are relatively isolated from the rest of the audio circuits.

    With previous work on the amp done by someone with unknown skill levels, the range of possibilities for what's wrong is pretty wide.

    One thought: the field-coil speaker dissipates a considerable amount of power from this amp: >30 watts. If you run the amp without the field-coil load on it, the B+ voltage is going to be a lot higher than normal due to the reduced current draw. If you have an original 30-30-10 can capacitor that you didn't check with applied voltage, that could be enough to kill the capacitor.

    By "check with applied voltage," I mean using something like an old Heathkit Condenser Checker that will run a capacitor up to full rated voltage. At the very least, a can capacitor like that might need to be reformed at this point before being used. And even if you can use it at the moment, it's past the age where you'd want to trust it.

    Removing output tubes one by one is also not a good idea since they are all in parallel. How old are they?

    On some older tubes, you can get bad connections inside the tube pin sleeves that might have to be resoldered. The amp does rely somewhat on the output tubes being balanced, and if one is out of circuit due to a bad connection, the amp will hum. Least hum should be with 4 well-matched 6L6s in it.
    I'm David. 'Dave' is someone else's name.


    • #3
      This probably has nothing to do with the cooked can capacitor, but your input jig needs to be grounded to pin 2. If you are applying signal with a transformer, and this transformer's secondary has a center tap, the center tap of the transformer is the signal ground, and should be attached to pin 2.


      • #4
        I guess it's fixed....
        I'm David. 'Dave' is someone else's name.


        • #5
          David - thank you for the guidance and the 2 speed conversion alternative (surely others looking to do this will come across this thread). I did not check the can with the applied voltage (guess I don't need to do that now!), I ordered a new one. As well I will replace the one tube socket as it is delaminating and loose. I'll also replace the mismatched 6l6's that came with it, they are also quite old. I'll report back. Thanks again for the help.


          • #6
            The phenolic wafer sockets can become brittle with age and heat. If one is bad, check the others.
            I'm David. 'Dave' is someone else's name.