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M100 with AO-44 reverb amp - no load when reverb tabs off?

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  • M100 with AO-44 reverb amp - no load when reverb tabs off?

    I just finished restoring an M100 with AO-29 and AO-44. The AO-44 was in a bad way and had a new output transformer fitted at some time. The reverb sounds great now, but I noted that there was no load on the amplifier when the REVERB I & II tabs were both off. I wonder if that is why the original output transformer failed? I fitted a 47 ohm, 2 watt resistor across the output tags to give it some load when the tabs are off. Does anyone know why Hammond thought it was OK to operate a tube amplifier without a load?
    "There's not much wrong with it ..."

  • #2
    Originally posted by ssh-boom View Post
    Does anyone know why Hammond thought it was OK to operate a tube amplifier without a load?
    You're certainly correct that it's not a good idea. I suppose Hammond took a gamble that they could get away with it in a fairly low-powered amp. I haven't checked the M-100 diagram to verify. In the A-100, there is always some load on the amp, even though it's 108Ω with Reverb turned off.
    I'm David. 'Dave' is someone else's name.

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    • #3
      Have you looked inside the switch rail at the back side of the 'verb tabs?
      IIRC the load resistors are on the back of the switch assembly?
      A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

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      • #4
        David - I think you are correct about Hammond's attempt to save the cost of one resistor. The A100 has a 100 ohm variable resistor in series with an 8 ohm speaker so there is still a 108 ohm load when the REVERB control is minimum, or OFF as it is labelled.

        Pete - Check the manual, Figure 47. There are three resistors mounted on the switches, but when they are off, they are off.

        If anyone is wondering why this is important, read Hartley Peavey's account of how an output transformer reacts if there is no secondary load:

        https://peavey.com/support/technotes.../Chapter_7.pdf
        Last edited by ssh-boom; 03-23-2018, 01:30 PM. Reason: To remove SkimWords hook!
        "There's not much wrong with it ..."

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        • #5
          The most frequent victims of output transformer meltdown with no load are Marshall heads. All it takes is a faulty speaker cable. I have also seen it happen to a Leslie 122 amp run with no speaker attached.

          Once, a teenager decided to plug his father's 1965 Fender Bassman into a subwoofer to see how that would sound. He didn't blow the output transformer (about which both he was very relieved), but one output tube socket arced and burned between pins 2 and 3 (where it usually happens), creating a conductive carbon path that put B+ voltage on the heater winding and burned up the 100Ω heater balancing resistors.

          But he had a load on the amp, right? Yes and no. What he didn't realize was that the subwoofer had a pretty severe passive low-pass filter between its input and the speaker, so at most audio frequencies, the passive sub's input impedance was so high that, from the amp's point of view, it looked like there was no speaker attached.
          I'm David. 'Dave' is someone else's name.

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          • #6
            We're going 'off topic' but I think it's a useful reminder which will possibly save AO-44 users and other tube amp users from damaging their amps.

            A friend and colleague (A100 owner) pointed out that the RS 'Heavy Duty' output transformer fitted to my AO-44, as a generic replacement back in the day, is now highly sought after, and the transformer is probably worth more than the entire M100. I hope not so now it's all working properly!

            I am restoring a Fender Deluxe Reverb (silver face era) and that needed a new reverb driver transformer. I don't know why that transformer failed, and it is on a much smaller scale, but I'm going to fit a resistor across the output of the transformer in case the reverb spring's drive coil goes open-circuit or the RCA plug is disconnected.
            Last edited by ssh-boom; 03-24-2018, 01:02 PM. Reason: Wrong kind of sought!
            "There's not much wrong with it ..."

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