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Magnatone M15A speaker-driven reverb tank?

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  • Magnatone M15A speaker-driven reverb tank?

    I'm working on a late 1965 Magnatone M15A amp, and the output transducer on the presumably-original Gibbs spring tank is open. On this amp, the tank is driven in series with a 100 Ohm resistor from one of the 7189A ultralinear output channels, each of which drives a single 16 Ohm speaker.

    The tank has the letter "F" printed on the inside in the same place where I've seen "4V" on other tanks from that era, but the input winding measures 1.2 Ohms DCR, indicating an 8 Ohm input. The F tank is supposed to have a 1,475 Ohm input impedance.

    Does anyone know exactly what the original tank specs were for this version of the Magnatone M15A? There doesn't seem to be much documentation on these amps other than the Magnatone site, which I have read thoroughly. I want to make sure I get the correct replacement tank.

  • #2
    Originally posted by Rhodesplyr View Post
    The tank has the letter "F" printed on the inside in the same place where I've seen "4V" on other tanks from that era, but the input winding measures 1.2 Ohms DCR, indicating an 8 Ohm input. The F tank is supposed to have a 1,475 Ohm input impedance.
    That old F designation is not the same as the newer 4AB types codes. There used to be two tank codes Type F and type C.

    I was told a long time ago that the F stood for Fender and Fender type drive circuits, meaning 8 ohm impedance driven by a speaker output or directly from an output transformer. There was a C type tank which was for capacitor driven circuits, directly from a tube plate like an old Ampeg style circuit.

    Hope this helps.

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    • #3
      So based on the Magnatone site it sounds like you have the "second M15A" variant? Does it have the tone and echo controls? The schematic they have for the late 65 M15A has a part number that is almost readable, 25-000x, where x to me looks like a 5 but the only valid similar number I can find for a tank is a Hammond 25-0002, which would be a higher impedance tank.

      Just thinking about it from a voltage divider perspective, though, if you have a 1.5k ohm tank in series with a 100 ohm resistor, and those two in parallel with a 16 ohm speaker the voltage to the input of the tank is going to be almost as big as the voltage on the speaker, which seems too high a drive level to me. An 8 ohm tank would give something more like 8% of the speaker voltage at the tank input, which may be too low? With the way the circuit is designed it seems like the input impedance will not hurt the amp either way, but may be too much or too little drive to give a pleasant reverb effect.

      https://www.magnatoneamps.com/M15reverb.html

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      • #4
        Yes, I have the March 1965 to January 1966 version in which the tank's output transducer appears to function as the cathode resistor for the first recovery stage.

        Looking at the schematic glued inside this amp, the reverb unit is clearly "25-0005," and there is a 25 ________ part number on the reverb tank. The rest of that number on the tank itself was eaten away by adhesive foam applied to the tank.

        I guess you could play with the 100 Ohm series resistor value to get more reverb if you desired it, or you could use the resistor-lamp limiter bridge from Hammond's AO44 reverb amp.

        The information I'm getting seems to confirm that the "F" found on 1960s reverb tanks does not correspond to the later F code for impedance ratings.

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        • #5
          The fliptops info for the Ampeg type F agrees with the 8 ohm input impedance:
          https://www.fliptops.net/catalog/p-1...-ampegs-fender
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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          • #6
            Originally posted by g1 View Post
            The fliptops info for the Ampeg type F agrees with the 8 ohm input impedance:
            https://www.fliptops.net/catalog/p-1...-ampegs-fender
            Yes, I think this one will end up being the 4AB3C1C, vertical-mount.

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            • #7
              FWIW I did something VERY similar with my stand alone reverb amp. It was built as an adjunct to a head and cabinet that I made for a customer. The initial head and cabinet were intended for cranked amp tone so a built in reverb wasn't suitable for the amp (because the reverb would be distorted). I used a 100 ohm series resistance for the input to the reverb tank to be driven by the "host" amp speaker output. I also had a VU meter so that an ideal drive level could be dialed in for amps of any wattage and protection diodes in case this was ignored by the user But the concept and the basic circuit seems to be about the same. I don't remember getting the circuit values from any Magnatone schematic, but it was some time ago and I did a lot of research to build that thing. It works PERFECTLY. I guess my point is that a 100R resistor in series with the input of an 8 ohm tank off an 8ohm amp output worked for me to achieve a good drive level. IIRC this would have been for amps from 30W to 50W. The VU meter and adjustment circuit I used allowed for dialing it in for amps from 5W to 100W, but the "nominal" range, with just the 100R resistor in series was good for an average stage amp of 30W to 50W.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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              • #8
                The M15A uses two channels, each with one pair of 7189A running ultralinear fixed-bias -- or rather a hybrid fixed-bias since each pair has a 100 Ohm cathode resistor in addition to a negative grid bias.

                I would guess no more than ~19 Watts clean per channel. The reported specs on Magnatone's site say 2 x 38 Watts peak, which I suppose could be true for a brief transient on a good day with a tailwind ;-)

                Magnatone gets the reverb signal from one output channel and then sends the reverb signal to the other channel, probably to avoid feedback issues.

                In Hammond A-100 organs with AO44 reverb amps, the reverb tank is driven from a 17 Watt amp designed to run a 4 Ohm load, and there is a limiter bridge composed of two lamps and between the amp and the tank. The bridge allows more signal to the tank at lower volume levels, but limits it as increasing signal heats the lamp filaments.

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                • #9
                  Originally posted by Rhodesplyr View Post
                  In Hammond A-100 organs with AO44 reverb amps, the reverb tank is driven from a 17 Watt amp designed to run a 4 Ohm load, and there is a limiter bridge composed of two lamps and between the amp and the tank. The bridge allows more signal to the tank at lower volume levels, but limits it as increasing signal heats the lamp filaments.
                  Oh yes. I remember that one well. Enzo almost talked me into using it. I might have except that I'm not clever enough to know what specific lamp requirements to look for. Plus, I wasn't concerned about keeping the reverb drive regulated since the unit I built had the drive level control. So whatever amp setting your using you could still adjust the dive level and final reverb level.

                  Are you thinking of trying the lamps, or is this just an observation? Because I'd be interested in following if you do lamps.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                  • #10
                    I could try the lamp limiter circuit. I'm a Hammond tech, so I actually keep the #12 lamps in stock. After years of use, they do burn out.

                    There are screw-base lamps with the same filament specs as the #12s, and that's all that matters.

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