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Tube intercom to SE guitar amp

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  • Tube intercom to SE guitar amp

    I have aquired a cute little desktop intercom unit which I'd like to convert into a SE guitar amp.

    It seems to have no power transformer, a 35w4 rectifier tube, a 50C5 power tube and a 12au6 pre-amp tube. (all are 7 pin tubes.)

    It seems to have 2 output transformers. I'll try and post a schematic when I have one drawn.

    Any suggestions on conversion for guitar use?

  • #2
    I have one too and thought of making it useful. I've never peered at the guts yet. One is usually very limited in available mods for amps that rectify the AC line voltage. I haven't look up your tube data but I'm guessing that's what you have.


    • #3
      Careful, those transformerless amps are death traps.
      Lots of guitar amps & small radios were made like that in the
      50's & have have killed a few people! Do search & you'll see
      what I'm talking about. You can make them safe by using a 1 to 1
      transformer. Shouldn't be too hard to convert just make sure you
      read up on them before you do. I've got a guitar amp but never use it.
      Some day I'll either grab a 1 to 1 or just chuck it! ;-)



      • #4
        I was thinking today that if I take out the (for my purposes) useless ~4" speaker there should be room for a PT, perhaps a little creativity in mounting will be required.

        So then it's just a matter of finding a good sounding circuit using 7 pin recto, power tube, and preamp.

        Got one?


        • #5
          I'd like to convert into a SE guitar amp.
          I've had a couple 'rubber sneakers' type amps, the chair kept pinching me though, microphone?POWW!Big shot in the face.
          'Hook me up to the power supply' also had hum problems, got literally sparky with other guitar hardware/electronics.
          It seems to have no power transformer,
          That puts you in direct connection with one of those two top prongs on the AC line, one of those is called hot, it was probably constructed with the intent that no contact with the chassis connected 'gnd' items is possible.
          a 35w4 rectifier tube, a 50C5 power tube and a 12au6 pre-amp tube. (all are 7 pin tubes.)
          I have a 50C5 in a 2prong amp, replaced tubes and caps even.
          I would take a look at the data sheets before choosing a power transformer for this.
          I bought new tubes and recapped a ''2pronger''.
          It seems to have 2 output transformers.
          These are always nice ! Stereo or dual mono amp ?
          I'll try and post a schematic when I have one drawn. Be sure to unplug AC and Drain DC.
          Any suggestions for guitar use?
          Don't plug a guitar in it.


          • #6
            Check out this schematic

            I have an amp simular to the Kent 2198 amp . I installed a standard fender 68k resistor with a 1 meg to ground at the input of the 12au6 . I also replace the 1 meg resistor that goes to ground at the input of the 50C5 with a 1 meg potentiometer. The amp sounds great. I bought 2 12v x 120v transformers from Radio Shack and wired the 12v sides together then hooked the power cord to one side of the 120v taps and the other 120v taps goes to rectifir and ground/switch. this will seporate you from the 120v AC coming from the wall if anything should fail in the amp. The amp is pretty simple to work on and i bet you amp is pretty close to the schecmatic i posted. i have an old phonopraph amp that is almost identical to the Kent 2198 amp so this design must have been used quite a bit. I would change the E-caps in the amp also. Jam at your own risk
            Ihope i helped

            Attached Files
            Last edited by baddog; 04-09-2009, 10:28 PM.


            • #7
              You also mentioned 2 output transformers. Im gessing that one of the transformers is being used at the input of the 12au6 as an isolation transformer. It isolates the guitar or mic from the amp to reduce the risk of a shock if somthing where to fail in the amp. I have seen this on old film projector amplifiers from the 50's. this is just a guess withuot having a schematic for referance.
              As always be careful.



              • #8
                just a guess here too.
                the one transformer should be switched when the talk button is pushed.
                I've seen old intercoms where the speaker is the mic and the second transformer is to match the speaker impedance to the circuit.


                • #9
                  to stingray

                  I'm pretty sure you're right, stingray. There is a DPDT slide switch marked "talk" and "listen" and the transformers are connected to the switch.

                  I started drawing a schematic, but... I can trace a circuit but getting it legibly on paper is another story. I stopped working on it before I got to the DPDT. But eyeballing it I'm pretty sure that's what's going on.


                  • #10
                    to baddog

                    hey baddog, thanks for the link to the Kent schematic. I had a Kent when I was in JR High School - maybe even the same model - the era is right.


                    • #11
                      Can anyone explain to me how having a power transformer makes the amp safer? Can one not still get a shock from the voltage on the secondary?


                      • #12
                        In theory the chassis is attached to the neutral of your mains.

                        it CAN be reversed

                        screws that attach the chassis to the cab are now at line potential

                        so is the outer sheath of your cable and consequently your strings and anything else connected to the ground on your guitar

                        if you become the conductor between this and a proper ground you get the full 15 amps (or what ever the the circuit breaker is rated at)

                        using an isolation transformer eliminates this situation. the circuit would not complete from the chassis to a different ground. it can only complete inside the chassis.

                        at least this is how i understand it works

                        If I'm off I'm sure someone will chime in



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