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converting radio to guitar amp, need advice (pics)

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  • converting radio to guitar amp, need advice (pics)


    I recently discovered that this is possible and found an old working tube radio/vinyl player at my parents house. So i brought it home and just connected my guitar into the place where the gramophone seemed to connect to the amp and it worked. The guitar signal was too weak to distort so i connected a stompbox distortionthingy (Peavey Dirty Dog) in between and with some tweaking found some great sounds playing throu the built in speaker.

    Ive decided to put some time and effort into turning this thing into a decent amp while learning and need help sorting out what parts belong to the radio (so I can remove them) and if there are tweaks to be made to make it sound even better when replacing the old components with new ones (caps, etc). Do I need to build an op-amp? Maybe the vinyl player has one that isnt working, I noticed one of the small tubes seemed broken none of them lighted up... I also wanted to ask if you guys would like to help me insert a tone control thingy (read about the Fender Champ one that was fairly simple to build).

    Thanx in advance

    Attached Files

  • #2
    That's a really unconventional circuit to learn on-

    Also there's a very good chance it won't be safe. I'd reassemble the neat antique radio and keep it for what it is- a neat classic piece.

    Clearly you're a novice, so there's a few things you have to learn about these tube 'thingies'. The first thing is that vacuum tube devices generally contain lethal voltages that can quickly kill you if you make a false move.

    These old radio circuits often used the AC mains with no power transformer isolate them from the wall, which leaves the chassis at full potential if you ground yourself. This is automatic death when you are handling a guitar that has the strings grounded to the chassis of the amp.

    If you're interested in building a small amp, seriously consider finding a kit for a fender Champ. This is a classic circuit, simple, there are many inexpensive kits, and it's a very good moddable (is that a word??) little amp. You can learn a lot from it. All of the building blocks of the larger amps are contained within it, and on top of that- they also sound quite good.

    Please read about safe practices while working on tube amps. We all have to start somewhere, and you'd be hard pressed to find someone on here who hasn't been zapped. But it's best to avoid it.

    Be very careful, do a lot of reading, and think about a more practical project using a tried and true circuit for your first build.

    Once you've learned the fundamentals, what each part does, and how it does what it does, then you'll be more able to mess with more obscure circuits.


    • #3
      Originally posted by akimball442 View Post
      There's a very good chance it won't be safe. I'd reassemble the neat antique radio and keep it for what it is- a neat classic piece.
      It looks in great shape and will never be what you want it to be. Put it back together and look for a friendlier donner. Someting with a power transformer, heater taps, and conventional tubes.
      Last edited by tboy; 04-06-2010, 04:05 AM. Reason: quote repair


      • #4
        Originally posted by akimball442 View Post
        ...Also there's a very good chance it won't be safe...These old radio circuits often used the AC mains with no power transformer isolate them from the wall...
        Uh, isn't that large square thingy a transformer?

        If not, what is the job of the AZ41 ?


        • #5
          Though it does appear to have a transformer and rectifier, I would agree with olddawg that it will not be suitable for what you are doing. Without schematics it will be impossible to tell you what are the radio parts "to remove". Some of them will be part of the metal chassis and hard to remove. You will not be able to tailor the circuit to guitar use without schematics and will have to completely gut the circuitry and start from scratch. There are much more usable "junk chassis" available for learning on.
          It looks like a real nice piece and you said it is working. I will plead with you to keep it that way. If you stick with it, one day you will know enough about this tube stuff to really be able to appreciate that piece for what it is. It may also become valuable some day.
          Most of us on here have probably had something long ago that we say "man, I wish I had kept that old..."
          "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


          • #6
            Yes, it has a power transformer, so should be relatively safe to use. (I remember getting some nasty shocks from things like this as a kid, but nothing fatal.)

            The tubes are a standard lineup for a radio, but non-standard for guitar amps, which will make your life harder.

            Tubes you're currently using:
            AZ41: Full wave rectifier with 4 volt heater
            EL41: Power pentode similar to EL84
            6AT6: Dual diode/triode (only the triode section is in use)

            Radio frequency tubes that aren't relevant:
            6BA6: Remote-cutoff pentode
            ECH42: Triode-heptode frequency changer

            If you rewire it so your guitar goes straight to the grid of the 6AT6's triode section, that's probably about as loud and badass as it's going to get.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


            • #7
              Thanx alot

              I want to thank everyone for responding and the mod who moved the thread to right part of the forum.

              You are right about me being new to this but i have some basic knowledge about electronics and what components do.

              I respect all opinions and restoring the radio has been an alternative but tbh I would never ever use it as such and that would be more of a waste imo. The radio produced some really sweet guitarsounds, it just needs some tweaking and perhaps proper grounding.

              I'm at work now, just dropped in to thank you guys but my plan is to start replacing caps, one by one and try the sound after each replaced part to see if I can notice any improvement. I like working methodically, perhaps I'll document every step and post about my findings here... Last night I was trying to figure out where each tube leads but drawing a scheme would require a complete disassembly. Now I know what each tube is supposed to do (Steve Conner <3), so the task does'nt seem impossible.

              Last edited by comae; 04-06-2010, 01:27 PM.


              • #8

                It sounds like you have a positive approach to this - nice to hear. No reason you can't tinker with the radio as long as you follow proper safety procedures. You will learn a lot in the process, guaranteed. Along those lines, to "see" what caps and resistors you are changing, I recommend getting the correct schematic for this radio.

                It is probably out there because these things were meant to be serviced. Google the manufacturer, model and the word "schematic" and see. There is also a great resource, the Antique Radio Forum, whose members are very helpful. They are most familiar with American radios, and yours looks European, but give them a shot. You might also try, but I believe there is a fee to join and get schematics there.

                With the schematic in hand you can, among other things, focus on the audio amplifier circuit and leave alone the radio circuits. Then you can gradually make adjustments, especially to the input so that it's a good match for guitar. Before long you'll be repurposing all kinds of forgotten treasures. I happen to like organ amps, but tape recorders, radios and record players are all fair game.

                Good luck!


                • #9
                  What is the make and model? Maybe we can find a schematic which will save you a lot of time. For example, if you are replacing all the caps one at a time, some of them will only affect the radio. With a schematic you could ignore those etc. You can also post the make & model in the "schematic requests" forum.
                  It will be great if you document everything you do, and draw out any changes you make. If ever you want to go back to stock you will be able to. Also, if something stops working or sounds bad, you can go back to when it worked/sounded better.
                  "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


                  • #10
                    In terms of the schematic, since you are really only concerned with the AF section, you don't need to work out the entire radio, just the last two tubes and the power supply. The AF section is one triode and the power tube, if you diagram those two tubes from phono input to speaker and if you work out the power supply nodes that service these tubes, you should be good to go. All the rest of it you can pretty much ignore. In my opnion you will not need to disassemble the radio to diagram it. Back in the days before the internet, when you had to go find schematics in a SAM's manual at the library (you can still do that today, too!), or order them by mail, I did several antique radio schematics by hand for myself rather than pay. You will actually learn a lot by doing this.


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