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Old Grundig Conversion

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  • Old Grundig Conversion

    This would be my first attempt , should I build a kit first to get my feet wet or is this doable ? I'm going for an old Fender Deluxe or Champ tone . I can take some pics of the inside after work .
    Last edited by tboy; 12-03-2008, 11:42 PM.

  • #2
    My advice would be to go for a kit first.

    I'm about to fire up a BF Champ (AA674) SE some time over the weekend.

    This is also my first build and I've been looking at schematics and layouts, and reading up, and I wouldn't venture out in converting a old radiogram into a guit amp as a first project.

    I collected various "bits'n-pieces" from all over: The chassis was an old LAN hub, the valves and speaker I got from my son-in-law, and the transformers were wound locally, the case is plywood left over from woodworking project...

    I've used the Ceriatone layout:

    Another project to try is the 5E3 Fender Tweed Deluxe. Push-pull 15-18watt 6V6


    • #3
      Depends what you find inside.

      For a champ-style amp, you'll need:

      a 10W-15W (4R, 8R or 16R) guitar speaker, and cab (sized to suit), a solidish metal (steel, brass, or aluminium) chassis big enough to fit everything into comfortably with the correct diam. tube socket holes in it (most peeps can make or find the other holes that are required - otherwise you can find existing tube socket holes in an existing chassis to suit), a power tranny with a 240ishV-0-240ishV* to 320ishV-0-320ishV* 60ish-90ish mA HT secondary and a 6.3V 2A heater secondary (and a 5V 2A secondary if you're using a tube rectifier), a single-ended output tranny of about 5-15ishW with a 6-10ishk primary and 4R, 8R, 16R secondary(s), an output tube (like a 6V6 or an EL84) & socket, a pre-amp tube & socket, a rectifier tube (like a 5Y3GT) & socket /or 1A 1000V diodes, one (or two) 1/4" input socket(s) with a tip-ground switch and (ideally) a 1/4" output socket (and TS plug), three 8-20ish uF 450V min. electrolytic caps for the power supply, two 22uF-25uF 6V-25V electrolytic caps for the bypass caps (all these electrolytic caps should probably be bought new - old ones are unrealiable), two 400V-630V coupling caps (about .02uF), about a dozen resistors of assorted sizes, a 1MA vol control pot and knob, and assorted bits of insulated wire, something to mount the resistors and caps on (like an eyelet or turret board, with standoff fasteners to enable it to be bolted to the chassis), a power switch, a fuse holder and 2A slo-blo fuse, an AC mains cord, a pilot lamp and lamp holder (good safety feature), some cable ties and clamps/fasteners, a few nuts and bolts, leaded solder (unleaded is a PITA), plus tools

      (I don't think I missed anything out?)

      * = depends whether you're going to use tube or diode rectification

      Check out the 5F1 schematic and layout.

      If your radio thingy has got some of those things, then fine, but you may find that have to buy some of the bits (like new electrolytic caps and resistors) tho'.

      You also need to know how to safely discharge filter caps (even old caps can store harmful charges) and work on tube amps without getting yourself killed, otherwise there is no point right?
      Last edited by tubeswell; 10-31-2008, 03:54 PM. Reason: missed some stuff out the first time round ;-)
      Building a better world (one tube amp at a time)

      "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo


      • #4
        Thanks tubeswell ! Here is what's inside .


        • #5
          Before you pull it apart, check and see if the thing all lights up and works when its plugged in and switched on. That's the easiest way to tell if the tubes and transformers are all working. (But don't poke your fingers in the back until you've discharged the filter caps.)

          You need to (carefully) pull the tubes out and see what they are. Its hard to tell from this angle - I suspect you've got a small signal pentode (the little Seimens), but I can't read the markings on the bottle clearly. There also looks to be another small tube (possibly a dual triode?). Its hard to say what the larger bottle is - could be a miniature beam pentode (EL84?).

          The National Valve Museum site has pretty good info on older tubes. You should check it out

          Building a better world (one tube amp at a time)

          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo


          • #6
            Ok I chickened out on the build , but I did pull some tubes and this is what I found .
            EL 84 Telefunken
            ECH 81 Telefunken

            SCC?? Telefunken
            EF 89 Seimans
            EABC Valvco


            • #7
              Instead of converting that German radio (which is a cool piece the way it is--- if it works) to a Champ-type build--- why not start with an amp that is closer to a Champ to begin with.
              Like this one:

              Of course this one uses a 6L6 output and a 6bw4 rectifier (replacements are $3 at AES for this rec tube--- specs here: )---- Good iron for a 6L6GC Champ build--- amp like this or Bogen CHB10A's (uses a 7868 output tube in SE--- can be easily converted to the readily avaliable JJ 7591S or 6L6GC if wanted--- EH makes new 7868 tubes) are on ebay quite often---they usually have OT's with 4/8/16 ohm taps so the head can be used with almost any guitar cab----or the chassis can be built into a combo cab.
              It's easier to start with something closer (like this PA head) to what you want to end up with than to convert a radio like that Grundig to a guitar amp---especially for a 1st build.
              Or you can do what most other people do and assemble a kit--- IMHO you might as well buy an Epi Valve Jr instead of going the kit route----- no imagination needed whatsoever and you'll end up with an amp like everybody else has--- and you don't need to assemble it.
              I checked out the "modded, upgraded Valve Jr" at a guitar show a couple of yrs ago---- a company was selling Mercury Mag upgrade kits for Valve Jr's and had both stock and upgraded Valve Jr's to demo----i soon realized that i had several Bogen and Knight lo wattage conversion heads at home that blew both Valve Jr's away, and cost very little to buy and convert to guitar heads--i gave up on Valve Jr's right then--- the old (60's-70's) PA/Commercial tube heads have high quality iron and well designed power supplies----amps like this were designed to be in constant use so are built in a very sturdy, reliable manner---- your favorite Classic guitar amp circuit will sound great built into an amp like this................gldtp99
              Amp Clips:


              • #8
                It takes some time to figure out what to build, especially when you want something special and unique.

                Here are some comments:
                The Bogen amps are very common starting points, which usually drives the price up.
                New 7868 tubes are available, but did you see the price? (Very expensive!)
                I doubt the 6bw4 rectifier will work well under pressure, and the result will be poor tone

                IMHO, your interest and excitement is fantastic, but you need guidance.

                If it were me, I'd get a standard Hammond aluminum box, new transformers from Musical Power Supplies or Edcor, and either make your own board or get one from Turret Boards. Either a 5e3 or Champ, although there are a lot of other ones.
                See the birth of a 2-watt tube guitar amp - the "Dyno Tweed"


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