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Tybe radio conversion to guitar amp

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  • #16
    Tbh I was just lazy drawing the input. It does go to ground and the volume works. It’s a bit hard to decipher what’s going on in the original schematic. To get signal in I just crocodile-clipped to the gramophone input.
    Are the fixed resistors at the input on fender amps just for the voltage divider when 2 inputs present or should there always be a fixed resistor somewhere?
    also volume pot on input seems wierd. 5e1 champ has it between the amp tubes does it matter?

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    • #17
      Wiring the pentode as a triode? WHy not try it, you have it there. Or replce the triode with a pentode.

      The extra filter stage is there to remove hum in this single ended amp. Push pull amps cancel power supply hum, single ended do not.

      Look at Champ 5C1. It has a single pentode preamp and of course the 6V6 output. Look at Champ 5E1, They replaced your resistor with a choke. And now the pentode is two triodes. And Champ 5F1, they added a B+ node - your resistors again - to lower the 6V6 screen voltage.

      So to get Champish, you can either use one pentode or two triodes as a preamp.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Look at any basic Fender amp with two-jack input. It is not intended for plugging in two instruments. The resistors only form a voltage divider when you use jack 2. So the two jacks are basically higher and lower sensitivity. In many amps these days they label them Active and Passive (pickups) inputs.

        Volume control wasn't on the input in the original circuit, it was halfway through the system. Yes, the volume control should be after the preamp tube somewhere. Look at those various Champ versions.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          Here is the 5C1 again. You can see it has a 500 ohm and a 25K instead of the two 10W resistors you asked about. Not really a good idea to bypass those.
          If there really is a 47K to ground from the input, that should not be. It will decrease the signal too much.

          Click image for larger version

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          Attached Files
          "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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          • #20
            Originally posted by gilmo789 View Post
            Are the fixed resistors at the input on fender amps just for the voltage divider when 2 inputs present or should there always be a fixed resistor somewhere?
            The input resistors serve voltage divider and grid stopper purposes. With a single input you don't need the voltage divider but you might add a 33k grid stopper at the tube socket.

            (The 6Q7G is no pentode. A pentode would not amplify without screen supply. When wired as triode, the pentode's screen needs to be tied to the plate. But generally pentodes have more gain than triodes.)
            Last edited by Helmholtz; 07-19-2020, 10:57 PM.
            - Own Opinions Only -

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            • #21
              His 6Q7 is no pentode, but the previous tube in his original circuit, V2, is a pentode and would be worth giving a try. Wire it like the 5C1 pentode.

              g1, when he said 47k to ground from the "input" I think he meant the grid of the power tube. Y'know the power tube "input". he said that along with telling us to move the cathode bypass parts from that grid to where they belonged.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Enzo, g1, that’s exactly what I meant. Grid not input. New to tubes! Don’t quite have the terminology down
                Last edited by gilmo789; 07-20-2020, 12:26 AM.

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                • #23
                  R 18 in the original schematic (470K) is a grid leak resistor.
                  - Own Opinions Only -

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                  • #24
                    thanks for all the suggestions guys, I'll have a play about and see what I can make work.
                    the pencil diagram should now be correct btw

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