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  • Tweed Champ build - just checking

    I posted here recently about helping a friend build a champ for his son - he ordered a 5F1 kit from Mojotone. The amp needed some troubleshooting and got it (with some advice here) and she sounds superb. I did one mod - simply added a 22uF electrolytic cap to the cathode bias on the preamp circuit so it can be switched on or off. With the cap connected (I guess like the 5e1 champs had) she has a great fatter tone. Before I give it to them I just wanted to check here.

    The amp gets quite hot after a couple of hours. Much hotter than my 5e3s.

    I checked the voltages in relation to those specified on the Mojo drawings. The B+ readings seem very close to what's specified. The readings from pins 6 and 1 of the 12AX7 are quite a bit higher - about 20% higher. I checked the grounding and all is good.

    Here's what I get on the meter.

    B+ at 16uF filter cap - specified on drawing 370v/ actual 379v
    B+ at 8uF filter cap after 10K resistor (pin 4 0f 6V6) - specified 325v/ actual 334v
    B+ at 8uF filter cap after 22K resistor - specified 275v/ actual 287v

    pin 6 12AX7 specify 165v / actual 189v
    pin 1 12AX7 specify 165v / actual 194v

    pin 3 12AX7 specify 1.65v / actual 1.46v


    As I say, she sounds superb. Should I let it go or do I need to make any changes?

  • #2
    Originally posted by d. spree View Post
    I posted here recently about helping a friend build a champ for his son - he ordered a 5F1 kit from Mojotone. The amp needed some troubleshooting and got it (with some advice here) and she sounds superb. I did one mod - simply added a 22uF electrolytic cap to the cathode bias on the preamp circuit so it can be switched on or off. With the cap connected (I guess like the 5e1 champs had) she has a great fatter tone. Before I give it to them I just wanted to check here.

    The amp gets quite hot after a couple of hours. Much hotter than my 5e3s.

    I checked the voltages in relation to those specified on the Mojo drawings. The B+ readings seem very close to what's specified. The readings from pins 6 and 1 of the 12AX7 are quite a bit higher - about 20% higher. I checked the grounding and all is good.

    Here's what I get on the meter.

    B+ at 16uF filter cap - specified on drawing 370v/ actual 379v
    B+ at 8uF filter cap after 10K resistor (pin 4 0f 6V6) - specified 325v/ actual 334v
    B+ at 8uF filter cap after 22K resistor - specified 275v/ actual 287v

    pin 6 12AX7 specify 165v / actual 189v
    pin 1 12AX7 specify 165v / actual 194v

    pin 3 12AX7 specify 1.65v / actual 1.46v


    As I say, she sounds superb. Should I let it go or do I need to make any changes?
    First, congratulations on a job well done! Your friend's son will have a fine introduction to the world of hollow-state!

    How hot is hot? The numbers don't seem too out there to me. Even if the V1 is running hotter than specified, that's only a fraction of a milliamp. Where are the dissipation figures for the power tube? Check the bias voltage for that. My champ circuit runs at or above the dissipation ratings for a 6V6, but has never made the iron too hot to touch, at idle or after screaming-loud continuous playing.
    Is the speaker impedance matched? Just thinking out loud...
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

    Comment


    • #3
      Thanks Eschertron. yes, I was amazed how great this little amp sounds especially when using the 22uF cap at the cathode of the V1 tube. But both ways it sounds great. Really great for morning playing as it isn't at all overbearing. To think that I went from a Twin Reverb to building 5e3s and now this. But it will go to my friend's son and he will hopefully love it - especially as he wired up the board.
      not sure that I know what dissipation even is, never mind the figures for the tube. something to do with heat dissipation I presume? Its a JJ 6V6. The one shipped was bad so I used one I had laying around. The speaker impedance is good, matched. My only concern about the heat was tube life for the power tube.

      Comment


      • #4
        Originally posted by d. spree View Post
        My only concern about the heat was tube life for the power tube.
        Right. If this is your first single-ended amp, then realize that the power tube is blasting away at full power all the time. If the idle bias is too hot, the tube will suffer - even when (especially when) the amp is sitting there quietly minding its own business. Measure the cathode voltage on the 6V6 and we can get an idea of how much power it's sucking up (sucking down?).

        Bottom line; if either of the trannys are hotter than the tube, there might be a problem. I'd expect the tube to be hot - but not redplating - while its on.
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

        Comment


        • #5
          Originally posted by eschertron View Post
          Right. If this is your first single-ended amp, then realize that the power tube is blasting away at full power all the time. If the idle bias is too hot, the tube will suffer - even when (especially when) the amp is sitting there quietly minding its own business. Measure the cathode voltage on the 6V6 and we can get an idea of how much power it's sucking up (sucking down?).

          Bottom line; if either of the trannys are hotter than the tube, there might be a problem. I'd expect the tube to be hot - but not redplating - while its on.

          No sign of redplating. Voltage at pin 8 of 6v6 is about 21.7v

          Comment


          • #6
            Originally posted by bestamp-guitar
            sometimes the plate is quite hot, i am thinking if can change some not easy hot plate to furnish.

            shenzhen grand musical instruments co., ltd. - pro accessories, tuner/metronome, guitar effect pedal, amplifier, drum pad, other accessories
            ouch! Spammed redux!
            If it still won't get loud enough, it's probably broken. - Steve Conner
            If the thing works, stop fixing it. - Enzo
            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

            Comment


            • #7
              Despite not being on the official Fender schematic and layout, you'll find that 25uF bypass cap on many, if not most (maybe all) vintage 5F1's.

              Comment


              • #8
                Originally posted by wyatt View Post
                Despite not being on the official Fender schematic and layout, you'll find that 25uF bypass cap on many, if not most (maybe all) vintage 5F1's.
                yes, I've seen both layouts, also I've seen schematics with the bypass cap. I love the way it sounds with the bypass cap 9of course I used 22uF cause it's what I had) - seems to "thicken" up the sound in a great way. Unfortunately (for me) I've passed the amp on to my friend's son who loves it. I'm back to my 5e3s.

                Comment

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