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Imitation Fender Bassman 5F6-A

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  • Imitation Fender Bassman 5F6-A

    So I am attempting to recreate the Fender Bassman 5F6-A amplifier using http://www.kbapps.com/audio/schemati...bassman5fa.gif as the schematic.

    I am still planning everything out and figuring out exactly how much money this is going to cost. While planning out the parts I ran into some trouble. The capacitors and resistors are very poorly labeled. The two 470 resistors near the two 5881 tubes are labeled as 1w but thats all. I know that tube amps can run at around 600V, and typically resistors come in 1/4w so the 1/4w compared to the 600V just seems as if the resistor would be fried right away. What wattage resistor do you guys recommend for this?

    Now the capacitors. Connected to the ground switch is a capacitor labeled at .05-600. I have determined that this mean a .05uF 600V capacitor. Now go up by the inductor and you see another capacitor labeled 20-600P. I am led to believe that the P signifies 600pF but then the farads and the voltage have switched places. And that seems to happen several times. Is there something I am unaware of or is it just an inconsistency in the schematic?

    Thank you,
    -Spingk

  • #2
    Originally posted by spingk View Post
    The two 470 resistors near the two 5881 tubes are labeled as 1w but thats all. I know that tube amps can run at around 600V, and typically resistors come in 1/4w so the 1/4w compared to the 600V just seems as if the resistor would be fried right away. What wattage resistor do you guys recommend for this?
    Doing the math:

    (about) 10 mA x 470 Ohms = 4,7 Volts

    4,7 Volts x 10 mA = 0,047 Watts.

    1/4 w resistors should be sufficient.

    Now the capacitors. Connected to the ground switch is a capacitor labeled at .05-600. I have determined that this mean a .05uF 600V capacitor.
    Correctly.

    Now go up by the inductor and you see another capacitor labeled 20-600P. I am led to believe that the P signifies 600pF but then the farads and the voltage have switched places.
    These caps are polarized caps, electrolytic caps. 20 µF / 600 volts. The + side is up, the - side goes to ground.

    cu,
    Ralf

    Comment


    • #3
      I'd actually regard 1W as the minimum for the 470ohm (screen grid) resistors (even these burn up with some regularity). 1W carbon comp are widely available, nothing to stop you using 5W wirewounds.

      Save yourself some potential hassle and fit 1.5K/0.5W grid stoppers to your build (see AB763 socket wiring).

      1/2W carbon comp resistors were what was used on the 5F6A, unless otherwise stated (e.g. 1W at the screen grids and in the power supply). You'd be asking for trouble by using 1/4W for plate resistors on the 12AY7. I'd use 1W carbon film resistors for the preamp tube plate resistors (see pins 1 & 6 of the preamp tubes.

      You will be lucky to find 600v caps for the power supply that will actually fit into the amp, assuming that you are using a tweed style chassis. I would use 2x100uf/350v caps in series for the main B+ node. Each 100uf cap should be bypassed with a 220K 2W resistor (see a Super Reverb schematic). This is the way that better quality reproductions are built, as well as the RI. If Fender had continued to make the 5F6A into '64, this is also the way that they would have done it.

      Connect the main filter cap assembly to the hot (rectifier) side of the standby switch to prolong rectifier life. 20uf 500v caps will be fine everywhere else in the power supply, assuming that you are running typical 5F6A B+ voltage (450-475v).

      The schematic you are referring to is rather vague regarding grounding, Doug Hoffman at hoffmanamps.com has a useful grounding scheme (as well as everything else you need for a 5F6A chassis) in his library of info.

      I know that the planning aspect is half the fun but seriously weigh up the costs against getting a kit from Mojo or similar. By the time you've sourced all the parts at single component pricing, you may find that a kit is cheaper?

      Mount the 68K resistors on the input jacks, not on the board opposite the 100K plate resistors. Use shielded wire from the inputs to V1 pins 2 & 7.

      Comment


      • #4
        Originally posted by MWJB View Post
        I'd actually regard 1W as the minimum for the 470ohm (screen grid) resistors (even these burn up with some regularity).
        Would you agree, that they burn up, because something else failed? Or could a bad quality be the reason for them to burn up?

        Regards,
        Ralf

        Comment


        • #5
          Well, all sorts of different brands and constructions still burn up at 1W, so I'm, not sure that quality per se is an issue. I'd presume that most times failure is linked to failing screen grids.

          Comment


          • #6
            Originally posted by MWJB View Post
            Well, all sorts of different brands and constructions still burn up at 1W, so I'm, not sure that quality per se is an issue. I'd presume that most times failure is linked to failing screen grids.
            I.o.w.: It makes no difference, and 0,25 W can be used as well, since - in case of failing screen grids - 1W burns as well as 0,25 W.


            Right?

            Greet's
            Ralf

            Comment


            • #7
              Hi Ralf,

              Well, if regular and repeated changing of your screen grid resistors is a particular hobby of yours, then fit the .25W. :-) The tubes will outlast the resistors.

              If you actually want to play the amp fit 2W-5W. If you want to "recreate" a 5F6A, visually, use 1W carbon comp.

              The screen grid resistors take the highest voltages in the amp (save for main stage filter cap smoothing resistors if fitted). 1W may be just about at their limit in a high voltage 5F6A (500v+), so it might not take much to push them over the edge.

              Comment


              • #8
                Dont use 1/4 watt resistors any where in a tube amp.Fender did this with the '59 Bassman reissue on the preamp plates and it was a total failure.It is the single most common repair on these amps.I especially would not use anything lower than 1 watt on those screens.They are there to keep the screen from conducting current,they do this by conducting current themselves,you put a 1/4 watt there and it wont last an hour.Dont be fooled by calculating the idle current,when a signal is applied and the amp is cranked things begin to change drastically.Although it doesnt say it on this schematic most schems will say "all resistors 1/2 watt unless noted".Using 1/4 watt resistors anywhere in a tube amp is asking for trouble.On the screens go with 2-5watt for added safety as MWJB pointed out.

                Comment


                • #9
                  Originally posted by MWJB View Post
                  Well, if regular and repeated changing of your screen grid resistors is a particular hobby of yours, then fit the .25W. :-)
                  Hey, great idea. I could build up a board with a bunch of 0,25W resistors, and have a rotary switch connected to them, so i could quickly switch, if one fails.

                  If you actually want to play the amp fit 2W-5W. If you want to "recreate" a 5F6A, visually, use 1W carbon comp.
                  Naa, i'm not doing such a project in the moment. I own a Bassman '59 RI, that's enough 6L6 things for the moment, and what i usually play is my Alligator amp - you'll find it on http://www.harper.amplifier.ch It's frankensteined Vox AC30 final stage with a paraphase PI and a 12AT7 in the preamp, first stage running on about 45 volts. Those EL84 are gritty power punks, but are a bit sensitive, too - but i hadn't any troble with this amp since about 1 year.

                  The screen grid resistors take the highest voltages in the amp (save for main stage filter cap smoothing resistors if fitted). 1W may be just about at their limit in a high voltage 5F6A (500v+), so it might not take much to push them over the edge.
                  I think, voltage is only one thing, it's rather the dissipated power what kills resistors.

                  cu,
                  Ralf

                  Comment


                  • #10
                    Originally posted by stokes View Post
                    Dont use 1/4 watt resistors any where in a tube amp.Fender did this with the '59 Bassman reissue on the preamp plates and it was a total failure.It is the single most common repair on these amps.
                    Strange. Does that happen often? I'm using a '59 RI from the early '90s - the model with Eminence Blueframe speakers, and there never was a problem. The former owner - a friend of mine - used it much more, on a regular base, about twice or three time per week, and he never had a problem. Each day more increases the probability for a failure, i think! ;-)

                    Dont be fooled by calculating the idle current,when a signal is applied and the amp is cranked things begin to change drastically.
                    Aaaah! How stupid i am! Claro! I didn't think it as running under full blast...

                    I'll do some measurements next days. Now it made me curious!

                    Although it doesnt say it on this schematic most schems will say "all resistors 1/2 watt unless noted".Using 1/4 watt resistors anywhere in a tube amp is asking for trouble.On the screens go with 2-5watt for added safety as MWJB pointed out.
                    For sure i do, since it makes no difference - i'd have to order them either way!

                    cu,
                    Ralf

                    Comment


                    • #11
                      I believe Fender corrected the problem with the 1/4 watt plate resistors,but I am not sure when.If your amp has the 1/4 watts I would suggest changing them to 1/2 or 1 watt before an inevitable problem arises.What happens is the 1/4 watt is a marginal value and over time they dry up from getting too hot,this is indicated by a crackling or hissing from the amp,then eventually they just burn out.As for the screen grid resistor frying,it is usually caused by a malfunction,but that is assuming the proper rated resistor is in there.If you have a 1/2 watt resistor there it will likely burn out,if you look at a tube manual you will see the 6L6 design max for the screen is 5 watts,while having a lower rated resistor will act like a fuse to some degree,1/2 watt will likely blow everytime you turn the amp up near full.always go for the 2-5watt here.

                      Comment

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