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  • Pilot lights - 120V vs 6.3V

    Hello gents, I have a question...

    I'm building a bone-stock 18w Marshall TMB amp for a friend. Three 12AX7s, two EL84s.

    He doesn't like the typical Marshall mains powered pilot light. He wants a Fender style 6.3v light so we can use a custom jewel color and swap colors at will.

    Besides drilling the chassis to accept the larger Fender style pilot light, can it just wire into the filament string like it would on a Fender?

    The PT specs on this Marshall are roughly the same as a Deluxe Reverb...which has many more tubes than this 18w Marshall, so I'm thinking this Marshall PT is capable of having a little tiny light bulb in it's heater circuit.

    Am I right?


    Attached Files
    Last edited by Greg_L; 10-29-2020, 02:55 PM.

  • #2
    The way I figure it....the PT provides 3A for the heater circuit
    Three 12AX7s at .3A each = .9a
    Two EL84s at .750A each = 1.5a
    That's 2.4 amps the heaters are using up.

    The Fender lamp is like .15 amps or so? That would put me at roughly 2.6 amps. Am I cutting it too close?

    Comment


    • #3
      Greg, you can order LED versions of the typical #47 bayonette lamp, and they only draw 20mA.
      --
      I build and repair guitar amps
      http://amps.monkeymatic.com

      Comment


      • #4
        Originally posted by xtian View Post
        Greg, you can order LED versions of the typical #47 bayonette lamp, and they only draw 20mA.
        Yeah but don't they need low DC voltage? I don't wanna redesign anything. I just want to hopefully stick a Fender style light into a Marshall on the heater string....like a Fender.

        Are my calculations correct?

        Comment


        • #5
          I can't imagine those few extra milliamps will be the difference between transformer failure or not. And yes, if you want a 6v bulb like Fender, then you wire it in like Fender.


          As to redesigning anything. An LED lamp does need LED, so we put a diode in series with it - voila, DC.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Originally posted by Enzo View Post

            An LED lamp does need LED, so we put a diode in series with it - voila, DC.
            As an LED is a diode, it rectifies by itself. An extra series diode is only required if the applied peak voltage exceeds the LED's PIV.

            Of course any LED needs a current limiting series resistor (often already built into the light).
            - Own Opinions Only -

            Comment


            • #7
              Originally posted by Enzo View Post
              I can't imagine those few extra milliamps will be the difference between transformer failure or not. And yes, if you want a 6v bulb like Fender, then you wire it in like Fender.


              As to redesigning anything. An LED lamp does need LED, so we put a diode in series with it - voila, DC.
              Right, thanks. I didn't think it would be a problem, but I didn't know for sure. I know there are a few milliamps to spare. I just don't wanna cut it too close. I don't know how close is too close.

              The LED is probably not an option. My guy really likes the standard Fender jewel look and set up.

              Comment


              • #8
                Related question....did I do my math right up above? I'm still learning and working through this stuff. Did I calculate total heater draw correctly???

                Comment


                • #9
                  Originally posted by Greg_L View Post
                  Related question....did I do my math right up above?
                  Yes.

                  My EL84 datasheet specifies heater current as 0.76 +/- 0.04A. So worst case you will have 2.5A total.
                  Like Enzo, I don't see a problem adding another 0.15A for a GE type 47 (1W) pilot lamp.
                  Last edited by Helmholtz; 10-30-2020, 03:08 PM.
                  - Own Opinions Only -

                  Comment


                  • #10
                    [QUOTE=Helmholtz;n916830]
                    Originally posted by Greg_L View Post
                    Related question....did I do my math right up above? QUOTE]

                    Yes.

                    My EL84 datasheet specifies heater current as 0.76 +/- 0.04A. So worst case you will have 2.5A total.
                    Like Enzo, I don't see a problem adding another 0.15A for a GE type 47 (1W) pilot lamp.
                    Excellent, thank you!

                    Comment


                    • #11
                      Originally posted by Greg_L View Post
                      Yeah but don't they need low DC voltage?
                      No, they're designed as drop-in replacements. E.g., https://www.ebay.com/itm/44-Or-47-LE...e/203057471669

                      --
                      I build and repair guitar amps
                      http://amps.monkeymatic.com

                      Comment


                      • #12
                        Originally posted by xtian View Post

                        No, they're designed as drop-in replacements. E.g., https://www.ebay.com/itm/44-Or-47-LE...e/203057471669
                        Haha you know that's funny...I talked to the guy this amp is going to and he bought a bunch of those on his own. I didn't even know about them. So I'm gonna install a standard Fender pilot light assembly and try those out.

                        Comment


                        • #13
                          Just in case you want to use the Type 47 Incandescent lamp....it only has a life-hour rating of 3000 hrs. Chicago Minature's CM755, also 6.3V/150mA has a life-hour rating of 20,000 hrs. Same T-3 1/4 Bulb miniature Bayonet base like the Type 47. I've been using teh CM755 in all of our Fender rental inventory for years. Same or similar price.....been buying them thru Mouser Electronics.
                          Logic is an organized way of going wrong with confidence

                          Comment


                          • #14
                            Originally posted by nevetslab View Post
                            Just in case you want to use the Type 47 Incandescent lamp....it only has a life-hour rating of 3000 hrs. Chicago Minature's CM755, also 6.3V/150mA has a life-hour rating of 20,000 hrs. Same T-3 1/4 Bulb miniature Bayonet base like the Type 47. I've been using teh CM755 in all of our Fender rental inventory for years. Same or similar price.....been buying them thru Mouser Electronics.
                            Cool thanks for the tip!

                            Comment


                            • #15
                              When I was in the arcade industry, we bought #44 and #47 by the thousands. I could easily go through 100 bulbs in a pinball machine cleanup. A couple hundred pinball machines, and...BIG box of bulbs. I still have a bunch.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment

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