5e3 build- Mojotone kit w/ robrob mods. 6.3V Center tap question

Discussion in 'Shock Brother's DIY Amps' started by quaterstaff, Oct 18, 2020 at 8:26 AM.

  1. quaterstaff

    quaterstaff Tele-Meister

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    So, I am Using a Mojotone 5e3 kit, modded to include the RobRob Deluxe layout. I bought the circuit board from Hoffman's using the RobRob DIY file.
    Can someone check me on my plan? The Mojotone kit power does not come with a a 6.3 center tap. There is an alternative for an artificial center tap in the RobRob notes under 5F6A mods. Can I apply this mod as shown? In lieu of the 6.3V center tap from the PT it connects 100ohm resistors to V3 pins 6&8 and 8&7.

    I have also decided to put the master volume in the low side of the normal channel. Can someone verify I have the leads correctly laid out?

    This is my first amp build. Really enjoying it. I am going slow as to learn as much as i can. I did a bunch of reading before beginning, but at some point you need to jump in and do it.
     

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  2. Nickfl

    Nickfl Friend of Leo's

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    I don't see any mistakes on that layout. I'd remove the standby switch and use that hole for something else if I were you.
     
  3. quaterstaff

    quaterstaff Tele-Meister

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    Thanks Nickfl, I appreciate you reviewing.
    I thought about moving the master volume into that section, but as I've read it does get a bit noisy there.
    Any suggestions given the mods I have identified,or other?
     
  4. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I did the 10% mod some while ago. You probably don't really need a master volume too, not unless you have really scary neighbours.

    Those 25w resistors are pretty big, and there's not going to be much space in the chassis, what with the other switches you have planned. I mounted them on the back wall, removed the standby altogether and used the redundant hole for the 10% switch.
     
    Last edited: Oct 18, 2020 at 12:48 PM
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  5. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Here's a photo of the build with the resistors on the back wall of the chassis-

    P1120629.JPG
     
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  6. quaterstaff

    quaterstaff Tele-Meister

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    Thanks, seems straight forward. As a 1st time builder, I am going to question myself on everything.
     
  7. quaterstaff

    quaterstaff Tele-Meister

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    I got brave and mounted the 10% resistors already. Lol. I had to stare at it for quite awhile to figure how to squeeze all that in there.
    I'm going to consider eliminating the standby, thanks.
     
  8. dan40

    dan40 Tele-Afflicted

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    The end of the artificial CT resistors can connect to ground (chassis) near the pilot light or you can elevate the CT by connecting it to the power tube cathode. In a 5e3, v3 would be a 6v6 power tube so your connection points for the resistors would be across pins 2 & 8 and 7 & 8. Pins 2 & 7 are the heater pins and 8 would be the cathode. By connecting the resistors here you would create an artificial CT and you will also be "elevating" it which will help reduce hum.

    https://robrobinette.com/5e3_Modifications.htm#Heater_Artificial_Center_Tap
     
  9. chas.wahl

    chas.wahl Tele-Meister

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    An alternative is to go adjustable with the center tap, by using Merlin Blencowe's (valvewizard.co.uk) "humdinger" design: a 250 or 500 ohm pot with the end lugs connected to the two heater conductors, and the middle lug connected to 6V6 cathode (pin 8), provided it's cathode biased. The humdinger allows one to adjust the "center tap's" resistance between each conductor (which, according to Blencowe, is never actually equal for least hum of that system).
     
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  10. quaterstaff

    quaterstaff Tele-Meister

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    Thanks, I did write that incorrectly.
     
  11. quaterstaff

    quaterstaff Tele-Meister

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    I thought about this on my way to work. I appreciate the explanation, very helpful in understanding what is happening and why this works
     
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  12. quaterstaff

    quaterstaff Tele-Meister

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    I appreciate this, very helpful in understanding some of the different ways to approach this. I'm starting to really see what is happening here between this and dan40's explanation.
     
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