I want to add 1 ohm resistor for ease of biasing

Discussion in 'The Workbench' started by Rotorcraft230, Apr 7, 2020.

  1. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    CCC6EEF9-F896-413D-8A72-A87CAF82059B.jpeg CA8CD3D1-3739-42F1-A16B-D8B5DDBABC37.jpeg I would like to add a 1 ohm resistor between pin 1 and 8 then pin 1 to ground for ease of biasing. The amp is a 1959 first reissue from 1988 S/N beginning in “W”. It does not have one and was wondering if there are any affects to the amp tonaly or anything else?
    Thanks Mark
     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    No change to the tone will occur. Personally, if that were my amp, I'd upgrade to ceramic octal sockets.
     
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  3. marco_giampa23

    marco_giampa23 Active Member

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    I have been told there are no downsides or tonal changes, havnt gotten round to doing this on my jubilee but some others on the forum will have some more info for you
     
  4. Chris-in-LA

    Chris-in-LA Well-Known Member

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  5. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Chris-in-LA likes this.
  6. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    9361822F-A5A7-4DDF-AB8B-74FF9BEE0803.jpeg I have this. One plug is the socket and the other plug is the pin(they are NOT connected inside the socket). It is pin 8 and it allowed me to measure actual mA. It works well but it takes forever to do one tube at a time. Just looking for something easier.
    And from a resale perspective would the added 1 Ohm resistors affect the value?
    It’s all stock except for a 1 wire mod that someone did which I removed. The circuit board appears to have never been removed until I did to replace a wire properly.
     
  7. RobS

    RobS Active Member

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  8. neikeel

    neikeel Well-Known Member

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    As per our JTM45 conversation
    use 1 watt 1R 1% resistors.
    Snip the wire that goes to ground on the output socket. Do not disturb the link between 1&8. Then solder your 1R resistors between the ground lug and pins 1&8 on each socket.
    Done.
     
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  9. Im247frogs

    Im247frogs Well-Known Member

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    It's a pretty handy feature and as long as it's clean and looks factory I don't think anyone ie future buyer is going to freak out about it.
     
  10. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Thanks Neikeel, if I do it I’ll do it like that. I can’t imagine it changes anything but making biasing easier. What type of resistor?
    I can make it look very nice, it’s not my amp it’s my Sons and he has been adamant about leaving it stock.
     
  11. neikeel

    neikeel Well-Known Member

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    I would suggest 1w 1R flame proof metal oxide, but make sure that they are 1% tolerance.
    Flame proof because if a tube shorts it will typically act as a fuse.
     
  12. South Park

    South Park Well-Known Member

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    I would get the tube socket to bias the amp .
     
  13. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Ok thanks
    Im weighing my options.
     
  14. anitoli

    anitoli Well-Known Member

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    You could use a scope......................................:D
     
  15. neikeel

    neikeel Well-Known Member

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    you are a naughty man!
     
  16. American Viking

    American Viking Well-Known Member

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    You can add the 1ohm resistor between pins 1 and 8 and you won't have to desolder anything. You would just have to clip the solid wire between the two pins.
    I use a desoldering vacuum and rework it all and position the 1 ohm resistor in a way where i can get my meter leads to it easily. Quick and dirty works too. You've got two unused holes in those pins ready to take the new resistor.
    I've done this to all of my amps whenever I have to bias new tubes. Makes the process way easier.
     
  17. neikeel

    neikeel Well-Known Member

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    See the JTM45 build post where this method was discussed.

    Cathode to G3 and then via 1R to ground is probably a more purist approach vs cathode to 1R to ground with G3 link but it really does not make any difference - what works for you.

    Either way you are going to need to cut a wire and solder a resistor in on each socket so no big deal.
     
  18. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    21C4B671-D268-4EF7-A0B9-8F10C8E02873.jpeg
    thanks fellas, both are easy ways to add the resistor for convince of biasing. I’m not very happy with what I’m seeing as far as the mA. We bought the amp online and it’s a great amp but I think the seller must have put his stash of miss matched tubes in.
     
  19. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    I have them together as best I can in pairs. V4,V5. Average 23 mA
    V6,V7. Average 23.43mA

    I have biased the highest tube V5 to 35mA and the pot is all the way up.
    Should I have more adjustment in the pot than that. 25X.7=17.5. 17.5/460=38
     
  20. neikeel

    neikeel Well-Known Member

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    I think that is the right way to go with the halves of the OT roughly in balance. It is a guitar amp not a hifi and that mismatch should not bother you at all, and these settings are at idle so you don't really know what each tube is doing when you are hammering it with signal.

    Ideally you want a bit more range on bias adjustment but that will involve generating a little more negative voltage on pin 5 and you will need to either play at piggy backing resistors over the existing 'stop' resistor or pull the pcb to sub a different value. What is in there now 47k or 56k are typical (the resistor in series with the bias pot)?
     

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