Conversion Issue: Help/Advice/Opinions needed by the experts

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mooncusser, Mar 21, 2006.


  1. Mooncusser

    Mooncusser Member

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    OK guys,
    I need the low down here.
    I bought a 1959 SLP reissue that had 6L6GC's installed into it.
    It was done by an amp tech, so I am assuming he did whatever procedure was nesessary.The owner of the amp sent the original Svetlana EL34's that it originally came with. I remember him saying that If I ever wanted to switch back to the El34's, a slight bias adjustment from your local amp tech is all it will take.
    1. Is it possible that the amp tech just installed them and madse a bias adjustment to rate with the 6L6's, is that possible?
    2. If that is not possible, has there been damage done? (although the amp does play beautifully with no issues whatsoever.)
    3. What procedure is most common when installing 6L6's in an EL34 driven amp as I have, in other words, what do you think (or guess) the tech did?
    4. What would I have to do to reinstall my EL34's back?
    Now, I generally do all of my own amp work, and have a good knowledge about their wiring and so on. I am not an ~amp tech~, but have been liscenced for 14 years now in electrical and mechanical applications.
    I wanted to get some feedback here from you, the experts as to what would be best to do.
    Can I reinstall the EL34's myself, following some procedure in regards to biasing?
    I really appreciate your help here, and I know it may not be a common question you encounter.
    Fire away, I am all ears.
    Best regards to you all,
    Mass
     
  2. electroid

    electroid Member

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    All you have to do is plug in the EL34s and re-bias. It is possible that the tech had to change a resistor to get the bias adjustment to go far enough. If he did, you might have to change it back.

    There is lots of information floating around about how to bias an amp so I won't repeat it.
     
  3. bob-i

    bob-i Member

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    :angel
    There's a few thoings that needs to be done to sub 6L6's for EL-34's. The good news is that this won't affect the ability to install EL-34.

    6L6's need pins 1 and 8 tied together. EL-34's have this doen internally, so once these pins are tied together either will work. The bias adjustment is no big deal, a replacement resistor will allow enough range for either.

    Yes, possible and common, no damage done.

    Get a bias probe, install the EL-34's and adjust according to EL-34 specs. Nothing more needs to be done.
     
  4. John Phillips

    John Phillips Member

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    It is certainly possible that all he did was install the 6L6s and rebias it with the trimmer. That would usually leave the tubes running very hot, because the trimmer doesn't have enough range to get up to the voltage that 6L6s normally need, but if so the only likely damage would be to the tubes, not the amp.

    If no components were changed, you can reinstall the EL34s and bias it correctly with no trouble at all.

    You should be able to tell reasonable easily - look at the end of the circuit board nearest the power switches. You should see four diodes in a group, and a single one on its own. Near the single diode there should be a 27Kohm resistor. If it's been correctly converted for 6L6s it will most likely have a lower value, probably 15K.
     
  5. Mooncusser

    Mooncusser Member

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    Well gentlemen,
    I can't thank you enough for all of your help.
    I got every question answered more thoroughly than I had hoped.
    ..hope I can return the favor someday.

    If I want to switch back to the EL34's, and there IS a 15k installed, I have to simply switch back to the 27K, and rebias accordingly. May I ask if you can buy a 27Kohm resistor from...radio shack, and if so, is there any particular type to the resistor, or are all of these types of resistors the same?..(in other words, a 27K is a 27K is a 27K....I hope I worderd that correctly)
    I ask this because most of my electrical work I am lisenced for is very high voltage work, and I am not so familiar with these resistors.
    Thank you for such wonderful help and advice.
    Take care, Mass
     
  6. electroid

    electroid Member

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    Bob-i,
    You've got it backwards. The 6L6s have the pins internally connected, EL34 need the sockets to be wired to the connections. Therefore, if the amp originally had EL34s, the sockets will work for either tube.
     
  7. Dave C

    Dave C Gold Supporting Member

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    Yes Radio Shack might have that. It won't be the best quality but should at least be adequate. What you want is a 27k 1/2 watt resister, it'll probably say flameproof on the packaging. If they don't have that value , try online where you can find better quality anyway.
    Dave C
     
  8. Wakarusa

    Wakarusa Member

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    Radio Shack may have them. The original was probably a 1/2W or 1W carbon film. You usually can't buy just one.

    It's not the cheapest way to get one, but if you send me a self-addressed and stamped envelope and a nickel ($0.05), I'll send you back a resistor.
     
  9. Mooncusser

    Mooncusser Member

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    Dear Wakarusa,
    That is a very kind offer I will take you up on.
    I visited your web page...very impressive.
    Is the address on your web page the address you want me to send the envelope/ money to?
    I'll have it in the mail tomorrow.
    Thank you very, very much!
    Best regards,
    Mass
     
  10. Wakarusa

    Wakarusa Member

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    The address on the web page is the one to use.
     
  11. Mooncusser

    Mooncusser Member

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    Wakarusa,
    I sent it out to you today.
    I want to thank you again for your time, efforts, and help.
    I have thoroughly been through your web site, and I can tell you, I may be requesting your services in the future. I am currently self training myself to become proficient in what you do for maybe a side business venue, or for my own personnal enjoyment. I am reading everything I can get my hands on, and staying very dedicated.
    I will most certainly spread the word about your specialties, business, and most of all....your support, which is what everyone looks for in the first place.
    In my current line of work, I run into NOS/ used componets all the time, and if there is something that I know you might be able to use for your business, I would like to contact you and see if you would be interested in just accepting some of these items, as a show of thanks for your help, and also maybe you could help me to learn by letting me know a bit about the materials.
    I am a 14 Year lisenced plumber, gasfitter, steamfitter,oil burner tech....it goes on. I always hate to run into something that I know that someone lke yourself could use, but I can't as of yet because I am still very much in the learning phase, so it goes to waste. Let me know if that interests you.
    Thank you again, and the best of luck to you.

    By the way, do you sell the resistors that you can install for biasing with a multimeter only? (1 ohm I think, please correct me if I am wrong.), and how many would you need for a 1959 SLP?
    Take care,
    Mass
     

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