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Fender '64 Vibroverb Custom: help me bias the amp please

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by echo unit, Jan 17, 2006.

  1. echo unit

    echo unit Member

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    Hi,

    I hust got a '64 Fender Vibroverb Custom used and it had the original tubes in it so I bought some new and better tubes, TAD 6L6 Power tubes
    EH 12AX7 preamp tubes and the others are Sovtek.

    Anyway, I want to know how to bias this amp with these tubes. I do not have a bias kit and do not want to buy one. I know that I can do it partly by ear (not sure what the listning method is yet) and that I can do it partly by measuring the Voltage.

    Where are the test points for my volt meter on this amp and how do i use my ears to cet the bias as well.

    Is there an external bias adjustment or does the chasis have to come out to do it by ear?

    Any help would be awesome! I do not have a schematic.

    Thanks
     
  2. echo unit

    echo unit Member

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    I would like to know where the bias control is on this amp? Is it availble on the backside or underside of the chasis without having to take the entire chasis out of the cabinet?

    I think I can Bias it by ear if i can just locate the bias adjustment spot.

    Also what should I do to protect myself against electricution while performing the bias adjustment?

    thanks for your help!
     
  3. Dave C

    Dave C Gold Supporting Member

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    The bias control on a Fender amp is located on the bottom of the chassis towards the front in the neighborhood of the filter choke and is usually accesible using a flat blade screwdriver without removing the chassis. However I strongly suggest you do some research on biasing your amplifier, acquiring some instrumentation and learn how to use it properly both for your safety and your amps !
    Dave C
     
  4. echo unit

    echo unit Member

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    Could you guys please give me the simple summed up version of what to do in order to not kill myself while turning the bias adjustment with the amp turned on?

    What's the procedure?

    Do I just need to touch my hand on something metal before and during the turning of the screwdriver?

    i do not plan on opening the amp up. I want to leave the chasis in the cabinet. Put the new tubes in place with the amp turned off. Turn the bias control all the way down, turn on teh amp an while dude is playing the guitar, turn teh bias control up until the tone hits the sweetspot where there is no crossover distortion happening, where the volum doesn't jump from nothing to something when turning the amp's volume pot up from 0. right????

    Anyways, just tell me how to not end my life and I will love you for it.
     
  5. deeval

    deeval Supporting Member

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    I kept a Hand in my pocket,But it felt so good I forgot what I was Doing-HeHeHe!:Spank
     
  6. Sean

    Sean Supporting Member

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    There are 2 point on the end of the board (with the knobs facing you, on the right side). You can follow the two wires from the power tube sockets to these points. I forget which pin they go to on the socket. These are your test points for setting bias in your amp.

    Pick up a $20 digital multimeter from Radioshack. Set the dial to volts (mine self sensing or whatever it's called. If yours isn't, set in to the mV setting). Put the black probe in one of the screw holes on the top of the chassis, touch the red probe to one of the test points, then the other. You can turn the bias pot to achieve the proper bias that you are looking for. You can also see if your tubes are indeed matched. There is an acceptable range I guess for 6L6 bias. Don't go too much higher than 36mV.

    If you're unsure about where the tests points are, take a picture of the chassis and I will show you.

    I have the same amp and recently learned how to bias it as well. I am no electronics guru, so if I can learn to do it, anyone can.
     
  7. echo unit

    echo unit Member

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    Sean,

    THANKS A LOT FOR YOUR HELP SO FAR!

    Anyway, I wanted to know, do I have to take the chasis out of the cabinet to get to the two test points that I will touch the volt meter leads to?

    Or are these tests points accessable on the outside of the amp without taking the chasis out of the speaker box. If so where are the test points? Are they at certain tubes?

    I would like to bias the amp as much as possible by ear and use the volt meter just to check an see where the numbers are sitting, unless i have toi take the amp apart and then I will just do it by ear and say to hell with numbers. As long as I am not getting any weird distortion and the plates in the tubes aren't glowing orange, I should be ok right?

    TYhis amp is brand new with the stock groove tubes and i am putting better tubes in it to improve the sound of it.

    I was thinking about turning the amp up to say 3 on the volume pot, recording myself playing strumed chords as hard as I can and then, after I put the new tubes in, I would record myself palying the exact same thing throught the amp again, without moving the mic. I would turn the bias up or down unit I get the same record level as before with the stock tubes. That would mean that the origianl volume at 3 with the stock tubes would be matched with the new tubes. This would probably get me in the ballpark of being at the right bias correct?

    Sort of a funny way of setting bias but I think it could work.

    Thanks again for your time and help!
     
  8. Sean

    Sean Supporting Member

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    To be perfectly honest, I would leave the GT's in there. I thought they sounded pretty good in this amp. I can't remember what you said you were going to put in (TAD's?). I thought the GT6L6's were better than TAD's. I've tried both.

    At any rate, the test points are inside the amp on the tag board. Either get someone to show you what to do or leave it be. Please don't go probing around inside this amp with a multimeter blindly. You can get zapped fairly badly.

    The ear thing is sketchy at best. YMMV and I don't know enough to know for sure, but i wouldn't do it myself. Good luck.
     
  9. Resonator_Duke

    Resonator_Duke Member

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    Seeing how y'all are talking about biasing, and the '64 Vibroverb was mentioned, I'm curious if anyone can answer this? I have a '63 Vibroverb Reissue that should be on it's way to my doorstep. I know for a fact that this amp doesn't have an adjustable bias pot in it. I installed an adjustable bias pot on a friend's Custom Vibrolux Reverb, and the bias resistor in the CVR was R59. Now, how similar are these amps inside and circuitry wise? What't the BIAS RESISTOR in the '63 Vibroverb RI? I know once I crack the amp open, I can track it down. But, curious to hear from anyone that's owned or worked on both of these amps, and can tell me any similarities or differences y'all have run into. I've noticed how many folks have converted their CVRs to Vibroverb specs, but have left the Vibroverbs alone. Thanks y'all.
     
  10. vibroverbus

    vibroverbus Member

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    What most of the posts are getting at is there is no such thing as 'bias by ear' unless you don't mind potentially having your amp setup such that it burns through tubes ("buy a multimeter and learn to do it right" is what we're saying). But if you don't mind the potential for torching tubes as disposable parts, then it's your choice of course.

    Glowing orange generally happens WAY AFTER the tube is over recommended specifications by the way, 'not redplating' is absolutely no indicator of safety.

    Most importantly to your comment above the rule is ABSOLUTELY DO NOT TOUCH METAL WITH YOUR HAND. You want to be as insulated as possible at all times. Rubber soled shoes. Use only one hand on a tool in the amp at a time (hence the "other hand in your pocket"). There's lots of sources of info for this - search this forum and the web in general, buy a book, etc...
     
  11. Steve Dallas

    Steve Dallas Supporting Member

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    PLEASE take it to a qualified tech for biasing. It shouldn't cost more than $35. That's cheap insurance on your life and on the lives of the tubes.
     

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