Can a novice safely re-bias a Vic 50212 (Tweed Twin)?

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


  1. AbstractLunatic

    AbstractLunatic Member

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    I have to re-tube a Victoria 50212 and was told it will need to be re-biased. I understand the operation and the steps I need to take to do it. But I'm not an electrician so I don't know what parts are what with the rest of the innards so I... well... I don't want to die over an amp.

    What advice can you guys give me about gathering the knowledge to re-bias an amp safely? I'm not in a big hurry.

    Thanks,
    Morgan
     
  2. brad347

    brad347 Member

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    Two options:

    Find the bias pot in the amp, if there is one. (sometimes one is accessible externally, on the old BF and SF Fender amps there was a bias trimpot accessible on the underside of the chassis behind the output transformer). Buy a weber Bias Rite (or similar device) which plugs into the tube sockets, then you plug the tubes into the BR. These are not usually too expensive. Then, by looking up the numbers you want, you turn the bias pot carefully until you get there (this has to be done with the amp on and open if there is no external bias pot, dont touch anything but the bias pot and it won't hurt you).

    Option 2: Take it to a tech and pay them 25-50 bucks to bias it for you and show you how to do it yourself (many will do this, some won't). They will have to see if there is a one ohm resistor going between cathode of the power tubes and ground, if not then they will probably want to add those for you. This will not change the tone or feel of the amp at all. All they do is just provide a test point to measure the bias.... then you will put your multimeter + on that test point and the - on the chassis (ground). Do this for each tube and split the difference to get the best average bias point for the two tubes.

    It's really not hard using either of these methods. There are other methods but they are considerably more dangerous if you should slip with the meter while working. I'd sort of recommend the first option. Good luck.
     
  3. deeval

    deeval Supporting Member

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    With a bias meter you can do it yourself because there is a bias pot on the Victoria,Just use an insulated screwdriver and adjust to your settings,and as stated before if you are not comfortable doing it pay some one,Its very eazy on the vicky amps.
     
  4. Guinness Lad

    Guinness Lad Silver Supporting Member

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    You don't need a bias meter because if the twin is anything like the bassman there is a 1 ohm resistor coming off the power tubes which goes directly to ground. What you do is take a ohm meter, set it to read dcmv and put the positive lead on one side of the resistor and the negative lead on the other side of the resistor. What you will be reading is dcmv, this reading correlates to the current draw of the tubes. Mark at Victoria designed the amp so anyone could bias the amp without dealing with lethal voltages. Just make sure you don't touch anything but the wire leads on the resistor. I believe this resistor is tied to pin 8 off the power tube but I don't remember. If you put the leads on the resistor the wrong way you will read a negative number, if so just switch them and the number will be positive. The negative lead should be between the resistor and ground. Once you get your reading move the variable resistor on the board until you get the sound you want to hear without exceeding the bias capability of the tubes. Personally I liked mine set right before the tone thickened up, this was usually right about 31 to 32.5 ma depending on the tubes. If your worried call Mark at Victoria, he told me how to do mine.
     
  5. Phil Harmoneeek

    Phil Harmoneeek Member

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    Hi Morgan ... I have the exact same amp & would be happy to guide you through it ... just like Mark Baier did for me. PM or email me & we can get together by phone (the best way I should think).

    Randall
     
  6. Phil Harmoneeek

    Phil Harmoneeek Member

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    Hey I just saw harryjmic's post & that is exactly how to do it ... nice post.

    Randall
     
  7. AbstractLunatic

    AbstractLunatic Member

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    I dropped by my local radio shack and got some help learning to identify parts. I know which parts are capaciters now. I also picked up their book, Getting started in electronics. Just a little education and I will lose the fear and replace it with respect. Thanks for the replies.
     

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