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Alembic F-2B help

Discussion in 'Amps and Cabs [BG]' started by timre, Apr 3, 2002.


  1. timre

    timre

    Jul 9, 2001
    Vancouver, BC
    I need a little bit of advice with my Alembic preamp. I just bought this guy used and I decided to change the tubes since #1 They seemed to be crackling and humming and #2 they shoot out a LOT of gain!
    I decided after doing some research that a 12AU7 might be a more polite sounding tube for my pre, so I bought one and put it in last week. It sounds really good and the gain is now at a level out of the pre that I can deal with. I now have the idea of running a 12AX7WA that I just bought in the other channel and running the 12AU7 in the other. Is there a problem with this since they both run off the some power supply? Is there any need to do a bias adjustment and is the pot on the circuit board for bias adjusting or is it for something else?
    The concept I'm thinking of here is to have a two channel pre with two different settings or even cascade one into the other and get some cool overdrive sounds that are tamer than cascading two 12AX7's. Am I out to lunch with this or is it even useful to do? I do like the 12AU7 already and I've heard others complaining of the gobs of level that this pre spits out. The 12AU7 only provides 20X gain compared to 100X gain in the 12AX7 so it is better when level is an issue.
     
  2. Chuck M

    Chuck M Supporting Member

    May 2, 2000
    San Antonio, Texas
    It is not a problem to run different tubes in your F-2B. There is no bias adjustment needed for preamp tubes.

    I've played a lot with tubes and think you might like the tone of a 5751 better than the 12AU7. The 5751 has 70% of the gain of a 12AX7. Not sure what the gain spec is on the 12AU7.

    The best sounding currently manufactured preamp tube that I've tried is the Sovtek 12AX7LPS. They are prone to be microphonic but sound good when you get one that is not microphonic. I still prefer the older tubes and use mostly RCA and Sylvania 7025's.

    I have a brand new in the box Sylvania 12AU7. If you are interested in it email me.

    moses@compuvision.net

    Chuck
     
  3. timre

    timre

    Jul 9, 2001
    Vancouver, BC
    I changed out the old ruby tubes 12AX7A for a JAN 12AX7WA (phillips) that I bought yesterday and it sounds pretty good though I can't put the gain past 1 or 2 without it clipping the input to our Mackie 24 board. At least with the 12AU7 I can play with the volume a little more. A slight turn ofthe dial makes such a big difference when you have so much gain. I'm thinking of making a little resistor network pad up in a project box to run between the amp and the DI that way I can use a tube based on its sound and not its gain. I wanted to get a 5751 tube but I couldn't find one, so I went with the 12AX7WA for now.
    When I changed out the tube, I looked at the 12AU7 I put in last week and it has a localised blackish bluish spot right on the top. It's different than the more uniform silvery appearance where the rest of the tube gets hot. Am I really cooking this tube somehow or is it normal? Though I am a technical guy, I am relatively new to tubes and I amy be concerned about nothing, if anyone knows more about these wonderful glowing tubes of glass, I'd love to know!
     
  4. timre

    timre

    Jul 9, 2001
    Vancouver, BC
    By the way, does anyone know what the pot on the circuit board of the F-2B is for? I was looking at it yesterday and it seems to be right after the big caps to provide the 300V supply. Is this a bias adjustment? WHy would you need it on a preamp?
     
  5. Chuck M

    Chuck M Supporting Member

    May 2, 2000
    San Antonio, Texas
    There is no bias adjustment for preamp tubes. Only pairs of power tubes need to have bias set.

    I had an F-2B and do not recall a pot on the PC board. I suggest you email Ron Wickersham at Alembic and see what he has to say. He designed the preamp and is very helpful.

    The Phillips tube that you put in your preamp is actually made by either Sylvania or RCA. I have examples of this tube that are identical in construction and performance marked GT 12AX7A, Phillips JAN 12AX7WA, RCA 7025, RCA 12AX7A, Sylvania 7025 and Sylvania 12AX7A. This tube is one of the best sounding 12AX7's ever made IMO.

    A better solution to matching the levels would be to use a direct box. Some include a switch or control to pad the input signal. I'm assuming you already have the input level on the Mackie set as low as possible.

    FWIW I've tried most brands of tube preamps and hve not found that the Alembic preamps had an unusually hot output signal. The older Demeter preamps do have a very hot signal, however, they also have a very nice built in DI which will go directly to a board with no matching problems.

    If you would like to try to locate some 5751's, email me and I'll help you find some. I've discovered a lot of sources for tubes.

    moses@compuvision.net

    Good luck.

    Chuck