If I were to "Blackface" my SF Vibrolux Reverb, what schematic would be the one to use?

Discussion in 'Amp Input - Normal or Bright' started by BlacknBlue, Sep 10, 2020.

  1. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    Now that I have my Allen Accomplice build finished I'm needing to do some work to my Vibrolux Reverb. It is a 77 and all caps are original, the filter caps are dated 1975. It had been modded at some point as it had a hole drilled in the back of the chassis but whatever was done was removed and plugged. As far as I can tell it was put back to stock as I haven't seen any deviations from the schematic.

    Since the amp has already been modded and it isn't a particularly desirable year I was thinking about changing it to Blackface specs but I'm not exactly what schematic to use.

    Thoughts?
     
    Nate D likes this.
  2. stevierayfan91

    stevierayfan91 DEEPLY SHY.

    Messages:
    10,768
    Joined:
    Feb 20, 2011
    Location:
    Private.
    Hi, @BlacknBlue

    See if this AA964 Blackface VR schematic is good early year one.

    Hi @Jimgchord -Buddy -Do you know about vintage Fender amps?
     

    Attached Files:

    Last edited: Sep 10, 2020
    Nate D likes this.
  3. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    The ago old problem if you will with blackfacing a Silverface amp is what to do with the rectifier tube!

    The V+ voltage between both amps is a good amount different, with the BF amp being lower.
    The lower the V+ voltage the less highend ( smoother tone when clean and cranked) the amp will have, the less gain the amp will have, the sooner it will distort and the less wattage outout it will have.

    This means that just the common mod's done to the SF amps phase inverter section are not enough if you are really going for the essence of a BF amp.

    Here are some details!

    Both amps use the same power transformer, yes the same power transformer!

    The SF amp schematic shows a AC output of 345 volts from the power transformer, it uses a 5U4 rectifier and had a D.C. v+ voltage of 465 in standby.

    The BF amp schematic shows 315 volts out of the PT and uses a GZ34 rectifier and a V+ DC voltage of 410 in normal play mode/ amp fully on!

    So in short the key to that BF amp tone and feel is getting close to what I found over the years of being in that 410 to 425 VDC range when you do the conversion from SF to BF.

    Please note that the GZ34 recto has less internal resistance so what compared to a 5U4 recto will output a higher level of V+ for any given voltage input from the power transformer.

    The GZ34 also has the nice power supply filter saving benefit of a slow warn up which the 5U4 does not have.

    If you care to know how to deal with this then reply back.
     
    Last edited: Sep 11, 2020
    Nate D and Robins like this.
  4. Triple Jim

    Triple Jim Guy Who Likes to Play Guitar Silver Member

    Messages:
    6,134
    Joined:
    Feb 27, 2018
    Location:
    North Central North Carolina
    Does the PT have different taps to enable this difference?
     
  5. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    Yes I would very much love to know any information you could give me.
     
  6. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    If you can track down a nte brand 5275ak Zener diode then it can be installed thru a hole you make in the chassis and wired up to the V+ center tap and it will knock out about 38 volts D.C. That the amp run's on.

    You will need to drill a 5/8" inch hole near the PT to mount it.
     
  7. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    You don't happen to have a link to one of those do you?
     
  8. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    NTE parts direct has them for 31 bucks.
     
  9. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    OK Thanks.
     
  10. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    Another mod needed will be to beable to check he amps output tube bias since you will be lowering the amps V+ the bias will need to be reset and your amp only has a bias balance adjustment, not a bias level adjustment.

    First you will need to get two 1 ohm 2 watt 1% resistors.

    You will then unsolder the ground wire from pin 8 of each output tube and solder in series one of these resistors.

    This will allow you with a voltmeter set or D.C. Volts to read the Miliamps that each tube is idling at.

    You will likely not be able to get the tubes to idle hot enough, if so that is due to too much bias voltage.

    You will likely then have to increase the value of the resistor feeding the input side of the bias diode.

    If you really want you do the job right you can install a bias adjustment pot and then your really golden with having both pots in the amp!

    When you get the resistors installed you want to read .035 to .038 VDC on the meter when it's hooked up across pin 8 of each tube and chassis ground.
     
  11. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO

    I didn't realize this was a balance pot and not an adjustment pot, your the first reference to that I've seen. Adding an adjustment pot would certainly be a good idea.

    I'm familiar with how the 1 ohm resistors work for checking bias, that is how my Allen amp is setup, sure makes checking bias easy.

    Question about the diode, which direction do I install it? Or do you happen to have a schematic with one installed?
     
  12. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    It's a balance pot if it's on the rear of the amp yes, if it's under the chassis near the front of the amp then it's the old style bias adjust pot.

    The diode is already there other wise you would have no negative bias voltage now.
     
  13. Triple Jim

    Triple Jim Guy Who Likes to Play Guitar Silver Member

    Messages:
    6,134
    Joined:
    Feb 27, 2018
    Location:
    North Central North Carolina
    I think he's asking about the big zener to drop the B+ you suggested.
     
  14. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    Yes
     
  15. Stevem

    Stevem Senior Stratmaster

    Messages:
    1,212
    Joined:
    Sep 3, 2014
    Location:
    NY
    Oh, sorry!

    It can only bolt down one way and that grounds it and then the transformer V+ center tap gets moved from its chassis ground to the solder lug on the top of the Zener.
     
  16. BlacknBlue

    BlacknBlue Strat-O-Master

    Messages:
    831
    Joined:
    Apr 14, 2012
    Location:
    Jefferson City, MO
    OK, thanks. Just want to make sure I don't mess up the polarity!