Ampeg SVT/V4B mod

Discussion in 'Amps and Cabs [BG]' started by Skullheadrac, Aug 15, 2021.

  1. Skullheadrac


    Jun 13, 2018
    So Cal
    I came across this video and it really got my attention. The guy modded an SVT to have a master volume, turning the volume knobs on the front into preamp knobs.
    Has anyone here done this?
    Also, do you all think the result would affect a 70s V-4B in the same manner?
    It has me tempted to take my v4b in and have the same mod done.

    Rodslinger likes this.
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Ampeg sold a stand-alone master volume pedal for the amps in the 70s, just goes in the pre out/power in jacks. You can do the same with a volume pedal or simple pot in a box kind of pedal. Would work on the V4 as well, yes.
    Jim C and beans-on-toast like this.
  3. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    I think the results would sound a lot like the SVT-CL :D
  4. beans-on-toast


    Aug 7, 2008
    What Corey mentioned is good. It’s easy to build a master volume control in an external box and requires no holes drilled in the chassis - the volume controls are good to keep as they are.

    If you want more dirt, why not simply use a good pedal at the input to overdrive the first stage.
    Jim C and MattZilla like this.
  5. MattZilla


    Jun 26, 2013
    0:40 in and I’m wondering how many long-term SVT users crank the bass and treble knobs and cut the mids as their regular go-to tone?
    Jim C, JimmyM and beans-on-toast like this.
  6. spankdaplank


    Jan 19, 2003
    Back in the day (70's) I rigged my SVT with an old Morley volume pedal, running the pre amp out jack through the pedal back to the power amp in jack. Worked like a charm. This was one of those old cast iron pedals with the crinkle finish. It was great having the control in a pedal on the floor.
    Jim C likes this.
  7. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    Out there!
    Guess what? Those have always been preamp knobs. ;)
  8. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Right, but you can't turn the power tubes down and get preamp dirt at a reasonable volume since single volume amps have the power section fully up all the time.
    MattZilla likes this.
  9. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    Out there!
    Are you implying that there are actually bassists out there that want to turn their amps down? :D
    JimmyM likes this.
  10. Do not want. Never been a fan of preamp distortion anyways.
    Jeff Scott likes this.
  11. VitoMB

    VitoMB Supporting Member

    Sep 28, 2019
    Quebec, Canada
    I have a 1977 SVT that I also would like to add a pedal volume. Do we know if it needs passive 250k or active 25k pedal volume ?

    From the amp picture of the back, I am not sure in which jack to plug in and what the « 0.25 volts for full power » means ?

    Attached Files:

    Last edited: Aug 26, 2021
  12. Wasnex


    Dec 25, 2011
    The Power Amp out Preamp In on this amp works sort of like a series Effect Loop.

    This is an image from the 75 schematic:

    Normally the signal flows uninterrupted through the switching contact in J2. If a plug is inserted into J2, the switch contact is lifted and the signal is interrupted. Then the expected signal path is out through J1 to some sort of processor (or master volume), and back into J2.

    The 0.25 for full power is quite literal. It means a signal of .0.25V RMS is required to push the amp to 300W. The is a low line level signal that is equivalent to about -10dBu.

    Older SVTs had two Ext Amp jacks. My understanding is they could also be used as input and input or output slaving amps, but they would not function as a series loop because the jacks were in parallel.

    This is from the 69 schematic:

    Notice J1 and J2 are connected in parallel, which means the cannot carry different signals.
    VitoMB likes this.
  13. Old Fart

    Old Fart Supporting Member

    Mar 11, 2011
    I have a V4 and a VT22, both from the last 1970s I believe, and both of them have a Master Volume. I leave those master volume knobs on "11" because I do not care for distortion and I particularly do not care for 7027A distortion. I doubt I would like 6550 distortion any better. I enjoy a bit of 'growl' in my bass tone, but this is not nearly the same thing as distortion, to my ear.

    I probably agree with beans-on-toast who suggested, above, that a pedal probably makes the most sense for someone wanting bass distortion.
  14. Wasnex


    Dec 25, 2011

    In my experience the original preamp in the SVT does not develop any sort of drive until it is almost maxed out (if at all).

    AFAIK, The SVT CL has a gainier preamp that is designed to breakup easier. So using a Master on a vintage SVT is not going to give you the same sound as SVT CL with the preamp gain cranked. Although the CL preamp is designed to have essentially the same voicing as the original, it's actually a somewhat different design.

    In particular the SVT CL has only two tubes in the preamp. So the signal goes through 4 triodes before it exits to the output section.

    Channel 1 of the vintage amps has three tubes. Additionally a 6C4 tube is used to mix the output of channel 1 and channel 2. So channel 1 sends the signal through seven triodes before it exits to the output section.
    VitoMB likes this.
  15. Old Fart

    Old Fart Supporting Member

    Mar 11, 2011
    Here is a basic question: When a full-tube amp has a Master and a Gain knob, and the operator chooses to get distortion by cranking the Gain and turning the Master down, what tubes are distorting? The preamp tubes or the output tubes?
  16. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    Out there!
    The preamp tube(s).
    Wasnex likes this.
  17. Old Fart

    Old Fart Supporting Member

    Mar 11, 2011
    Alright. Does this mean that in order to get output tube distortion, you would do the opposite? Crank the Master and lower the gain?

    I am struggling to understand this. It had been my belief that the sound of rock and roll guitar is the sound of EL34 output tubes being killed. And that the way to do this is to feed them lots of preamp gain. I had been under the impression that guitar players got distortion by turning down the master and turning up the gain.
  18. Jeff Scott

    Jeff Scott Rickenbacker guru..........

    Apr 11, 2006
    Out there!
    Not really, as it requires a signal that is hot enough to get the output tubes "moving" so to speak.
  19. beans-on-toast


    Aug 7, 2008
    Master volume is simply a preamp attenuator. It allows you to crank the preamp into distortion and then attenuate it or turn down the overall level, then feed it to the power amp. You allows for get preamp distortion, not power amp distortion.

    To distort the power amp, you have to drive it hard and loud.
    VitoMB and Wasnex like this.