Advice on modding an SVT II (extra effect loop and tuner out)?

Discussion in 'Amps and Cabs [BG]' started by Rattanee, Feb 8, 2018.

  1. Rattanee


    Oct 31, 2017
    EU, Hungary
    Hey folks,

    After realizing today that the existing 'effect loop' on this amp is not suitable for my intended use with a compressor (because it's between the pre-amp and the endstage, changing volume directly affects the output level, and hence requires constant re-adjustment of any effect in and output levels), I've come to the conclusion I need to do some modding. I hope there's some like-minded folks here who can chime in on this as I'm more at home with digital circuitry than analog.

    Now before anyone cries 'don't ruin it', I'll state that I want to keep everything fully reversible.

    What I want, is to add a buffered tuner out jack, and a fixed-level effect loop that is in front of the volume pot. What would be the best way of going about this?

    I was considering simply removing the original pre-amp out and power amp in jacks, and installing wired jacks upside-down, but I suspect there would not be enough clearance between the PCB and the jack housing. What I am thinking about, is to replace the rear grille on the unit with a custom one, that has space for mounting three jacks (and possibly a speakon to update the speaker output from Jacks).

    Now creating a good quality voltage follower with an op-amp for the tuner out is no big deal. However, the effect loop is more interesting. I would think most rack compressors would require line level, not instrument level signal, so another OP-amp circuit pair seems to be in order, (one to amplify output to the compressor, one to attenuate the return signal without degradation) however what I would like to do, is that if the loop is not used, the op-amps should stay out of the audio path.

    Any chance any of you guys has done something like this before? What is the best way to go about this circuit-wise?

  2. ThisBass


    Aug 29, 2012
    Tuner-out can be easily realized with an AB-Stompbox.

    About 20 years ago I had some issues to have enough loudness in a stupid loud band with my 70's SVT powering a 212 cab.
    To fix the issue I plugged the bass guitar into a dbx266 compressor and the output of the compressor into the normal input of the SVT.
    The threshold was set in a way just to left some dynamic feel of still "alive" performance with ratio about 4:1 at most
    Its also possible to use the send-return effect loop of the SVT but then the compressor needs a different adjustment for threshold. As with the 70's does not have a master volume its more practical to use the input jack to feed the processed signal to the (70's) SVT.

    As your SVT II provides a master volume you may check both possibilities to check-out which set-up is best for your needs
    Last edited: Feb 8, 2018
  3. Dave W

    Dave W Supporting Member

    Mar 1, 2007
    Westchester, NY
    I have nothing useful to add, but the SVT II has no master volume.
    Rattanee likes this.
  4. Rattanee


    Oct 31, 2017
    EU, Hungary
    I have the original SVT-II not the PRO version. I only have a preamp volume, no master volume. My problem with using the existing loop is exactly the fact that it is a preamp out, which means every single time I adjust volume, the required threshold setting changes. IE, if I raise the volume, I need to raise the threshold as welll, otherwise my signal gets compressed way beyond what I like. I pretty much just use the compressor as a soft limiter, to take off the occasional hard peak, and an expander as a noise gate.

    I have an Ashly and a TC dual compressor. Currently using the TC, and I am not sure if an instrument level signal would be enough to use it (prety sure it would be too quiet for the Ashly), but I suppose it's worth a shot. I'd just need to add a 1U panel with a jack because all the inputs are on the back for both compressors. I'll ry and see what happens.
  5. BassmanPaul

    BassmanPaul Inactive

    Get yourself a clip on tuner. Run the compressor in front of the amp. Do not modify the amplifier unless you are conversant with electronic technology.
  6. Rattanee


    Oct 31, 2017
    EU, Hungary
    We're on speaking terms yes :D
  7. seamonkey


    Aug 6, 2004
    It would be best to use the effect before the amp if you want the most from the pedal.

    If you follow the threads on the diodes in the signal path of these amps, then the Pre-amp out and Amp-in jacks are nowhere near that of any standard Effects Loop in any other amp.
    The levels would be much lower than the levels a bass outputs by itself.

    Otherwise, you could run the amp-out jack into op-amp circuit with a boost up to standard levels. Then return after the pedal with attenuation to the amp-in. A small mixer could probably do this.
  8. Xandrell


    Aug 23, 2014
    I went through many trial and error inserting a compressor into a SVT II Pro effects loop. I only had a Warm Audio WA76 and Distressor to work with. Long story short, If you crank up the pre, drive, eq etc.. you may get close to a line level needed for the compressors line input, but this probably isn't the sound your after even without a compressor. A line driver fixed it for me, in my case a Lehle Sunday Driver between the effect send and compressor. Worked perfect for the WA76 but still wasn't enough for the Distressor. See the info in the post below.
    Ampeg SVT 2 Pro Send Levels ?