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Thread: Ampeg SVT 400T Limiter doesn't work

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    Ampeg SVT 400T Limiter doesn't work

    I bought this amp a while ago and for the biggest part it works perfectly but i recently found out that the built in limiter (which is turned on and off by pushing and pulling the master volume knob) doesn't seem to work.. I will post some links to a manual and schematics of the amp below. In the manual it states that when the limiter is turned on it should limit the overall volume to just below the clipping point (fully undistorted ). Ive tried multiple settings to test this but it doesn't matter if my gain is turned all the way up or just far enough that it clips a little. The Limiter doesn't do anything and the LED which should go on when the limiter is active is no different.
    Even though Pulling the master volume knob should turn it off i've tried pushing and pulling but it makes no difference.

    Manual:
    https://ampeg.com/pdf/SVT400T.pdf


    Schematics: (the limiter is found on the bottom right of the third page)
    https://ampeg.com/support/files/Sche...ematics_PL.pdf

    Please let me now if you know anything

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    Supporting Member nevetslab's Avatar
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    [QUOTE=Sjaky223;526250]I bought this amp a while ago and for the biggest part it works perfectly but i recently found out that the built in limiter (which is turned on and off by pushing and pulling the master volume knob) doesn't seem to work.. I will post some links to a manual and schematics of the amp below. In the manual it states that when the limiter is turned on it should limit the overall volume to just below the clipping point (fully undistorted ). Ive tried multiple settings to test this but it doesn't matter if my gain is turned all the way up or just far enough that it clips a little. The Limiter doesn't do anything and the LED which should go on when the limiter is active is no different.
    Even though Pulling the master volume knob should turn it off i've tried pushing and pulling but it makes no difference.

    Manual:
    https://ampeg.com/pdf/SVT400T.pdf


    Schematics: (the limiter is found on the bottom right of the third page)
    https://ampeg.com/support/files/Sche...ematics_PL.pdf

    Please let me now if you know anything[/QUOTE)

    Welcome to the Forum! The Q17 J112 JFET 'Limiter' circuit doesn't look like it will do any serious Anti-Clipping, though the other Limiter JFET Q1 DOES look more traditional. Q1's circuit at least has a Bias Adjust to control it, and looks like it would do more dynamic limiting than that of Q17, which is there to prevent hard output stage clipping. Hard to hear that as a function. Is this a new amp, or used? The bias for setting Q1's limiting character may need to be adjusted.

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    Old Timer J M Fahey's Avatar
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    What a mess of a schematic, chopped ll over the plce at random.

    Only way to use it is to print all pages exact same size (so NOT using Musicparts "advice" of using "size to page" printer setting) and then yape them together to get a blanket size (not kidding) paper schematic.

    Or re-"print" it (using Cute PDF Printer or similar) to get a non-"secured" version which allows this poor excuse for a schematic to be turned, page by page, to a large mosaic/tiled .gif where you can follow signal paths.
    What a mess.

    That said, your confusion comes from the fact that "SVT" 400 (not a tube in sight) has 2 *two* completely different and independent "Limiters".

    The first one is fully contained in Page 2, is NOT to avoid power stage clipping , it has NO CLUE as to what is happening there, and insteas is equivalent to a compressor effects pedal, only there to increase Sustain ... and in fact so labelled.

    How it works:
    IC1a is the variable gain input stage, its output goes to compressing gain cell: R7/Q1 .
    IC1b recovers compressed signal, sends some to IC2b through P2 "sustain" (compression threshold), itīs amplified 30X by IC2a and rectified by D3.
    Attack time: R13-C8 ; release time: R13+R16 - C8
    That control voltage varies Q1 bias; the more positive=th more it attenuates.

    To test it, feed a relatively strong signal into J1 (say, 200mV) , rise gain until yopu get a *strong* signal at TP3 , say anything from 5V RMS to Op Amp clipping (about 9V RMS) and then start rising "limiter" (P2? ... muddy), signal should bve compressed down to, say, 1 V RMS or thereabouts, and further raising "gain" should have little or no effect.

    Not your main problem, but check it so afterwards you donīt worry about it.

    Later Iīll search fo the real power limiter, so far only found half of it, a LED which is part of an Opto ... which is the Crate/Ampeg favourite way .
    Havenīt found yet the matching LDR which "should" be limiting power amp input.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Later Iīll search fo the real power limiter, so far only found half of it, a LED which is part of an Opto ... which is the Crate/Ampeg favourite way .
    Havenīt found yet the matching LDR which "should" be limiting power amp input.
    Found the Opto Cell.....OC1. The LDR is in the feedback loop around IC1, and the LED portion is in the base circuit of Q1, between D1 & D2.

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    Old Timer J M Fahey's Avatar
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    Thanks nevetslab.
    Lost some time finding it (and thinking WTF?) because they use IC1 designation twice; once in the preamp, once in the power amp.

    Ok, looking at the power amp, IC1b is in the power amp proper; IC1a feeds it and is gain controlled to compress/limit.

    Signal level at IC1b output, pin 7/Tp3 , is normally mild until when clipping it tries to compensate and level jumps through the roof.
    That signal goes to the bridge surrounding OC1 Led and turns it on.

    So of course it wonīt react to the Preamp "Limiter" potentiometer.

    If you want to check itīs working tack solder a red Led in parallel with OC1 Led.
    Dive amp to clipping and it should light up (weakly).
    Try both positions of the "Limit" switch, in one of them Led should light and illuminated LDR should limit gain.

    Amp will clip "a little" or you would not have error signal to turn Led ON , this is not a Studio quality limiter but meat-and-potatoes musical instrument grade.

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    Last edited by J M Fahey; 03-29-2019 at 12:16 AM.
    Juan Manuel Fahey

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