Johnk_10's SVT VR relay mod.

Discussion in 'Amps [BG]' started by Rickett Customs, Jan 11, 2010.


  1. Rickett Customs

    Rickett Customs

    Joined:
    Jul 30, 2007
    Location:
    Southern Maryland
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    Luthier: Rickett Customs...........www.rickettcustomguitars.com
    I've had 2 gigs so far, where my SVT VR would stay in standby.
    The first time, I didn't know what had caused it, so in TB fashion, I posted a thread asking about it. A fella named Johnk_10 had mentioned a mod, that he had done to his VR.

    Link to his page on this (he's more detailed with the info on it): http://johnkvintageguitars.homestead.com/SVT-VR.html

    Apparently, if the voltage is under 112 volts, she won't even come out of standby. After the second time at a gig that happened to me, that was it. With a few more questions I asked John, I was ready to do this.

    First off, and very important, I needed to discharge the cap. Set up my rig in the living room and got it all hooked up.It was told to me that one way to discharge it, was to power it on (of course for a minute or so, take it out of standby and unplug the power cord, while playing the bass, it fades out, distorts, etc, then leave it for about 10 minutes or so. That would at least take any of the "lethal" voltage from the caps and leave a few volts or so.


    The Amp:

    [​IMG]






    After taking the amp out to my shop, I would need to make a "Y" cord first, so I got 2 male connectors and a female connector. Ok, so I cut 2 short lengths of solid core 16ga copper wire. Soldered that up.

    [​IMG]





    I took the screws out of the bottom, that holds the chassis to the enclosure, left one in and took the last one out on the edge of the table (sitting flat).

    [​IMG]






    Then, I took the back grille off:

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    Once you take the connections off the fan, I put this aside. Not pictured: I also took the chrome corners off the lower back side.

    [​IMG]






    I then carefully slid the chassis out of the back and disconnected the 2 cable plugs. Admission of guilt: I did leave the tubes in, when I did this, but I did not move this around too much.

    [​IMG]






    Once I liberated the chassis, I flipped it over (The transformers sit a little higher than the tubes, so the tubes didn't touch the tabletop. The small circuit board on the upper lefthand side is the one we're looking for.

    [​IMG]

    [​IMG]





    I cut the wire ties, that were holding the wires all together.

    [​IMG]






    At this point, this is where the "y" cable come into use, I unplugged the yellow wire @ "J-26", connected it to one of the male connectors on the y and unplugged the black wire from "J-27" to the other male connector. I plugged the female connector to the "J-27" on the circuit board. Then I wire tied it back up and re-assembled it back together.

    [​IMG]




    Took it back into the house, plugged it back in and what do you know? It worked. So, Johnk_10, you da man!
  2. Rickett Customs

    Rickett Customs

    Joined:
    Jul 30, 2007
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    ...........Just need to try it at one of these low volt gigs now :)

    The lowest so far I've seen at the offending places was 109v
  3. TheDialog

    TheDialog Supporting Member

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    Hey, I see you are located in MD, is he located in MD as well?
  4. Rickett Customs

    Rickett Customs

    Joined:
    Jul 30, 2007
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    Luthier: Rickett Customs...........www.rickettcustomguitars.com
    I think Johnk is in California, I think.
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  6. 6jase5

    6jase5 Mammogram is down but I'm working manually Gold Supporting Member

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    San Diego/LA
    JohnK is indeed in SoCal and has a wealth of SVT knowledge.
  7. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
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    Apopka, FL
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    Endorsing: Ampeg Amps, EMG Pickups
    made it look easy!
  8. nysbob

    nysbob

    Joined:
    Sep 14, 2003
    Location:
    Cincinnati OH
    If I owned a VR, I'd do it as that seems to be the only consistent reliability issue with the thing.

    Posting clear instructions and good pics is a real public service - so kudos to you and Johnk...:cool:
  9. Jerrold Tiers

    Jerrold Tiers

    Joined:
    Nov 14, 2003
    Location:
    St Louis
    They ought to do that mod at the factory........ i.e. get rid of the relay.

    One day I'll ask George M why that circuit is even in there.

    BTW, if it was still SLM and not Loud, there would be a Service Bulletin on that, very possibly to just bypass it. Way I see it... forty years of SVTs can't be wrong.
  10. Tim1

    Tim1

    Joined:
    Sep 9, 2005
    Location:
    New Zealand
    Only trouble with doing it as soon as you get it is that you will probably void the warranty - the only reason why I have not done mine. Mind you, we are on 240volts here and so far I have not had a single problem.
  11. JimmyM

    JimmyM Supporting Member

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    that mod look non-intrusive enough to where you could put it back to stock. still, as you said, probably not an issue for you anyway.
  12. Rickett Customs

    Rickett Customs

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    Yep, it's reversible, with no physical cuts to any wire.Just the wire ties were cut ;)

    OTOH, you could just leave the original ties alone and still do it too..., then remove it with no trace.
  13. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    Feb 16, 2008
    Location:
    Thousand Oaks, CA
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    John K Custom Basses
    Glad it worked out for you!
    I ran mine for a very short time, to test it on my variac with as low as 87 volts and it still worked fine, albeit with lower output power. however, i DON'T recommend running any tube amp on a variac at low voltages, since the heaters will not be getting the correct voltage.

    IMO, it is a silly circuit and i think a candidate for extraction in all of them, since it really has no business being in there. and, as stated above, it is completely reversible. after the mod, it takes all of the worry out of it going into standby on the gig, and IMO, makes the amp exactly what it is, a great sounding SVT.
  14. username1

    username1 Supporting Member

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    Dec 28, 2005
    Location:
    alberta canada
    Jonk10, Do you know if this mod will work on an SVT classic or would the power section be different than the vr? Thanks..Dave
  15. JimmyM

    JimmyM Supporting Member

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    unnecessary on the cl. the vr is the only one with that problem.
  16. username1

    username1 Supporting Member

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    alberta canada
    I have actually had that problem a couple of times in our rehersal room. Its an old building. Thankfully it hasn't happened yet on a live job but my old V4B has never had a problem plugged into the same outlets. I would just like the assurance it wont happen during a job. I'm not concerned about not having protectection cuircitry in the amp as i always maintain my equipment well and my old svt's never seemed to have problems without that cuircuit.
  17. JimmyM

    JimmyM Supporting Member

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    hmmm...could have sworn they said it wasn't necessary on the cl. could be another problem, though. i've played a couple problematic cl's from loud, but not that problem.
  18. johnk_10

    johnk_10 vintage bass nut Supporting Member

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    Location:
    Thousand Oaks, CA
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    John K Custom Basses
    CL's definitely don't have the bias relay circuit, or at least, every one that i've seen doesn't. AFAIK its exclusive to the VR.
  19. username1

    username1 Supporting Member

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    Dec 28, 2005
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    It must have been a surge through the wall outlet then but it hasnt done it for over six months now I just assumed mabe the classic ran the same relay cuircuit but I really dont know alot about it compared to you guys. Thanks for your replies and info, nice to have people on here that know ampegs as well as you guys. A great place to come for info and great having guys like you on here. Thanks again.
  20. Rickett Customs

    Rickett Customs

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    JohnK,
    I'm going to do some more research, but I thought about later on (maybe in a year or 2) trying to set up this amp as an actual "2 channel amp" where I could use a foot switch, to change the 2 channels and perhaps make channel 2 a little less cleaner than channel 1. is it a crapshoot or something I should do a helluva lot more research for? I don't plan on selling it, so no worries about getting rid of it later.
  21. smo

    smo

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    A Sandgropers' City
    Thanks for the tutorial dude ;)

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