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Got 1969 SVT back from the shop...almost fixed

Discussion in 'Amps and Cabs [BG]' started by JimmyM, Mar 21, 2013.

  1. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Well, I am bummed. Just brought out my SVT to its first gig since getting it back on Monday. It has been to two different techs twice now, and they both tell me it's fixed when it's not. I can't blame them, though, because they actually have been fixing things along the way. Like this time, he fixed a problem operating with the REDDI where it would make a ground noise that even adding an external pin 1 lifter to the REDDI couldn't get rid of. I needed to use the ground lifter tonight, but the hum cut down when I added it tonight, so it's not like he's jerking me around.

    But there is still a hum. At most settings it's kind of faint but noticeable. It's in the midrange circuit somewhere. I know this much because when I boost the mid knob or when I back off of it, it hums more. At first I thought it was just boosting mids, but it does it when you go the other way, too, starting at around 9:00 down to all the way off. It's especially bad when you set the mid selector to 200hz. And after 4 trips to two of the best techs around here, it's still not acting right, even though they tell me it's fixed. Yeah, they did fix most stuff, but this is the one thing still not fixed, and it's been driving me batty for over a year now.

    So I'm at a loss as what to do next. Do I keep taking it to these guys till one of them hits it? Do I risk sending an old as hell and beaten-up SVT to a serious SVT expert? Does anyone have any clue what's going on here? Why, Lord, must thou punish me so???

    Oh, excuse me...sorry, just having a first world tantrum there. But I really would like this fixed. Any and all ideas are welcome.
  2. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Take it to the nearest old ham radio guru. He'll listen to it and say "Well there's your problem right there sonny." And he'll whack it with the side of his hand and it will sound like a dream. Seriously, old ham guys are known for being able to debug a tube amp. They analyse it more from a circuit analysis standpoint than a tone stack one. That takes the clouded view of an amp tech out of the equation. It's worth a shot.
    bobyoung53 likes this.
  3. spacebassed

    spacebassed Supporting Member

    Jan 31, 2009
    Everybody knows that God is a B15 guy... He's punishing you for straying from the true path. :D
    bobyoung53 likes this.
  4. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Bah, God doesn't care which Ampeg I use, just as long as I use Ampeg.


    Well I guess all good techs are early risers, so I guess I'll have to wait till tomorrow to see how this play turns out. Can't say I'm totally bummed. It was really noisy with the REDDI before and now it's not.
    bobyoung53 likes this.
  5. JOE EDO

    JOE EDO Supporting Member

    Aug 28, 2007
    Central New York
    Jimmy Did you try different outlets? My 73 SVT will buzz like a hornets nest when plugged into certain outlets in my house that have dimmers or rheostats in the curcuit. If that is not it, see if you can plug in direct to the power amp with you bass..buzz? Might help isolate the issue to either the power section or somewhere in the preamp circuit. You did mention the midrange circuit bringing out more buzz. Could be the inductor on the preboard? I would have a tech hit 9Reflow) every joint on the pre board and then use some hi-temp silicone to hold the pre caps and the inductor in place. All I got for ya.
  6. Send it off to JohnK?
  7. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Joe, I have tried in different outlets, as well as a stage where I never get buzzes from my gear. An inductor sounds like a possible culprit.

    CL, unfortunately, sending it probably isn't in its best interests, otherwise John would be one of the first people I'd ask ;)
  8. The hum is at what levels at what positions on the Mid control? I read at 200Hz is the highest level but pot position does what?
  9. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    It's at the higher and lower levels of the midrange volume knob, and the selector does 220, 800 and 3k. Hums on all settings but especially at 220.
  10. Interceptor


    Mar 29, 2005
    Madison, WI
    I couldn't agree more.

    Or, an old Radio and TV engineer. There's some of them retired in your area. Guys who worked places like NBC or CBS in New York. An SVT is child's play to them.
  11. jason weatherby

    jason weatherby

    Aug 30, 2012
    MOST people that don't know don't know that they don't know, ya know.
  12. I would be looking at R40, R41. The ground through the mid select switch (not knowing from here of course if the shift point are actually correct).
  13. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    R40 and R41, check. OK, so I have a little $20 multimeter. What do I do? ;)
  14. With the switch in position 3, mid pot at noon you should be able to measure the resistance from both resistors to ground. One side will be zero (or close to it, check your grounded reading by shorting the meter leads together), the other side of the resistors should be close to the same and less than 220K ohms. If you disconnect the wiper of the mid control you should be able to get a better read on the two resistors (closer to the value within the tolerance range).
  15. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    OK, so I set my meter on what and touch what color probe to what?

    Sorry Jon, but admittedly, I am not a genius when it comes to this stuff.
  16. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Hey Jimmy,
    Does it have a problem when you plug a lead straight in without the REDDI?

    Also, have your techs been troubleshooting with this DI plugged in and connected to a mixer?
    My guess is that the noise is based around 120 Hz - 600 Hz and that boosting the mids accentuates the problem.
    Or, I'm wrong.
  17. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Yes Jim, it does have the problem without the REDDI. But the REDDI communication problem got fixed, so that's not the issue anymore. You may be right about that noise. Definitely louder on 800 and 220 than 3k.
  18. Is your meter auto ranging in ohms? Color of the leads won't matter. The fact it nulls somewhat with the pot centered is leading me down this path.
    One meter lead will go to the preamp chassis (black if you like).
  19. andrew

    andrew Supporting Member

    May 20, 2000
    Vancouver BC/Pacific Northwest
    Endorsing Artist: Aguilar Amplification, Spector, Regenerate Guitar Works, Tech 21 NYC
    I have a similar gremlin in my '73 SVT that two very good techs could not find. Same thing, plenty other small things fixed along the way but not the hum culprit.
  20. hover


    Oct 4, 2008
    Just keep them away from anything that looks like a big battery....;)

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