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SVT issue

Discussion in 'Shock Brother's DIY Amps' started by Syrinx, Jul 22, 2020.

  1. Syrinx

    Syrinx Tele-Holic

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    Hey Guys- I was setting up the SVT stack for a bud to play at jam, and forgot the indicator light was supposed to turn green-and played with it red. After an hour or two amp shut down-no sound-red indicator light still lit. Manual says high power supply or tubes. I have 250$ in tubes coming-but wonder about power supply-the tubes light up when turned on-would they do that if there was no high voltage, or is the highvoltage on the output transformer side. Even though I have built a few kits- I likely will have this one to a tech-due to weight as much as anything.
     
  2. 6String69

    6String69 Tele-Holic

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    Go to talkbass.com and ask. There is a guy there who will know
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The SVT has circuit protection. Hopefully it protected the amp.
    What happens when the amp is turned *on* now?
    You say the tubes "light up" is that just the heater glow or do the tubes glow?
     
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  4. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    Yep. Ask @agedhorse. He’s an Ampeg design engineer and very helpful. He’ll even do repairs.
     
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  5. Syrinx

    Syrinx Tele-Holic

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    Thanks -when I turn it on now, the pilot light is red, the tubes are lit like on standby I can only see front 3 power tubes,but all visable pre and power tubes are visually on.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Tech time. Imho, any bass player who runs an SVT or a Sunn/Fender Bass 300 and let’s anyone else...other than a tech....set it up is asking for trouble. All of these amps should be biased before playing.... after every move that is. That is why there are external bias points. It becomes a ritual. They do not simply setup, turn on and play....or if they do, they will encounter problems quickly.
     
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  7. mojavedesert

    mojavedesert Tele-Meister

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    A power conditioner is good with those amps. If power is plugged into the same circuit as an air-conditioner unit that surges and you go too far below 115v or over they'll shut down. Also make sure right ohms match with speaker cab. Also not a guitar cable between amp and cab, you need speaker wire. That would shut it down.

    Plugged into a long, cheap extension cord also would shut it done.
     
    Last edited: Jul 23, 2020
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  8. tubelectron

    tubelectron Tele-Afflicted

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    @Syrinx,

    The plate voltage in an ampeg SVT is circa 600-650V, so be cautious when you work in it with the chassis opened and powered.

    I serviced and repaired several of them, from originals with 6146A "Rolling Stone Tour" to RI from USA, Viet-Nam and China : it's not always a pleasure...

    [​IMG]

    You can also check the screen resistors of the 6550 : they often burn or cut when PA has a failure - doing this, they also protect tubes and circuit from more severe failures.

    And yes : the RI ones have a protection circuit... That can be defective :mad: ! I saw that also...

    Good luck ! ;)

    -tbln.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The tube heaters are functioning but there is no power to the tubes. It is very similar to having the amp on *standby*. Since you have tubes on the way, say a hail Mary and give them a try.
    Unfortunately, I think you will end up with a tech and a lesson learned. (Cross your fingers the protection circuit worked.)

    Best of luck.
     
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  10. Syrinx

    Syrinx Tele-Holic

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    Thanks again! I dont smell anything fried-and I have a world class sense of smell! Agreed on the voltages-they are insane on this amp! I have more experience with electricity than I do with amp repair-so a tech will be employed before I do anything I dont fully understand-my goal is to live as long as uncle doug!
     
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  11. Syrinx

    Syrinx Tele-Holic

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    Right on 100%- it is on a power conditioner- and its own circuit. Using proper ampeg cab and insanely expensive speaker cables as it is speakon only as I recall. And also for Wally-it was biased regularly by myself as well as former owner.
     
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  12. tubeswell

    tubeswell Friend of Leo's

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    I presume its a SVT-CL? You need to dismantle the amp and get at the output tube board with your R-meter to check whether any Rg2 are open. If they are open, the current imbalance will likely cause the bias comparator circuit to go into auto-standby mode.

    A flaw with these amps is the liberal use of space connector terminals for the connection leads between the boards. The terminals for the main filament connectors to the output tube board see just over 11A in idle operation, and surge up to 6 x more than that when those 6550 filaments are cold at startup. Over time the contact surfaces of the spade connectors oxidise and this creates resistance across these connections. So at startup, this can create a large voltage transient at these spade connectors, and the poor conductivity results in sparking/arcing on the spades (which you will notice as charring on these terminals). These transients travel back through the filament PT's heater winding and are inducted into other windings from the same PT, one of which supplies the dual power rail for the bias comparator circuit. And so these transients can damage the PS for the bias comparator. So you may need to do a thorough check of that circuit as well. (BTW, the long-term preventative 'cure' for dodgy spade terminals like these, are to clip them off and solder the heater leads directly onto the terminal tabs on the output tube board, to ensure a good resistance-free connection)
     
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  13. The Ballzz

    The Ballzz Tele-Afflicted

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    Yes indeed, an SVT is a serious piece of machinery! I just bought a '97 SVT CLASSIC and the sound with a great P-Bass is the most classic bass sound you could ever ask for! On the other hand, the more I read about them the less I feel inclined to do any work on it myself! Luckily everything functions almost perfectly and it is visually/cosmetically pristine. The only sore spot is that the -15db input jack is intermittent. I'm suspecting a bad jack, or more likely a wonky solder connection. Externally cleaning with De-Oxit and plugging in/out repeatedly gave no love! I've seen enough documentation and gut shots to feel at least confident enough to pull out the preamp module and tackle the jack issue, but will not likely ever go into the power amp section. Kind of planning to sell the head (got a steal of a deal on the head/matching 810 cab) to help finance a V-4B. Should be able to get nearly as much for the head alone as I paid for the pair!
    Just Thumpin'
    Gene
     
  14. mexicanyella

    mexicanyella Friend of Leo's

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    If I’m not mistaken, agedhorse is affiliated with Mesa and Genz Benz/Genzler. Which is not to say he doesn’t know his way around Ampeg stuff; he very well might.
     
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