SVT - What do you think about this board?

Discussion in 'Amps [BG]' started by blockinlay, Oct 6, 2012.


  1. blockinlay

    blockinlay

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    77 SVT Not used in maybe 15 years. It biased fine with the mix match of vintage tubes that are in there. None of the resistors are bad, but they are not great either. The hole doesn't seem to affect the amp, but it sure looks like hell.

    [​IMG]
     
  2. CL400Peavey

    CL400Peavey Supporting Member

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    I think its tech time...
     
  3. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

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    Ampeg: the opposite of Hiwatt, inside.

    If it sounds good, it is good, but I can't imagine everything works properly. At least one trace looks like it is interrupted. This would be a good opportunity for some new filter caps, too.
     
  4. bigbottomend

    bigbottomend

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    I'm supprised it still works. It would be nice to find out if someone can make a replacement board.
     
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  6. BassmanPaul

    BassmanPaul Gold Supporting Member

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    I would contact the Ampeg guys and see if you can procure a replacement. Hodgy is the one to talk to.
     
  7. jmpiwonka

    jmpiwonka Supporting Member

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    it looks like someone extended that wire to replace that trace that got burned...
     
  8. B-string

    B-string Supporting Member

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    IMO you are correct. IMHO it's road worthiness is now questionable and that board should be replaced.
     
  9. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

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    Good eyes! I think you are correct. The amp will probably need some repairs in another decade or two. ;)
     
  10. blockinlay

    blockinlay

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    Played it for an hour yesterday through two 810's. I love SVT's! This is my second one. I am going to take it to a tech though. One of the plate resistors is bridged to the next one one side because of the hole. The bias was the same after an hour. Would be nice to see a new board in there, but it might not be necessary. I wonder what caused it. Maybe the previous owner bridged the fuse. He bought it used in 1980, and I just picked up for bargain. The tube sockets are all fine.
     
  11. beans-on-toast

    beans-on-toast Supporting Member

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    Here is your circuit board oriented as you have it in your photo. It looks like the burn is under three plate resistors, R27, R38, and R41. Fortunately, there aren't a lot of traces in that location. What you have isn't unusual although it is worse than what you normally see. The board can be cleaned up, patched, and the amp can be fixed.

    What happened is that a power tube blew, current surged through it and the plate resistor was taken out. Normally the resistor acts like a fuse, it burns a bit and blows open. Sometimes there is some scorching to the board but not like we see here. Normally the resistors are installed offset from the board a bit, it is important to do this. Also they if they are the cement block type, they are installed on their side so the cement is facing the board. If they blow, it protects the board somewhat. Some of these resistors have cement on three sides, the forth side is glue. The glue side should not be down against the board on these amps. If the resistors are the cylindrical type, there isn't much that you can do other than installing them with some space between the resistor and the board. Sometimes people make the mistake of installing higher wattage resistors than the 5W that should be there. A bad idea because this delays the resistor from blowing and causes more damage.

    I've never seen replacement boards available. It wouldn't be difficult to build one but it also wouldn't be cost effective. Finding a junker and salvaging the board is a possibility. I would clean the board and trim away the bad bits. The missing traces can be fixed with some bus wire. If you wanted to get fancy, a piece of G10 FR4 Garolite, a fireproof circuit board material commonly used in amp building, can be fit in to patch the board. This would make the amp more road worthy. Depending on the extent of the damage, it may or may not be necessary.


    [​IMG]

     
  12. blockinlay

    blockinlay

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    I like that schematic. Got any more like that?
    Here's another pic.

    [​IMG]
     
  13. beans-on-toast

    beans-on-toast Supporting Member

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    The six 5 ohm cement block resistors are the plate resistors.

    You can see the writing on all but one, which is upside down. The flames tend to shoot from the bottom, as seen on the upside down one. I install them on their side with a small offset from the board. That way you can still see the label but the dangerous part is facing away from the board.

    The two other block resistors near by, R35 and R36 are the 1 ohm 5 watt cathode resistors. These should be matched as closely as possible to ensure that the balance can be set accurately. When replacing the plate resistors I also match them. I buy extra, they don't cost a lot, and sort them with an ohm meter.
     
  14. Jerrold Tiers

    Jerrold Tiers

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    I think the board is ugly, would kill the resale value, and likely works fine.....

    Get rid of the burnt bits, they act like resistors (carbon). use the amp. Good luck finding a replacement board...... you'd have to buy a non-working "parts" amp to get one. I don't know of anyone making replacements, how many would ever be needed?
     
  15. blockinlay

    blockinlay

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    Tech has it now for a once over. New feet and power cord, clean up the hole and go over the amp. He doesn't like the odd mix of power tubes. The tubes in it now are three different vintage, and three of the same coke bottle 6550's made in Japan. I thought about maybe buying online some new JJ's that are burned in for 24hrs, they say, and then matched at BOI audioworks for $238. Any tube seller recommendations, or should I pay extra and get the same tubes from the tech?
     
  16. CL400Peavey

    CL400Peavey Supporting Member

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    I would worry about tubes after the tech gives you his ideas about the board. No reason to sink cash on tubes if the board is going to give your amp a limited life time.
     
  17. beans-on-toast

    beans-on-toast Supporting Member

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    A mix of tubes could be an issue but doesn't have to be if they are matched. Japanese tubes are very good.

    As for a reseller, I like tctubes. They match the tubes at the voltage that your amp works at. Most don't do this.
     
  18. jmpiwonka

    jmpiwonka Supporting Member

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    ^ i got some tungsol reissues from TCTubes, my tech didn't complain about the set...and a year and a half or so later i've had no problems with the power tubes.
    they shipped out real quick too.
     
  19. blockinlay

    blockinlay

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    Got the amp back from the tech. He went over the whole thing, and replaced three resistors, two output tubes, (one was bad), three pre-amp tubes causing noise, the fuse holder, two fuses, added new feet, and a new cord. He went over all the jacks and switches, cleaned up the wiring in the area where the hole was, repaired an issue with channel two, tested, and biased. The amp is working perfectly now and should be reliable. All I need now is a small piece of the blue and silver Ampeg speaker cloth to replace the rabbit cage wire that was on the front panel.
     
  20. beans-on-toast

    beans-on-toast Supporting Member

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    Glad that you got the amp resurrected and back in service. Always nice to see.

    Most places sell the grille cloth by the yard, but you can get it by the foot, which is the amount that you need, at fliptops.net, they also sell the frame with the cloth installed.
     
  21. jmpiwonka

    jmpiwonka Supporting Member

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    ^^ awesome. love to hear when people get the old gear running right again.
     

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