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Ampeg SVT Classic problem! Cannot get green LED's when Biasing! Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by gitfiddler, May 23, 2011.

  1. gitfiddler

    gitfiddler Member

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    I got a Ampeg SVT Classic and recently replaced the power tubes with a new set of EHX 6550's. I replaced the tubes because it would cut off after a while of playing and the standby light would flash red and green. I also could not get the old tubes to bias up to get the green LED's to light up. So after I put the new power tubes in, I still cannot get the green bias LED's to light up. Has anyone ever had this problem? If so what can I do?! Got some gigs coming up and I need the amp working. Any help would be much appreciated!
     
  2. ScottR

    ScottR Silver Supporting Member

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    Mrlnd
  3. gitfiddler

    gitfiddler Member

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    Tried new driver tubes and still no green LEDs. Any more suggestions?
     
  4. schmidlin

    schmidlin Member

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    Those amps intimidate even seasoned techs. I think it's time to find qualified tech and drop it off. You don't want 700 volts on you.
     
  5. WaltC

    WaltC Member

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    as for the green led stuff, time for a trip to the tech and measuring voltages and checking tube bias.

    but... for the other behavior, the shut off and intermittent play, that could be the result of excessive sensitivity to vibration. There is a "watchdog" IC on the main circuit board that is there to watch for critical faults (bias, current draw, etc.) that is know to be, on some units - not all, sensitive to vibration. So, if you set the head on top of an Ampeg 8x10 (or even 4x10 sometimes) cab and play moderately loud, it will trip the safety cutoff and it will shut down and you have to wait until it resets to play again.

    For older units there is a mod that can be done to reduce the sensitivity, but the mod is 4 years old now and all the newer models should have the mod installed from the factory. Worth looking at (again, see your local tech) to check.
     
  6. wizard333

    wizard333 Member

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    Thats typical of those amps if they have one or more blown screen grid resistors. Check them.
     
  7. Prairie Dawg

    Prairie Dawg Member

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    Correct.

    I just finished one last week that had the same complaint and it spurred me to finish constructing my tube matcher. The problem gets complicated because there's no such thing as a solid reliable 6550 any more. So when Ampeg says "swap in a new set of matched tubes-maybe it'll fix it and maybe it won't" you are out on a $200 limb.

    It had three open 220 ohm screen resistors. Change all six. They're on the tube board. While you're in there check all the heavy resistors on the main board.
    Once you're back together let it warm up real well before you take it off standby and then carefully adjust bias. If it's been set too far out of kilter it'll trip the "honey I've got a headache" relay.
     
  8. gitfiddler

    gitfiddler Member

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    You were right! Three of my screen grid resistors were open as well! So should I use a 220ohm 1/2 watt resistor or 1 watt? Also do you think I could find some at radio shack?
     
  9. Prairie Dawg

    Prairie Dawg Member

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    The Ampeg people suggest you use 1/2w flameproof resistors. Having said that, all I had was 3 so I went to Rat Shack and bought 1/2w carbon film resistors on the theory that new, on-spec carbon film resistors were better than off spec resistors that had failed so I stuck them in. If you wanna wait, order them from Mouser and get the nice 2w Dales if they have them-they're about the same size as the 1/2w Rat Shack carbon film numbers.

    Wasn't it fun getting in there? My back still hurts from hefting that bad boy.:mob
     
  10. gitfiddler

    gitfiddler Member

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    Thanks. I'll run to the rat shack and get some carbon film ( in a hurry to get it up and running right). And yes that thing is quite the gut buster to lug around!
     
  11. gitfiddler

    gitfiddler Member

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    I was able to get some 1 watt 220ohm flameproof resistors yesterday. I put them in and got the green LED's! So it's up and running great! Thank you everyone for you help!
     
  12. CaptainJake

    CaptainJake Member

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    I would replace them with 1k 5watt flameproof resistors
     
  13. phsyconoodler

    phsyconoodler Member

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    Using over spec resistors here can harm other components if a tube shorts.Not a good idea.
    They act like fuses in those amps.It's really not hard to change them once you've done it a few times.
    Don't try and re-design a poor design,just grin and replace them when they open up.
     
    Last edited: May 25, 2011
  14. jay42

    jay42 Member

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    I found 1/2W KOA Speer metal oxide (aka flameproof) at mouser. They're a different color, but the same dimensions and texture as the original resistors.
     
  15. Miami Mike

    Miami Mike Member

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  16. phsyconoodler

    phsyconoodler Member

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    Quote:"They're a different color, but the same dimensions and texture as the original resistors."

    The TEXTURE is important if you wanna manhandle those resistors!
     
  17. phsyconoodler

    phsyconoodler Member

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    By the way,that link ain't gonna help at all with the 'new style' SVT Classic amps.Totally different animal.
     
  18. wizard333

    wizard333 Member

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    Glad I was right and it helped.

    I'd never use 1/2 or 1W screen grids. 5W FTW.
     

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