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SVT-CL Bias Issue

Discussion in 'Amps and Cabs [BG]' started by viking67, Nov 1, 2013.

  1. viking67


    Sep 14, 2005
    Langley, Canada
    Tried searching but couldn't find any helpful information for my issue.

    After using Gallien-Krueger amps for years I found a decent price on a 2007 Ampeg SVT-CL from craigslist. Tried it out, the seller wasn't very knowledgable about the amp but all seemed fine, so I bought it. Looked up the user manual before my first gig with it and tried to bias. The left side works perfectly, with the dial affecting the sound and lights. I set the left side green light to just barely green, but the red light flash would come on when I played, as expected. But here's the issue: the right bias lights don't turn on and the dial doesn't affect the volume or tone when turned. Played with it, and the left side, for 10 mins and nothing changed on the right side.

    Talked to some gear heads hanging around and we thought it was ok to do the gig with it, and it worked great, but I want to get it fixed. I've heard everything from a bad set of power tubes to blown screen grid resistors. I'll obviously need to bring it to a tech, but I'm looking for more insight before the tech just throws in new tubes and hands me a bill for $400. Has anyone experienced anything like this? Any help would be appreciated!!

    Setup: Sadowsky RV5-PJ (passive, front P pickup only) -> SVT-CL (0db input, Gain @ noon, bass @ noon, mid @ 1, treble at 2, master at 2) -> speakon cable/Ampeg 8x10
  2. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Supporting Member

    Take it to a tech, he is not going to hand you a $400 bill, maybe $100. When I had a single bad power tube my whole amp stopped working, so I doubt you have any kind of tube power issue. I had a 2005 American one but I think internally your Vietnampeg is the exact same.
  3. If you take it in for one triplet not biasing and they just throw new tubes at it then they have not completed the contract. Have them demo the bias adjustment before paying or leaving the shop, if it won't they have done nothing for the concern you contracted for repair. At drop off be sure to say you want all parts removed returned to you.
    Shops get paid to repair, not to WAG (Wild A** Guess) your money away. ;)
  4. viking67


    Sep 14, 2005
    Langley, Canada
    When I called the only tech i know of in the Vancouver, BC area he quickly quoted me a new set of tubes and $350-400. I'll pay it if that's what's necessary, just looking for more confidence before I do that.
  5. I would call him back, yes he may be busy and quoted an off hand price. Explain to him the one side won't bias with the on board adjustment and that one side doesn't seem to affect sound, but amp seems to function okay otherwise. If the tech says they need to "start with a new tube set", spend some gas money to go elsewhere. You'll be money and time ahead.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    If he is going to quote you that kind of money without even looking at the amp, find another tech if you can. Vancouver has got to have some good techs.

    It might just need a new tube or a trio. If your power tubes are worn, maybe a new set. If you can't bias one trio, I wouldn't use the amp as it is. If it is limping along with only two tubes on one side, they are being pushed to their limit.

    The thing about the SVT is that it takes more time to open up to get to the power tube board. So it is going to cost more if they have to do that. Check out this video and you'll see what I mean. In the video they explain a bit about the bias in your amp.
  7. WretchedExcess


    Jul 29, 2013
    You've got a problem on the right side. It could be simple, like mismatched tubes or one dead tube, or more complicated, like a burned fusible link. Before doing anything else, let's review the bias procedure for a working amp, and then we can cover a non-working amp once we know you've got the bias set right.

    I would not recommend biasing your SVT according to the tone that it produces. Doing it that way offers the risk of biasing the amp way too hot, and that'd be a recipe for burning up your tubes.

    The idiot-light system with the red-green LED indicators actually covers a pretty wide range of safe bias, so you really don't want to go outside of the limits recommended by the red-green lights, regardless of how you like the tone that you might get.

    The endpoint that you're looking for isn't necessarily to have the green lights just turn on. The green lights stay off when the amp is overbiased (undercurrent). The green lights turn on when the amp enters the safe zone. The red lights come on when the amp is underbiased (overcurrent). Red on marks the end of the safe bias zone. You want to stay out of the red zone. The safe zone is the entire range of rotation during green without red.

    The endpoint that you're looking for with the idiot light system is not the threshold at which the green lights just turn on. That event only defines the beginning of the safe bias range. You need to continue to turn the bias control until the red light comes on to determine then end of the safe bias range. Then you need to back down to an intermediate point between the two events. That puts you in the middle of the safe bias range.

    Don't leave the red lights on, no matter what -- red lights on mean that the amp is underbiased and excessive current is flowing through the tubes. Although that will give a great edgy and attacking tone, it will ddramatically shorten tube life.

    Now, assuming that you've got the bias into the middle of the green zone on the first bank:

    when you go to do the second bank you should do the same thing. you might notice that when you're tuning the second bank, there's a wide range of adjustment that allows power supply hum to increase/decrease while you're in the green zone. my personal preference is to set the second bank's bias at the point in the green zone that minimizes hum. i think that's a good idea, though it's not mentioned in the manual.

    whatever you do, don't go biasing into the red zone, no matter how good it sounds. doing that will dramatically shorten your tube life and could cause the amp to fail.

    before doing anything else, I'd turn on the amp, let it idle for 20 minutes, and then rebias the left bank using the above procedure. then rebias the right bank, and report back.
  8. WE I know you are trying to simplify but please, "channels"? They are push/pull triplets, not channels. ;)
  9. WretchedExcess


    Jul 29, 2013
    brainfart! :oops:

    sorry about that -- brain is still in neutral. need more coffee. ;)
  10. Yes, been there also :).
  11. WretchedExcess


    Jul 29, 2013
    That's a new video -- just over a week old.

    Unfortunately, he never fixed the bias indicator circuit.

    I don't understand why he didn't address the indicator problem. To get a full set of schematics all that you need to do is to email ampeg a serial number and agree to their terms of use. Maybe he just didn't want to deal with understanding the comparator logic. IMO he got the amp working but did an incomplete repair.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I figure that he must have fixed it and just didn't include it in the video. It would have been nice to see what was needed.
  13. WretchedExcess


    Jul 29, 2013
    Continuing the hijack:

    I'm not sure if he understood the circuit he's dealing with -- I hope I'm not offending anyone here -- but the comments about poor quality control rubbed me the wrong way. He commented twice that he had doubts about Ampeg's QC, when it was obvious that someone had already serviced the amp and had replaced the fusible link screen resistors. That's not an Ampeg QC problem, that's a case where a tube failed and took out the fusible link resistor -- that exactly what's supposed to happen. And then the amp was modified by someone who replaced the fusible link resistors with larger resistors that look like they have a higher dissipation rating. That defeats the purpose of having a fusible link, and removes a safety mode from the amp. Those resistors are supposed to burn up to save the amp. the last thing that you'd want to do is to replace them with a resistor that won't burn up.

    It would have been interesting to see him demonstrate the bias control adjustment after he hand-matched the tubes. With the SVT's idiot light circuit, the duration of rotation during green without red is an indication of the quality of the tube matching. Well matched tubes have a long green duration, poorly matched tubes have a short green duration, and unmatched tubes have zero green duration.
  14. Didn't offend me. They are supposed to be metal oxide resistors and someone installed metal film. Ampeg (whoever owned them at that time) would NOT have done that.
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The spec is 220Ω ½W 5% flameproof for the screen, and 5Ω 3W 5% on the plates. Techs don't always seem to be aware of the concept of a resistor acting as a fuse and put all kind of stuff in there.

    The guy in the video is always willing to discuss issues. His email is available via his web site which is on YouTube.

    BTW, although Loud recommends that you let the amp warm up for 20 minutes before adjusting the bias, I find that it sometimes takes longer for the settings to stabilize if the amp is just idling with the standby off. Once the bias is set, I like to check it again a half hour later.
  16. DRafalske


    Nov 6, 2008
    Hebron, KY
    Hey, I hate to hijack, but could you expand on this a little? I have a new-to-me SVT-CL I just picked up a few weeks ago. Made in Vietnam - unsure the year (I'd be interested to learn how to date it though). No issues with it at all, and it sounds great, but I didn't buy it from the original owner. I bought it used from a big box music store we're all familiar with so I'm completely unaware of it's service history. The guys at the store said it was used in a church. It seems to have been very well kept and it's very clean.
    Anyhow, the bias lights on mine are REALLY touchy. They go from off to green to red with only the slightest turn of the bias pots. The amp does seem to bias properly, but your comment make me think maybe the power tubes are getting a little worn.

  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Reminds me of the used car salesman who would claim that the car was used by a little old lady who only drove it to church on Sunday.

    Dino has a how to bias an SVT CL video.

    There is hysteresis or slop in the mechanical pot. When adjusting the bias you should always back it down by a small amount and then turn the pot clockwise up to the bias point. If you go too far back it down past the bias point and turn it up again.
  18. DRafalske


    Nov 6, 2008
    Hebron, KY
    :D Well, I never said I believed him. Wherever this thing came from, someone took pretty good care of it. Theres hardly even a scratch on the tolex .
    Thanks for the video. It gives me something to compare mine to, and I think all is well.


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