Ampeg SVT-CL troubleshooting

Discussion in 'Amps and Cabs [BG]' started by hotlavaman, Oct 14, 2020.


  1. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    Bought a barely used svt-cl in late 2018 and had no problems with it until earlier this year. First, once when I was playing the volume suddenly drops significantly and most of the lows cut out. The sound could best be described as weak and distant with some fuzz/hissing. No squealing or popping however, and turning the volume up or down did nothing. About three months later I plug in the amp, it turns on but no sound whatsoever this time. I've replaced the fuse, but still no sound, so it seems a blown fuse wasn't the issue. In the three month interim it sat untouched except for a 1000 mile road trip that was definitely bumpy at times, but the amp was very well secured and encased in about a foot of padding all-around. Prior to these issues it was working perfectly.

    Anybody out there have any ideas? I am no expert on amps so am basically at a loss at this point. I'm on a budget and live a good distance from anyone who could potentially repair it so I would prefer to do it myself if possible. Any input is much appreciated.
     
  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Look for simple, everyday problems first. The first issue that comes to my mind is the poweramp in jack on the back panel. That jack has a switch in it to disconnect the preamp when you are slaving it to another head. It's not uncommon for that switch to develop some corrosion (especially if the amp is unused for a while) and cause the exact symptoms you describe.

    Connect an ordinary guitar cable or pedalboard patch cable from preamp out to preamp in and see if that helps.

    Beyond that - if you're not an "expert on amps" it might be time to find yourself a good tech. The SVT has voltages inside that can definitely kill you quickly.
     
    JB1962 and bobyoung53 like this.
  3. Have you looked at the bias lights during any of these episodes ? If you don’t have the owners manual, google “SVT-CL manual”. There are basic power tube bias and troubleshooting instructions on pages 9 & 10.
     
  4. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    The green "bias 1" light turns on once I play my instrument and stays on, but does not turn on when I first turn on the amp. I will take a look at the manual...
     
    Last edited: Oct 14, 2020
  5. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    The cable should be connecting the two jacks which are right next to each other, correct? I did that and still no sound. The jacks are labeled preamp out / poweramp in though. Not preamp out and preamp in.
     
  6. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    LOL! I meant to type "poweramp in", why didn't my fingers do that? :rollno:

    Yes, look at the bias procedure in the manual. As it will tell you, both green bias lights should be on when the amp is fully on (not standby) and warmed up, and with no signal (no bass plugged in, master volume turned down).

    If the amp won't bias correctly, I'd say there's one more thing you can safely do. Shut the amp off, let it cool down, and remove the metal grill on the back. There are three small tubes in there that are all part of the power amp. One of them affects the bias, and all three are in the signal path. Carefully wiggle them out of their sockets and then wiggle them back in. Be careful to reinsert them without rotating them and without bending any pins. This makes sure they are properly seated, and will help clean any corrosion on the pins. If you have known-good spares of the same type, you can try swapping them in, one at a time. (You should own at least one known-good 12AX7 and 12AU7 as spares, just saying.)

    There are more tubes in the preamp, but you have to start disassembling the amp and exposing potentially hazardous voltages to get to them. There are also connectors that could have come loose during that bumpy road trip. But try the safe and easy stuff before disassembling anything beyond that back grill.
     
    hotlavaman likes this.
  7. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    Scratch that. If leave the amp in standby for 3-5 mins the bias 1 LED turns green immediately upon putting the amp in ON mode/taking it out of standby. I then turned the bias 2 control very slightly and the LED turned red. I continued turning until it stopped, and still red LED. When I play my bass the bias 1 LED flickers red.
     
  8. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Directions for using the bias lights are in the manual for the amp. Flashing red while playing is normal. Not being able to get a green light on the other side is not.

    The timing of color changes when first starting up can mean different things. If the lights don’t come on at all change the 12AU7 drivers.
     
  9. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Do you mean the bias 2 lights went from no green LED directly to red LED? Please read the manual... that would indicate a problem.
     
    Funk With Me likes this.
  10. Yes, first read the procedure (page 9) for setting the bias. Not so much for setting the bias, but for how to and how long to warm up the amp. Then look at the troubleshooting table on the next page. This is all done with the amp at idle, without playing.
     
  11. Wasnex

    Wasnex

    Dec 25, 2011
    You really need to read the manual. The green light should come on before the red light or you have bad tubes. Is the amp producing a hum that is sort of a dull roar.

    The way the bias lights work: The amp has 6 output tubes arranged into two circuits. The green LED is on an AND logic gate. As you adjust the bias, the green LED will come on only after all three tubes in the circuit meet their minimum current requirements.

    The red LED is an OR logic gate. It lights when any of the tubes exceed their maximum current.

    If the amp goes to red before green it means the tubes are mismatched. The manual describes what to do. If the red comes on, but you can't get green it means one of the tubes is totally dead. By maxing out the control you are cooking the crap out of the two twos that are still working. SVTs frequently blow a screen resistor when a tube fails, so the problem may require attention from a technician.

    If you are going to pull the 12AU7 tube or output tubes, I suggest you set the bias for minimum current first. This to protect the amp.

    What you could try is to set the bias for minimum current on both sides of the amp. Let the tubes cool than swap the output tubes from one side of the amp to the other. The output tubes are organized into two sets of triangles.
    images?q=tbn%3AANd9GcRugG8iVaenKxRylo14xe8Sl8gfN_ViYjoY2g&usqp=CAU.jpg

    Go through the bias procedure and see if the symptom follow the tubes. If it does, you can get away with a new trio of tubes. A technician could possibly just replace the bad tube if they have a tube tester and collection of old tubes. The tech would need to find a tube that is reasonably well matched to the two good tubes in the circuit.

    If the problem does not follow the tubes, the amp will need to be opened up and repaired.
     
    hotlavaman, bobyoung53 and rickdog like this.
  12. JeezyMcNuggles

    JeezyMcNuggles Supporting Member

    Feb 23, 2018
    Santa Maria, CA
    I suck, but nobody really notices
    Rectifier?

    Have you replaced the tubes?
     
  13. JeezyMcNuggles

    JeezyMcNuggles Supporting Member

    Feb 23, 2018
    Santa Maria, CA
    I suck, but nobody really notices
    Okay, so your second tube set is bad. Buy a tube kit for your amp, and have a tech tune it up for you.
     
  14. JeezyMcNuggles

    JeezyMcNuggles Supporting Member

    Feb 23, 2018
    Santa Maria, CA
    I suck, but nobody really notices
    Dino says 20 minutes.
     
  15. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    Since first posting last week I returned the bias 2 control to its original position and the amp has been off since. This morning I turned it on, let it sit in standby for 5 mins and upon taking it out of standby both lights were green this time, however the bias 2 light is significantly dimmer than the other one. I'm guessing this is probably just an issue with the bulb itself though?
    Anyway...
    When I play my bass both red lights flicker but the bias 2 light flickers at a MUCH faster rate to where it basically appears solid- whereas the other light flickers for just a second. That is with gain at 50%. As I increase the gain, the light flickers longer.
    I feel like Wasnex and Jeezy probably solved the problem, but i thought I'd give this update since for some reason both lights are green this time even though I haven't messed with anything in the past week.
     
  16. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    Haven't messed with or replaced any components since I've had the amp. I did just notice that one of the tubes isn't glowing dull red-orange like the rest are...so its dead right? The third tube from the left when looking at the rear.
     
    JeezyMcNuggles likes this.
  17. JeezyMcNuggles

    JeezyMcNuggles Supporting Member

    Feb 23, 2018
    Santa Maria, CA
    I suck, but nobody really notices
    Most likely. Just based off the bias led, it's telling you that something is wrong there. See, those amps are built to play, and stay playing. Those lights are made so you (or your roadies) can set, correct, and troubleshoot on the spot. Know what I mean?
     
    hotlavaman likes this.
  18. hotlavaman

    hotlavaman

    Dec 15, 2019
    Washington
    Oops...I meant third tube from the right.
    So pretty much the whole problem here is the dead tube? If any one tube isn't functioning then the signal is interrupted and there will be no sound, correct?
    Yeah I got you. Makes sense. So its kinda hard on the tubes when the amp is turned on and off rather frequently?
     
    JeezyMcNuggles likes this.
  19. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    After coming out of standby, the amp needs to be in playing mode for at least 20 minutes for it to warm up. THEN look at the LED’s. Playing through the amp helps it to warm up faster.

    The color of the LED’s means absolutely nothing when you are playing so don’t look at them then. The lamps indicate bias at idle, they can’t be used for anything else. Idle is instrument unplugged. I like to set all the tone controls in the middle, boosts off, volume down, just to be consistent.

    The intensity of the Green LED that is dimmer could be due to the setting of the bias pot, it could also be the lamp itself.
     
    hotlavaman likes this.
  20. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Nope, pretty much impossible based on symptoms.
     
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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