Ampeg SVT-6 Pro Bias Help

Discussion in 'Amps and Cabs [BG]' started by KoolCharisma, Nov 18, 2016.


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  1. Just got my first tube (hybrid) amp, a brand new, used Ampeg SVT-6 Pro.

    It's obviously seen quite a few gigs, but for $350, I couldn't go wrong. As far as I know, it's working properly. I've got it opened up and I've checked all of the tubes for continuity. My last concern is the bias. I'm just wondering if I should attempt to check or adjust it. I've looked for more information online but can't seem to find much. It only has one bias adjustment and it doesn't have a bias tester spot like a lot of tube amps have. if I could take it to somebody and have it fully cleaned up and checked out I would, But I don't have the money. Any help would be greatly appreciated.
     
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    IMO, leave it alone. If you feel compelled to check it, find a qualified professional that doesn't have a reputation for turning otherwise functional amps into non-working junk.
     
    BasturdBlaster likes this.
  3. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    If it ain't broke, don't fix it.
     
    Aqualung60 and BasturdBlaster like this.
  4. I am getting a low hum out of the speaker which I wasn't getting before from my solid state amp. How much hum is to be expected from a tube amplifier? Is it possible that the filter capacitors need to be changed? It's not a hum that's so loud it irritates me, but I'm worried a sound engineer might not feel the same way.
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Very unlikely that it's the caps, this is probably a good time to consider finding a real tech who can properly troubleshoot and diagnose the problem. Only then can it be properly repaired.
     
  6. How
    But what do these guys charge? I mean I might be able to scrape together a diagnostic fee, but I'd have to do the work myself. Do repairman even offer a diagnostic service only?
     
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Every shop is different, but my policy has always been not to diagnostic work without completing the repair (with warranty). I tried the diagnostics only option and I regretted it every time. Worst was when somebody without experience tried to repair then claimed the diagnostics were faulty. They were just fine until the guy powered the amp up without recognizing they hadn't completed the repair and blew what was left to kingdom come. The SOB took me to small claims court and it ended up costing me a replacement amp because there was no way I could disprove the guys claim as he had destroyed the amp. That was the end of Mr. Nice Guy diagnostics. No good deed goes unpunished in the service industry. In general, DIY repairs are not just unsuccessful, but very unsuccessful. What makes somebody think they can fix a complicated piece of electronics without the equipment, education, experience and knowledge?

    The other problem is that I have seen an awful lot of really terrible techs in my time, so the other side of the coin is that the diagnostics may in fact be really bad if their troubleshooting skills lead them down the "impossible faults" road.

    The worst thing that has happened to this industry is the loss of really skilled techs, and they keep on leaving while many of the new "techs" (and I use this term loosely) can't do anything but guess and swap PCBs.
     
  8. I hear you man. I can't imagine the frustration of being forced to pay for damages for something completely outside of your control when you were simply trying to help people out. I started with zero knowledge and a need to fix some banged up basses. Soon I had a sweet solder kit and a pretty good proficiency for making connections. My new endeavor is using a multi-meter and learning to read schematics to work on two amps. One's a 1001RB that suddenly stopped working. The second is the above mentioned ampeg. I don't want to fry the Ampeg, but I want it to work right.
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Then I suggest that you get studying and learn how things work and why. You need to understand what to measure and what the measurements mean. An oscilloscope is another essential tool of the trade, so you can see what's happening rather than working in the dark.
     
  10. It's it possible though to bias by reading the Ma levels under the right input conditions? I understand you can't SEE what's happening, but at least you'll know what's happening assuming the input conditions are exact. I just can't find the specs I need to set up those conditions anywhere, or instructions on where to put the positive test lead. Obviously, Ground goes chassis.
     
  11. bb5000

    bb5000

    May 30, 2006
    Sundsvall, Sweden
    Setting the bias on a MosFET power amp is rarely needed unless you replace the transistors..as for the preamp and driver tubes, they are most likely cathode biased and need no adjustment. This is also true in all-tube amps, the bias is adjusted for the power tubes only in fixed bias amps..
     
    KoolCharisma likes this.
  12. BassmanPaul

    BassmanPaul Inactive

    Unless you know exactly what you are doing, leave the amp alone. A fractional move of the bias pot can turn the output devices hard on and into destruction in a split second.
     
  13. So I did actually adjust the lower bias pot. I went too far, admittedly, and I got a nice puff of smoke from the heatsink. After opening up the heatsink and taking the upper circuit board off, I realized there was an additional blue pot, that looks identical to the first bias pot. So now I'm confused as to which pot should have been adjusted. I'll be able to test and repair the circuit board. It doesn't actually look like anything got significantly cooked. I'll simply have to test each component.

    I'm thinking now that I should have adjusted the upper blue pot to change the bias. So when I adjusted the lower pot, it increased the wrong circuit flow causing the overheat. Is that correct?
     
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    No, "ground" or minus on your meter does not go to the chassis for bias measurements. There are different ways to measure or extrapolate current depending on the design of the amp.

    Input conditions are "always" no signal, and output conditions are "always" no load.

    There are many ways, or combinations of ways to measure bias, which is why I suggested that you learn more about how an amplifier actually works first.
     
  15. Thank you, that does help. I discovered the no load, no signal condition. I then found out that the bias needs to be adjusted based on the readings registering across the resistors coming directly from mosfets. I read those Mosfets and the average mV reading was around 21mv. They run optimally however between 25mV and 30mV. The majority of forum suggestions say 25, but I also read that factory is 30mV. I'll bump it to 25mV and leave it there as long as everything seems to be working correctly.

    Obviously, from here I'll have to diagnose what burned up. I've already looked at most of it without finding anything so I'm confident it won't be a big fix. Then I'll recheck the mV readings and adjust the upper bias control near the resistors. This is the other control that I failed to adjust last time. I'll go slow and maybe even a bit down first just to make sure I'm making the right adjustment.

    Now for the lower bias control that created burnup in the first place, I'm now confused as to what to check to get this reading.

    Any ideas?

    I understand that "learning the amp" prior to adjusting anything is optimal. I find however that tweaking, checking, and adjusting things is how I learn. I can read a manual over and over again but it won't make sense unless I "see" or "do it". I love youtube for this, but bias adjustment is very different for different amps so I can't find a silver bullet method of checking and adjusting.
     
    Last edited: Nov 21, 2016
  16. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    If I saw an average of 21mV on a bias check, I know enough to leave it alone... that's PLENTY close enough. Bias is a range of current that relates directly to conduction angle. It only needs to be enough to be greater than zero under all applicable conditions and is never an exact value since it changes with both temperature as well as current.

    The reason I suggest understanding how the amp works and why specific design features are used is so that you can recognize BEFORE you get into trouble. Seeing 2 (or more) controls tells me that one is not bias, and that nothing gets adjusted until you understand exactly what each control does. Apparently, the control you adjusted is the one that governs the magic smoke generator... ;)

    Do you have any AC power source current limiting, specifically a variac and a current meter (applicable for this particular amp, but not all)? It is likely that you damaged a fair number of components already, and unless you completely repair the amp, you are destined to continue to destroy more and more parts unless you take a step back and learn more about troubleshooting and repair.
     
  17. 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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