Svt-2Pro Power Tubes 5jj 1 Sovtek

Discussion in 'Amps and Cabs [BG]' started by matzingerz, Jul 29, 2019.

  1. matzingerz


    Dec 26, 2018
    I got myself a Svt 2 pro about 8 months ago. Through the back panel you can only see the JJ Brands on the outer 3 tubes. But I always thought that one in the back, when its quite dark in our rehearsal room, I can only see 5 glowing and one is kind of glowing and looks like its plugged in, in an different angle or something so I cannot see it right.

    Now I took a really close look with a flashlight and i could see that it says SOVTEK on the one that was seemingly less glowing.
    So my Amp i working just fine I think, but i never got it checked.
    If you read my other Posts, we always play really loud, so the Amp isn't fragile or something. I don't know if the previous owner just replaced a bad Tube or if this is a thing one can do soundwise for later breakup or something.
    Still cannot see if its glowing less or not.
    Also the green LED from the bias 2 never glowed, even though i could adjust it with just the red one and the noise coming from inside. I guessed the LED might be broken because it also was not in its right place just like the red one. it kind of popped in and moved a bit up so in can see it through the holes if i look from beneath.

    Can you tell me if this is really bad for my Amp or shouldnt I worry as long as everything sounds fine?

    thanks for reading
  2. If you cannot bias the bias 2 light into green then you have an issue. I would recommend taking it to a tech to be checked out. It could be either worn out tubes, or a bad tube on that side, or it could be a blown resistor or some other failed part. First thing I would do is open it up and make sure that the one that looks wrong is plugged in correctly. If you get it in correctly and you can get the bias green then you are good, otherwise take it to a tech. Both bias LED's need to be lit up green at idle. They will flash while playing, that is normal.
  3. As OneWayPunk mentioned, if the bias light is not turning green it is probably time to see a tech. The LED is mounted to the board in a way that when putting the board back in the chassis, it could be moved. But no green light probably means there is an issue.

    It would not hurt to pop the top off the unit and try re-seating the power tubes one at a time. You can pull the retainer off the top, pull the tube out and then place it back in the same socket. After you do this will all power tubes, try turning it on (after connected correctly to a cab) and see if you see all tubes glow now. If not, look at the (1) 12AX7 and (2) 12AU7's closest to the power tubes. Are all three of these tubes lit when on? These can be pulled and re-seated as well.

    The power tubes run (3) tubes per bank, six total. So each bias pot is biasing the (3) tubes for that side. It is best to get the (3) tubes matched per side if you can. They can technically can be any brand as long as they are 6550's. Actually, all of the power tubes can be different brands and the amp could work fine (and bias fine). But it is always best to get (2) matched sets of three power tubes for a total of six..

    Try re-seating the power tubes if you feel comfortable doing so. And then see if you can get a green bias light and if all tubes are showing a glow.
  4. matzingerz


    Dec 26, 2018
    thanks for your fast answers!

    i contacted the guy i boought it off and he said his tech only had this one in stock but its matched with the other 2 jjs. is there an easy way to check if the LED just isnt working? like just get a 9V Battery and connect it with a 220kOhm resistor and with the LED?
    Like I said the light didnt work when i first got it, but the guy had his tech check it before he sold it.
    At least that is what he says lol.
  5. Unfortunately, Ampeg removed the external bias test points on the 2 Pro. So the only way I know to test is to open up the amp. I would not suggest this unless you are familiar with working around high internal voltages.
  6. matzingerz


    Dec 26, 2018
    Nah I just want to check if the LED lights up when the amp has no connection to whatsoever
  7. ThisBass


    Aug 29, 2012
    you could check (quite easily) the LED for proper working (or fault) with a quality multimeter that provides diode (LED) level for voltage and current on the Voltage terminal.
    Not sure if you have got such an expensive multimeter by hand.

    In the 70's it was nothing unusual to mix different brands to get a matching set of power tubes.
    Tubes can glow way "differently" depending on perspective you view them, view from front end vers back side and left vers right side may make a noticable difference how a tube glows.

    Anyway if I was in your shoes I'd bring the amp to an experienced service tech.
    Last edited: Jul 29, 2019
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  8. Wasnex


    Dec 25, 2011
    The bias can be checked from internal testpoints using a multimeter. Basically the cathode of each output tube is connected to ground through a 10 ohm resistor. When the bias is set properly, the current passing through the cathode resistors causes a voltage drop of 0.24V. You can measure the voltage drop manually.

    The bias LEDs are driven by a set of circuits called window comparators. Basically the window comparators monitor the cathode sense resistors and looks for a range of voltages. When a defined condition is sensed, a LED indicator lights.

    The output section is arranged into two sets of three tubes. When all three tubes in one of the circuits are conducting in the proper range, you get a green LED. When the current is too high, you get a red and a green LED. If no LED is lit and the bias cannot be adjusted to correct the problem, either something is wrong with the amp, or you have a bad tube.

    To summarize: The bias sense circuit has three defined conditions low current (no LED), proper current (green LED), high current (red and green LED)...actually there are a few more codes so see page 7. of the attached manual.

    What you could do is turn the bias on both sets of tubes for minimum current. Next number the tubes with a sharpy and swap the first three tubes with the second three. Now turn on the amp and try to bias the tubes. If same LED does not work, the bias sense circuit is malfunctioning. Take the amp to a tech to have it repaired.

    If the problem follows the tubes, then you have a bad tube. Buy a new matched trio of tubes and reset the bias, and you should be good to go.

    Attached Files:

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  9. Wasnex


    Dec 25, 2011
    If only one of the bias sense circuits is functioning, it should still be possible to adjust the amp. The reason is because the amp will hum if the two sets of output tubes are not pulling approximately the same amount of current.

    If you bias the tubes with the working bias sense circuit into the green and the other set is biased for minimum current, the amp will produce a very noticeable dull hum. As you increase the current in the other set of tubes, the hum will decrease. Set the bias for the minimum hum and you should be close. For this to work you need to two good trios of tubes, and the amp must be properly functioning aside from the bias sense problem.

    Ideally each tube is supposed to pull 24mA of current. Each trio of tubes pulls 72mA total (3x24=72). But if one tube is not functioning, you will have to bias the remaining two tubes in the circuit to 36mA (2x36=72) to minimize the hum.

    Obviously it is not a desirable to run the amp with only 5 functioning tubes, so perhaps it is a good idea to take the amp to a technician for a checkup if one of the bias sense circuits is not indicating properly.
    Last edited: Jul 29, 2019
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  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    You may not be able to see the heater glowing on some tubes, depending on their construction.

    I suggest not messing with the amp unless you know what you are doing.
  11. matzingerz


    Dec 26, 2018
    Thank you for your responses!

    this is what i have done and it worked just like that.

    i adjusted bias one just like in the manual, also could hear the hum come and go while turning left and right.
    bias 2 (with the probably malfunctioning LED) was behaving the exact same humwise but the green LED never lit up. i will test it one more time tomorrow, when i see my amp. but I will definetly bring the amp to a tech sonner or later. Probably when I'm on holiday next month.

    this is what i will try. Gonna read some more about not electrocuting myself before that.

    I'm not igrnorat to your advices to go to an amp immediatly, but you see, I played the amp for 8 moths now 2 -3 times a week with no problems at all and also really loud, like wearing earplugs and its still loud.
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