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Marshall TSL122

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  • Marshall TSL122

    Hi again hope your all doing fine.

    I hope this to be a short post and a quick repair (not on your nelly) but i found it interesting so thought id ask you for your thoughts!

    I got the beast today, opened it up and found a Sovtek 6L6W in location V4 along side 3 JJ EL34s!!. So just to put that right, El34 X3 & 6l6w x1.

    I fliped the amp over and resistor's r6 and r9 are blown on the main board. No other damage just them two resistors, r9 is blown into two parts and lots of carbon on the board, r6 not as bad but is blown.

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    Q1. Would the incorect valve have caused the resistors to pop? it was the first thing to enter my head. Im not too clued up on valves so im hoping one of you guys can say ""hey yeah thats a stupid thing to put that valve in there and thats your problem pal"".

    Q2.Will i need to bias the amp after fitting the resitors and another valve and what values please.

    Q3. What do you recomend i do about valves? I have an el34 knocking around that i can pop in for testing along with the three that came with the amp, will that be ok or shoud i buy new ones first.

    I'v had a good read around about the boards being s**t on these amps and it looks like i do have the early board. But i wanted to ask your opinions on this one first.

    Thanks

    I think i have the new style board, dark green and has stoc number in right place
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    Last edited by MARSHALL BUSTER; 10-20-2011, 10:28 PM.

  • #2
    Those are the 1 ohm resistors in the cathode lead of the power tubes. It isn't the "wrong" tube that burnt them out, it was failed tubes. SOme tube shorted internally, causing massive current to shunt through the tub to ground. Those resistors ARE the path to ground, and so burnt up. I wouldn;t be surprised if the 6L6 was thrown in there to replace a tube that failed utterly earlier.

    If you don;t have four EL34s, use two, one on each end. Then when adjusting bias, set the voltage at the test point to half what it is supposed to be. HAlf as many tubes - half as much current - half as much voltage then.

    But before that, go through the amp. Pull ALL the power tubes. Each power tube has a fuse for its heater on the board, check for any blown fuses there. Ther is a 1k ohm 5w ceramic resistor for each power tube screen. They are raised off the board on long legs. Check each for open condition.

    After you replaced those two 1 ohm resistors, check from pin 8 of each power tube socket for continuity to ground - well that 1 ohm really.

    Once all that is OK, power up the amp, still no power tubes. Verify at EVERY power tube socket that ther is B+ voltage at BOTH pins 3 and 4 or each socket. I forget what B+ runs on these, something around 480-500vDC. Then also check pin 5 of each socket. There must be good bias voltage, something in the order of -40 volts or so. If that is OK, try it with some tubes.

    ALWAYS connect a speaker or load to a tube amp when it is running, exception being with all the powr tube removed.

    Let us not just blanket Marshall amps with the "boards suck." If you HAVE a biasing problem, the board may be the cause, but that is not the same as the boards are all bad. Most of them will not have the problem. It is not as rare as we would like, but it is not in every amp either.

    The bias adjustment is cconvenient on the rear of the unit, simple to adjust with a small screwdriver anad a voltmeter, so YES, rebias the thing when you change tubes or parts.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Thanks Enzo.
      Picking up parts in morning, i will let you's know the outcome.

      Comment


      • #4
        Thanks agin Enzo you were spot on. Thankyou so much

        I followed the instructions given and the amp works great again.

        I had some truble with the valves tho! i could only bias one side and the other was kinda all over the place and rising steady up to over 100mv. so i switched the valves around a few times and i located the wandering valve and changed it for one iv had here for a while. I still think needs a new set of valves to put it right. It was a long job to finaly bias the amp but im putting that down to running 'three' diferent makes of valves hahaha, 'O' and one is the 6l6 that came with the amp. The bias is still wandering abit between 89.5mv-91.7. Anyhow she works and thats all good.

        I hav noticed a blue glow in two valves, what does that mean?? Its like a blue neon or plasma, are they going bad.
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        Comment


        • #5
          Blue is good.
          Google it.
          Red is bad.

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