DSL 401 - no sound, but all looks good

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Motterpaul, Feb 10, 2012.

  1. Motterpaul

    Motterpaul Tone is in the Ears

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    I have a DSL 401 that was working fine. I wanted to install some different power tubes so I put them in, turned it on and heard two short static bursts, a sksksk - sksksk. Then no sound at all.

    Now I have nothing, no sound. The speaker is good, the F4 fuse is good, all tubes light up (though one power tube seems a little dim).

    I have had the bridge rectifier replaced and a fan installed. All the pre tubes were good when I made this change.

    It is just kind of like I can't get it out of standby mode, but the switch is connected just fine. I also get nothing out of the effects send or D.I.
     
  2. phsyconoodler

    phsyconoodler Member

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    Pull the power tubes back out and see if you bent a pin on one or more of them.
     
  3. Motterpaul

    Motterpaul Tone is in the Ears

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    Thanks - tried that and they all look good. In fact I even checked the circuit board and I see nothing wrong.

    Just because all tubes light up - does it mean one could still be bad (I am thinking preamp tube since I am getting no output anywhere?)
     
  4. tubetone74

    tubetone74 Supporting Member

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    yes, the light up are the heaters but tube could still be defective.
     
  5. Motterpaul

    Motterpaul Tone is in the Ears

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    Let me describe the sequence and ask you this...

    I had a fully working amp. I took out the four EL84 power tubes and put in some NOS tubes.

    I powered it up - and that was when I got the static and the amp died. All the tubes appeared to be lit (one was dim) but no sound at all.

    So - I put back in the working tubes - but the amp is still not sounding.

    Could one of the NOS tubes have had a problem causing something to burn out in my amp? (and by the way, I did look carefully at the NOS tubes when I took them out, no bent pins or anything).
     
  6. Prattacaster

    Prattacaster Member

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    Check inside the amp for open fuses. There are fuses on just about every secondary, if the HT fuse(F1) doesn't read very low ohms replace it.
     
  7. Markorock37

    Markorock37 Member

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    Did you replace the bridge rectifier before or after the tubes were changed? The DSL 401 is notorious for burning out rectifiers. I always leave them off the board a ways when I replace them and put a heat sink on them in this amp. Could also be the screen resistors gone bad.
     
  8. Motterpaul

    Motterpaul Tone is in the Ears

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    Yes, the Bridge rectifier is an inch off the board and I have a fan. This burnout happened right when I turned the amp on with a new set of EL84s. I will look up the screen resistors and if they are bad.

    And I'll try the fuse test suggested above.
     
  9. Motterpaul

    Motterpaul Tone is in the Ears

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    Found the answer - the screen resistors were all good - so I checked the output fuse with a voltmeter. Sure enough, it looked fine, but it was not conducting. I replaced the fuse and all is well.

    Thanks for the advice to check it with a voltmeter. I never would have thought of that.

    Now, I am afraid to try the NOS tubes again, afraid they might burn out the fuse again. Is there a way to test EL84 tubes (I don't have a tube tester).
     
  10. phsyconoodler

    phsyconoodler Member

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    They probably just draw more current in that amp.that would blow the fuse.
    They may be just fine with a bias adjustment.
     
  11. Prattacaster

    Prattacaster Member

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    Glad you found it, What is the corresponding schematic number for that fuse?

    Why don't you order a couple 3 fuses just in case in blows again?

    Another thing, you might have done it already but it is wise to change R36 to 33k. This resistor is part of the bias circuit. Raising the value gives you more negative voltage allowance so that the bias trimmer pot has a more ideal range. More negative voltage equals less current which equals lower bias. When biasing the 401 connect positive probe to con 4 - pin 1 or 3 and then your negative probe goes to con4 -pin 2. The reading will be in MV or it is still at the factory setting could be over 1V. You want the meter to read no higher than 1V. To calculate the mV for one tube divide the number on your meter by 40. So 850mV would be 21.25mV which is also 21.25mA per tube. 21.25mA x 330vdc = 7.2w or 60%. So any meter reading between 850mV and 1V would be fine.
     
  12. Motterpaul

    Motterpaul Tone is in the Ears

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    I'n not sure. I think it is just F1 or F2, there is an F4 inside the chassis.

    Yup, the tubes I was trying to use were Bugle Boys. But they were EL84s so the suggested bias setting should be the same, right? I am not that good at biasing Too much math), but the amp was just set up so the bias was just done for the tubes that were in it.
     

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