DSL40C

Discussion in 'The Workbench' started by Highroller54, Nov 10, 2020.

  1. Highroller54

    Highroller54 Member

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    Hello all,

    I having an issue with one of my DSL40C amps. I have a tech check for circuit board issues and there doesn't seem to be any, at least the best he can tell. I have been able to duplicate my issue twice and still the tech can't figure out the issue.

    So I purchased the amp used, when testing it I found a microphonic power tube. After getting the amp home, I purchased a new set of preamp and power tubes. after installing the tubes and setting the bias. The amp sounded very musical, harmonic and very playable. after about 3 to 5 hours of use, not all at the same time, I noticed as if someone flipped a suck-it switch and without any pops or noises the amp went to sounding extremely harsh and gritty.

    I checked all connections and pulled the amp apart to check the bias. The bias on one tube almost doubled in reading. I re-biased the amp but it still sounded awful. I lived with it for a few weeks and thought maybe I had gotten a bad tube, so I installed another brand new set of EL34's, again the amp sounded great, but then again after around 3 to 5 hours of use it went back to sounding like crap.

    I'm about to pull it apart again to check the bias. I think I have an idea of what the problem maybe, but I wanted your input. Thank You!
     
  2. Highroller54

    Highroller54 Member

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    Pulled the amp apart. Just set the bias at 37.5 & 37.5 yesterday, played a few hours yesterday until the amp started to sound shitty and now the bias reading are 39.5 & 35.6! Just as I thought, looks as if a bias issue, but what component is failing?
     
  3. Highroller54

    Highroller54 Member

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    This maybe normal, but I don't know. I reset the bias at 37.5 on each tube! Figured I would test the sound see if the gritty shittys went away, nope still there and notice as soon as a made a sound on my guitar the bias jumped up into the high 50's and when I stopped play it came back to normal! I think this is normal but not sure.
     
  4. Micky

    Micky Well-Known Member VIP Member

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    Tolerances for bias voltage are generally 5%, and the negative voltage will generally climb a bit and stabilize after playing for a while. What you are seeing seems normal to me.

    The bias is generally about balance. When the voltages are far apart, it will generally sound crappy and have a tendency to hum. This takes about 10 volts difference to start to be noticeable. This is irrespective of the level that has been set.

    I would look at other components, especially anything that would tend to drift as temps go up. If you have some cold spray you can probably identify the actual problem component. Start with the stuff that connects to the finals and work back from there. I hope you have and can follow the schematic.

    General disclaimer - If you are inexperienced or uncomfortable working around hi-voltage, it is best to take the amp to a tech. If you follow safety precautions and are not afraid to test things then by all means have at it.

    There are a lot of great people here willing to help, so don't be afraid to ask questions.
     
  5. J5684

    J5684 Active Member

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    I agree with Mickey.

    When biasing, you really should get the plate voltage before just saying 37.5ma is the correct setting, you should expect to see around 440vdc or better on the plates so as Mickey said if you are uncomfortable with high voltages go no further. 37.5 is probably not far off but you should always adjust bias at no more than 70% dissipation for your AB amp at idle, meaning no audio. It's normal for the bias to jump when you play something, that is it working (more or less). It will drift slightly as the amp warms up.

    As far as the sound issue, if your tech can't find it I'm pretty sure it'll be a hard troubleshoot over the internet. But I'd start by checking preamp, power amp voltages if all squares close to the schematics, move on to using a chopstick or similar piece of wood to poke around and maybe some freeze spray to check for component/connection issues. Then on to scoping the audio path to see what's going on we can't hear. Not saying anything against your tech but you may need to get another tech to look at it, sometimes another set of eyes can be the key. Hope you get it figured out.
     
  6. Highroller54

    Highroller54 Member

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    Well my original down and dirty quick plate voltage reading was 469v. After these issue I felt I better pull the tubes and check the actual voltage it's 500v so for starters it seems as if I have been biasing the amp on the hot side which might be at least one of the issues, we shall see. I'm letting the amp warm up and will now bias it at 35 instead of 37.5. so we will see.. I update after a test run, getting late here not she if the wife will let me fire it up this late so it may7be tomorrow before testing. Just hope I haven't torched a new tube already!
     
  7. Highroller54

    Highroller54 Member

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    Yes 500v plate seems a little high for a DSL40C, but here in the boonies of WV, I get 124.9 volts on a regular basis, might have to invest in a Brownbox.
     
  8. Highroller54

    Highroller54 Member

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    35 +/- .2 on one tube and the other 35.1 +/- .2 the best I can do trims are too sensitive, to get any closer, which doesn't really matter at this point.
     
  9. Highroller54

    Highroller54 Member

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    Well after properly setting the bias, and a couple days of testing, I've found the amp sounds better but not right on the gain channel. I change over to the clean channel, cranked the gain and was surprised how good it sounded. I plugged my multi effects into the front of the amp (usually use the loop) and now seem to have a really good sound, with none of the issues, I was having on the gain channel. So my question is, what component or tube is bypassed on the clean channel? Thanks
     
  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The voltage is always higher with the tubes out. You should be checking voltage with the tubes in.
     
  11. J5684

    J5684 Active Member

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    Hard to say about the multi effects sounding better. Not sure about your specific amp but the overall design is to be clean and generally speaking when you hit the OD channel it sends the clean signal through 1 or more gain stages which is generally another pre-amp tube side.
     

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