Tube amp problem... need help.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bobyjoes, Apr 17, 2015.

  1. bobyjoes

    bobyjoes Member

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    So , I have a lifco 1000 tube amp ( yba-1 clone) that is having a problem. The problem is when raise the 2 inputs volumes at the same time , and then turn the bass or low pots up, it makes a big electric sound like if something was shorting out, it even makes the power tubes glow more as the sound grows bigger... Its not doing it with all the tone stack turned down to zero, when i raise the treble pot no problem, only with the bass and low pots. How can this happens it looks like filament current was going up when turning those pots. I tried changing all pots with no success.... Could this be a faulty cap in the tone stack ? Thank you for helping me .
     
  2. xtian

    xtian Member

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    So this problem only happens with BOTH volume controls above zero, AND bass pot up?

    Does it work OK with Bass pot up, but just one channel volume up at a time?

    If true, the only thing I can think of is a leaky coupling cap. If leaky it can let DC through to the grid of the next stage, upsetting the bias...hmmm....my explanation is going nowhere because that still wouldn't affect the power tubes.

    However, if something was oscillating above 20KHz, you wouldn't hear it, but it would draw current through the power tubes.

    Anyway, sounds like time to take it to a tech.
     
  3. bobyjoes

    bobyjoes Member

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    Yes only with both volumes controls above zero, it does it with the bass bot and the low pot, it works ok with 1 pot a a time.... I've ordered all electolytic caps to replace them except the coupling caps.... Maybe i should replace those too... Also ordered new sockets for the preamp and power tubes cause the old ones dont look in good shape.... I wonder if a ground loop could do this... The sond grows up as i raise the pots.... so its like if i was letting more current passing trough the pots as it dials up... pretty strange, by the way i'm trying to rebuild it by myself, so dont want to take it to a tech for now... :)
     
  4. bobyjoes

    bobyjoes Member

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    Also would'nt the treble pot makes it too if it was the coupling caps ?
     
  5. xtian

    xtian Member

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    Seems like you're replacing components without testing. Do you have a multimeter? Check for DC voltage on the tone pots.
     
  6. zenas

    zenas Member

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    From what I can tell your amp is from the '60s or maybe the '70s so changing all the electrolytics is a no brainer.

    After that I'm thinking you might have more touble shooting ahead of you. Does this use those "musturd" coupling caps like the old Tranynors and Marshalls? If so you don't want to change them unless they're bad. If you look at gut shots of vintage Marshalls you'll see they don't go bad often.

    Unless the tube sockets are broken or have carbon tracks they can look pretty bad and still be good. Main thing is the metal part that actually make contact with the tube pins and the wires. New tube sockets aren't as good as the old ones for the most part.


    The trick to working on an old amp is replacing what needs to be replaced. Electrolytics are something that need to go and you've got those coming.
    The next trick is if those don't fix it nailing down the problem without rebuilding the whole amp in the processes and that's where it gets real tricky.

    A little light reading might help. http://www.trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf

    If you could post some gut shots a picture is worth a thousand words as they say.
     
  7. bobyjoes

    bobyjoes Member

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    Yes I have a multimeter, the reason i want to change the sockets is that tubes dont hold well in it, i agree with the fact if it isnt broke dont fix it. I'll check for dc voltage on the pots. And yes coupling caps are mustard caps. There were a couple of mods done to the amp like 3 prong power cord, i personnaly removed the ground switch with the death cap and added a bias pot. Just to be clear, the problem was there before i made any mods to the amp. For the pictures, i cannot post attachements here but you can go to http://music-electronics-forum.com/t39222/#post38147 i posted pics of the amp there. Thax for your time.
     
  8. zenas

    zenas Member

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    Try retensioning the sockets you just stick something pointy in and bend the metel in a little so it holds the pin tighter.
    They can seem loose but as long as the contacts are tight to the tube pins your ok.

    Before you do that make sure the voltage is drained !

    Looks like alot of things have been done in the amp. If it was an old Fender no big deal easy to find a schematic and a layout for those then you just go through and compare.
    Anything else gets complicated because your working from a schematic and sometimes companys printed one schematic but changed the amp every 15 minutes for the years they made them. Sometimes you just can't find a schematic at all.

    I'd wait till you get the electrolytics replaced then bring it up with a light bulb limiter. (google it if needed) The way the wiring looks reading up on lead dress might help while your waiting for parts.
     
  9. bobyjoes

    bobyjoes Member

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    So here's an update, i think I found part of the problem ,on the 3 prong power cord, the previous owner installed the ground wire is soldered to pin 8 of the second power tube so there is a short somewhere or dc leaking trough a cap and when i turn bass pot up, the dc flows trough the tube by the ground wire acting as a bulb limiter. I also made a limiter, and as soon as switch the standby switch on, i can see the filament lightning a little, not enough to light the bulb(300w) but enough to make it red.... so something is really shorted out in there. For the sockets, I already ordered them at the same time as my caps, i ordered from antique electronic supply (tubesandmore.com) I took the micalex ones , i wonder since i alredy ordered them if they really are of lower quality than the old ones, and while changing them i would also change the leads allowing me to maybe make better lead dress in the amp .... What dou you think ?
     
  10. zenas

    zenas Member

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    While pin 8 isn't where I'd put the ground wire it is hooked to ground. In other words not a problem.

    If you had a short the light bulb would be full bright so you don't have a short.

    Have you put all the electrolytics in yet ?
     
  11. bobyjoes

    bobyjoes Member

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    No still waiting them should be here by monday.
     
  12. zenas

    zenas Member

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    Wait till they get here and put them in. You've got two in the bias supply alone that can cause all sorts of problems.

    Had a tech friend call me the other day on a SF Princton Reverb he had in. I forget what was wrong with noisy or something but I asked about electroyltics. "All new exept the bias cap". So of course I said replace it he did and that fixed it.
    So I lost a few bucks ! But I was out of town anyway.
     
  13. donnyjaguar

    donnyjaguar Member

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    Line cord ground wire should be bolted to chassis. If you've ever seen what a high-current short does to a solder joint you'll understand why.
     

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