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Single Tube Amp (Distortion)

Discussion in 'Amps [BG]' started by Ron G, Jan 21, 2013.

  1. Ron G

    Ron G

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    I have an Ampeg BA-115 HPT with a single 12AU7 tube. I can't plug and play I have to let it warm up for about 5 minutes if I don't I get a lot of distortion and it will stay the entire time I'm playing. Ampeg replaced my first amp because it had the same problem, but I didn't think to let that one warm up.

    Is it normal to let tube amps warm up?
  2. Nashrakh

    Nashrakh

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    Power tubes need warmup. Preamp tubes don't. I find the problem you describe to be odd, not playing this amp but as a tube amp user this never happened to me.

    Edit: forgot to say, the tube you mention is a preamp tube. As such the amp would not need any warmup because it doesn't have power tubes. In my SS amps there's a bit of a delay between poweron and standby but again, distortion never happened to me there either.
  3. RickenBoogie

    RickenBoogie

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    The single 12AU7 in your BA-115HPT is only there for "flavor", and should not need more than a few seconds to be up and running. Also, it should not be causing ANY distortion. I suspect what you're hearing is your speaker. I also suspect you may be pushing the volume and the low band of eq, farther than that spkr can handle. What comes next, is a blown spkr. Your amp is capable of pretty decent power output, but the limitations on that will always be the single 15" spkr.
  4. Bassmec

    Bassmec

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    This is perfectly correct.:bassist:
  5. Ron G

    Ron G

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    This is my normal amp settings: volume 2 or 3, low;mid;hi 5, master volume 5.

    If I let my amp warm up I can play at all volumes.
  6. Bassmec

    Bassmec

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    You see we have no way of telling what the output of your bass guitar might be or how hard you are playing with it.
    Unless there is something wrong with the power supply to the tubes heater (normally at 6.3 volts) I can't see what could cause such an extended warm up time.
    I am afraid It's tech time unless you can get in there and measure some stuff for us.:bassist:
  7. Ron G

    Ron G

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    Thanks, Ampeg replaced my first amp with the same problem two repair shops could not fix it.
  8. B-string

    B-string Supporting Member

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    I would suspect the heater voltage to be more like 12-13 volts feed with heaters in series through a dropping resistor from the +15 VDC rail. The dropping resistor could be at fault, the B+ for the plate supply could be soft, the tube could be bad and many other possibilities.
    If 5 minutes are required for proper operation that gives a good amount of time for trouble shooting. The shop should be able to find the fault if they are any good at all.
  9. 5port

    5port

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    It couldnt hurt to do a line voltage check. Should be between 110 - 120 VAC. If its happening with two amps in a row I suspect something local. Then again the repair shops had the correct voltage.
  10. Mr. Foxen

    Mr. Foxen

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    Odd problem a local tech had in an amp, the heater fuse had gone resistive, so everything measured fine unloaded, but with all the valves going, there was a voltage drop that made it all go wrong.
  11. BogeyBass

    BogeyBass

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    or if you had this problem with ampeg before.

    then stop buying ampeg.

    sounds like a #### thing to say but it really can be that simple
  12. Ron G

    Ron G

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    The first one was under warranty.
  13. JimmyM

    JimmyM Supporting Member

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    Why couldn't it be a tube?
  14. B-string

    B-string Supporting Member

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    Not out of the realm of possibility.
  15. Ron G

    Ron G

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    I've replaced the tube in my first amp, the repair shop replaced the tube. It seems the only thing that works in letting it warm up. No more Ampeg's for me!
  16. 1958Bassman

    1958Bassman

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    Tubes are tubes and it doesn't matter if it's an output or not- the anode still has to be hot to start emitting electrons. Could be a bad tube, weak heater circuit, transformer winding, etc.
  17. B-string

    B-string Supporting Member

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    The anode (plate) does not need warming, the cathode does. Just to be clear. :)
  18. mjac28

    mjac28 50th Anniversary Ed Sullivan February 9, 1964 Gold Supporting Member

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    I agree with that one guy.
  19. gurensan

    gurensan

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    ALL tubes need to warm up. Not just the big ones.

    Ron, allowing time for the tube to heat up is par for the course. Instant power-on is something out of the SS world. Once the tube is ready the distortion should go away. If the heater's power supply is good it can still take 2-3 minutes for it to come up. This is normal.

    If the tube never gets warm enough, it'll distort like mad all day.

    I haven't taken the dive into that amp's schematic so I don't know for sure if it's got a regulated heater or not. if it doesn't, you could have soft power. If it does, you could have a bad regulator. There are a million things that can cause a tube to distort; a floppy heater supply is only one of them.

    Honestly I've never had an issue like this with any tube amp I've ever owned or built. Usually the distortion is on purpose!
  20. Ron G

    Ron G

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    Thanks

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