Just got my tube amp working (AQ-1002)

Discussion in 'Tube Audio' started by Ken Boyd, Jan 13, 2018.

  1. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    First hello to all in the tube section! My tube amp stopped working on one channel a number of months ago and being it was in a cabinet buried I didn't get to it right away. Well last night I dug it out and checked it out. Seems that one of the small 12aux tubes must have failed. It was an old Brimar set that I had purchased years back when I was doing a bit of tube rolling. So with a new pair of small tubes in place everything is working well. I did notice one of the power tubes plates getting a bit hot as it started to glow red slightly, so I shut it down got my meter out and set all the bias to .4mv DC which I hope is ok, the red plate cleared up.

    I was wondering about the chrome looking stuff on the tops of the main power tubes. I think they call that the getter flash, and I notice some tubes don't have as much on them as the others. Is this OK? I purchased this amp used have no idea the number of hours on the power tubes. I know I use the crap out of, everyday for years, at least a couple of hours a day for the most part some days more than that. They all seemed to bias ok, but how will I know when or if I need to replace them? It is a rather small amp at 50 watts a channel, and I use the 4 ohm output as it is driving a pair of Infinity Quantum 2 speakers. I don't over drive it rarely have I put the volume past 12 oclock. Is the lack of a log of getter flash an indicator that they are near the end of their lives. Right now it sounds great. This is the first time that I have adjusted the bias, but I have never seen the plates get red before.

    Thanks for any advice given. ;)
     

     

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  2. Duke58

    Duke58 Well-Known Member

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    It depends what day you ask and who is on the forum. Some days, people will advise you to replace your tubes, based on what they have heard or read here or somewhere on the inter webs.

    I have four 7591 tubes that I replaced recently. The tubes are in a Fisher KX-200 that I use Monday-Friday, 9:00 AM to 5:00 PM for the past four years. That's over 2000 hours. The tubes are rated for over 2000 hours and one pair on the right channel made the right channel sound lower in volume. I tried adjusting the bias and balance and couldn't adjust either correctly. So, I bought another quad of tubes. The two tubes in each channel are matched, but not the four tubes. Adjusted bias and balance for each channel with the new tubes. I also notice that the weak tubes had less gettering. I can also tell you that the amp sounds fine after the tube replacement.

    The amp doesn't blow up and catch on fire once a tube wears out. On my amp, the sound got weaker and I did notice less getter on the 7591 tubes on the channel that was sounding weaker and I couldn't adjust the bias and balance. Some people may say to replace the tubes and I say if you've got the money and the time then do it. Others may tell you to wait for tube rapture, that the signs of the end times for the tube are there.

    That's about all there is to shrimpin' and tubes for me. All I can tell you is what I experienced.
     
  3. crispycircuit

    crispycircuit AK Subscriber Subscriber

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    First check you have the correct fuse in place, in case the tube shorts out. A pic or amp model number would help us figure out the unit you have. Also how you measure the bias. When the getter is see thru or missing it's means the tube is close to dying. No way of knowing if it's gonna happen nicely or ugly. The fuse is the only protection.
     
  4. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    The Amp is an Antique Sound Lab AQ-1002, I put that on the thread title.
     
  5. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    Thanks for the info, very helpful.

    I also have a Fisher KX200 it was my first tube amp and my first restore job. I have it paired up with my Empire 598 TT and a pair of Altec Iconic Flemenco's. They all make up my most vintage setup. I have a X202 that I purchased to rehab, I just have not gotten to it yet, and a Fisher 400.

    I have another one of these Antique Sound Labs, the same model that needs some work. I just powered it up, but only got sound out of one side. I pulled the cover and I can see where someone has been inside replacing some resistors. Looks like a few fried at one time, and the repairs don't really look like a professional did it. I pulled out the main board and will try and source some new correct resistors and see if I can't clean up the board some and get it working.
     
  6. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

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    What are the power tubes for your amp? The need for frequent rebiasing is also an indication that tubes are wearing out. Remember tubes are like tires and wear out. How often that happens depends on how hard you run them. Again with the car analogy; racing motors will output many times the horsepower of a regular automobile engine. But their lifespan can be only a few minutes vs many years for your standard stock motor. Conservative biasing will make power tubes last longer but may not sound as good as hotter biasing. You can choose longer tube life or better tube sound by adjusting the bias current.
     

     

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  7. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    The amp has four 6550 tubes. I set the bias at .4vdc. My first post I mistakenly posted .4 mvdc.
     
    Last edited: Jan 14, 2018
  8. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

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    I don't know what the manufacturer recommends for that amp but .4vdc seems low. If the voltage sensing resistor is 10 ohm and both tubes are dissipating 40 ma of current at idle, that is low. For most push pull amps it would be at least 60 ma per tube. The GE tube recommendation for the 6550a in UL class a/b is 75ma per tube at 450 Av.
     
  9. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    Well I am going to order a quad of KT88's and start with some tubes that I know how many hours are on them. I have no way here locally to check these tubes.

    Somewhere hear I have the owners manual which would be helpful.
     
  10. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

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    KB, no matter what brand of tubes you put in your amp you need to know what the bias voltage should be. 40ma per tube is not the correct bias current for 6550 type tubes. For the Dynaco MK3 also 6550 power tubes the manual recommmends a total of 140ma bias current for 2 tubes or 70ma per tube.
     
  11. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

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    So looking at 70 ma you would divide 70 into 110 volts to come out at about .63 volts. A little higher plate voltage than what I have mine running at.
     

     

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