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Help needed ! Lifco model 1000 tube amp bias capacitors values and adding bias pot.

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  • #16
    The other option is the 1 ohm cathode resistor method. It's in several places on this forum that will explain it. Use the search box.
    Turn it up so that everything is louder than everything else.

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    • #17
      i just measured both sides of the OT with the method g1 suggested. Here's what i got :

      V4 side :

      1,34 VDC
      133 ohm

      V5 side :

      1.34 VDC
      153 ohm

      That means currents of 10 mA for V4 and 8 mA respectively... seems pretty low... Would that explain the lack of bass in the tone of the amp ??

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      • #18
        Yes, for those plate and screen voltages for 6L6s, I've usually found about -45 v to -50v to be closer. Start at that -61v and work to a lower neg. voltage. If you want to bias at say 60%, you take 30W rating of the tube, multiply by .6 and get 18W. Now divide 18W by plate voltage, 432v and you get 42mA. So you want about 42mA per tube. If you have EL34 tubes, do the same math except use 25 watt rating, multiply by .6 and divide the 432 v onto that to get bias current. Start with 15 W. About 35mA. My guess for EL34s, you might be at about -38 to -44 v bias.

        The difference in resistance of your OT windings is because many of them are wound with one half of the secondary winding on top of the other. The one on the outside has farther to go to wrap around the bottom one, so it is longer. Longer wire equals more resistance. It's the turn number that counts. They are the same. Maybe pick a point half way between your two windings to bias at.
        Turn it up so that everything is louder than everything else.

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        • #19
          In case you don't know how to change the bias, you can install a trim pot in series with a resistor to replace R30, or you can just change R30 value (by replacement or adding resistor in parallel to reduce value).
          Reducing the value of R30 will make the bias voltage less negative, which will increase the idle current.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #20
            Good catch, g1. I missed the fact that there wasn't an adjustable bias on this poster's amp. We changed to goltigut at post #9 and had it in mind there was an adjustable pot added. It was the O.P.(bobyjoes) who mentioned adding a trimmer.
            And to clear up what I said in my last post, I should have said " start at -61v and work to a less or smaller negative number" and not "a lower neg. number." This whole neg. bias thing can be interpreted two different ways. To increase bias, a larger neg. number(voltage) is used. This results in a tube conducting less than before. Most books I've studied and forum entries are consistent with this. He needed to decrease bias to increase his tube's current.
            Turn it up so that everything is louder than everything else.

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            • #21
              Here's a little update after some work :

              I put the amp back to YBA-1 original schematic

              - Changed all the electrolytics, filter caps from 40uF to 50uF, 80uF to 100uF (the previous 80uF cap had been wired in reverse polarity, blue mallory from my last picture). Bias filter caps back to the way they were supposed to be 2x 8uF
              - Changed the power tube screen resistors from 470ohms to 1K.
              - The power supply dropper resistors had been changed so i brought back R31 from 2K to 4.7K, R32 from 5K to 10K. That seems to have brough all the voltages back to normal and it fixed the plate current apparently. Here's the readings i got :

              V1 plates : 223 VDC

              V2 plate 1 : 188 VDC
              V2 plate 2 : 326 VDC (seems pretty high...)

              Power tubes :

              V4 plate : 410
              screen : 414
              plate current : 46 mA

              V5 plate : 412
              screen : 414
              plate current : 43 mA

              I get higher screen voltage than the plate voltage, what do you guys think ? And what about that 12ax7 326VDC plate voltage ? Should i install that bias pot ?

              Click image for larger version

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              • #22
                Try measuring the cathode voltage when you also measure the plate voltage.

                There is a lot to be told with those two measurements.

                V2 is set up as a cathode follower.
                The first triode is a 'normal' setup.
                The second triode is taking the signal off of the cathode.
                (Not too sure if your designators match the schematic. Try using pin numbers.)

                Look at the schematic & you will see that there is no plate resistor.(V2/ pin 6)
                But there is a resistor off of the preamp high voltage to the grid. (V2/ pin7)
                Also, take note of the large value cathode resistor.(V2. pin 8)
                Last edited by Jazz P Bass; 01-05-2016, 11:57 PM.

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                • #23
                  Originally posted by goltigut;407077
                  - Here's the readings i got :

                  V1 plates : 223 VDC

                  V2 plate 1 : 188 VDC
                  V2 plate 2 : 326 VDC (seems pretty high...)

                  Power tubes :

                  V4 plate : 410
                  screen : 414
                  plate current : 46 mA

                  V5 plate : 412
                  screen : 414
                  plate current : 43 mA

                  I get higher screen voltage than the plate voltage, what do you guys think ? And what about that 12ax7 326VDC plate voltage ? Should i install that bias pot ?

                  [ATTACH=CONFIG
                  37167[/ATTACH]
                  Ultimately, the screen power dissipation is the determining factor. I'm guessing that's what Jazz is after when asking for cathode current. This way you can determine screen current by subtracting plate from cathode current. Just a personal opinion I have is that I never run my screens higher than my plates. It just invites higher screen current possibility and that only takes away from power output. Lower screen current is best IMHO since it's just wasted power. I usually run my screens from 5 -10 volts lower than my plates, depending on plate voltage. There is really nothing to be gained by pushing the screens to their limit in the name of a couple more watts of power that you will never hear anyway.

                  And depending on screen current which is usually is ignored for bias purposes, V4 is biased at 75%. That's about max, again in my opinion. Combined with the screen 4 volts higher and that high of bias current, I wouldn't expect that tube to have a nice long life.

                  You are running EL34s, correct?
                  Last edited by DRH1958; 01-05-2016, 10:40 PM.
                  Turn it up so that everything is louder than everything else.

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                  • #24
                    I should have mentioned, i've got the 7027 version running on vintage american 7027a tubes. So should i get higher value screens resistors than the 1k i've put in ? What value should i use ?

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                    • #25
                      Glad to have straightened out that you have the 7027a version. Your bias current is fine since they are rated at a higher wattage, 30-35W. And your screen current will be less when under load because the 7027 is a beam tetrode/pentode and the EL34 is a true pentode. You might be fine as you are.

                      The 1k screen grid resistors are fine. You have increase the value from what the schematic calls for, 470 ohm. They really don't figure in when biasing because screen current isn't taken into consideration when biasing. They help safeguard the screens when signal is applied and the tubes conduct harder. I wonder if you should take a DC resistance reading of the choke in the power supply just to be sure it's OK. That is what drops the screen voltage at idle. It really should drop the screen voltage at least even with or a couple volts lower than the plates but since you have increased the screen grid resistors to 1k, you might be fine. Make sure you drain your filter caps completely before you do this.

                      Take Jazz P Bass's advice and see what the cathode current is if you can. Did you ever install the 1 ohm cathode resistors?
                      Turn it up so that everything is louder than everything else.

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                      • #26
                        I don't have 1 ohm resistors handy, unfortunately...

                        So i measured the choke DC resistance and got a reading of 94 ohms, and the voltage drop across it at idle is about 1.1 volts. I get a voltage drop of 1.6 volts across the screen resistors. But for the plate, the voltage drop on the OT's primary is around 6 volts for each tube.

                        Does it make sense ?

                        Oh and for the record, i had some pretty bad humming after i restored the dropper resistors to original values, the power section had way more gain. I looked really hard to finally discover that the problem was in the grounding of the volume pots. They were grounded right on the same point as one of the filter cap cans... I fixed it moving the ground point to the body of the pot itself ground to the chassis away from the filter cap. This Lifco 1000 was pretty badly grounded... Now it is pretty quiet !

                        Btw thanks guys, your input has been really helpful to me restoring this amp !

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