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Huge difference in bias readings from tube to tube on matched sets

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  • #16
    Down & dirty OT test, power down & unplug from the wall, pull the power tubes & simply measure resistance between the brown 6L6 plate wire to the red OT wire at the very bottom left of the main circuit board (enters the chassis with the 2 black choke wires). Then do the same from the blue plate wire, from the other 6L6, to the red OT wire.
    Last edited by tboy; 11-14-2010, 09:50 PM. Reason: typo

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    • #17
      Do not be alarmed if the DC resistance readings are not identical for each half. A similar transformer (A good one) that I measured was 43 Ohms for one half of the primary and 49 Ohms for the other half. That is normal.

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      • #18
        Originally posted by MWJB View Post
        Down & dirty OT test, power down & unplug from the wall, pull the power tubes & simply measure resistance between the brown 6L6 plate wire to the red OT wire at the very bottom left of the main circuit board (enters the chassis with the 2 black choke wires). The do the same from the blue plate wire, from the other 6L6, to the red OT wire.
        48 and 46 Ohms respectively...this looks OK because red is the CT and Brown / Blue are the start and end of the secondary...right?

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        • #19
          Originally posted by Bruce / Mission Amps View Post
          If you check the bias probe (I never use one) and it is working correctly it can only be a few things that can cause this..... I read you've checked all those.
          I didn't read if you cleaned, de-ox'd and retensioned the sockets. Make sure you do that.
          Anyhow, I've had this happen a few times over my career and when all the easy-quick stuff fails, and it sounds like you have failed to remedy the situation with the easy stuff, replace the tube sockets. I know, it's pain in the butt.
          Just because the socket has the same bias voltage delivered to each socket does NOT mean the tube in question is seeing the correct voltage.

          Added: Yes a bad OT can do weird things to your idle current ... but so can an ultrasonic oscillation on one tube that you can not hear but the tube is drawing "apparent" DC idle current, when in fact it is a lot of AC (ultrasonic audio) power going out the tube.....
          Thanks Bruce, I will make sure the sockets are clean and have adequate tension...not looking forward to the prospect of swapping the sockets out.

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          • #20
            Would seem OK in lack of any info to the contrary.

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            • #21
              I guess since everything looks to be 100% I will just bias it as best I can...most likely 1 at 70% and the other colder tube wherever it falls and see how much hum I get...who knows it may sound fine even fantastic.

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              • #22
                Things aren't 100% because you have a severe mismatch of plate current, a few mA wouldn't be an issue but you're telling us that one tube draws twice as much as the other. Your amp will still make sound, but it won't sound fine or fantastic.

                These matched sets that you have, do they read as matched (+/-5mA) in other amps?

                Have you changed the PI coupling caps?

                Have you changed the tube sockets?

                What are the PI plate voltages?

                Why didn't the tech who did the last work on the amp notice this or bring it to your attention? What do the tubes that he supplied read?

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                • #23
                  I read the whole post and my thoughts are the tube sockets also as Bruce suggested and really aren't that hard to change in these, in fact much easier than some other amps. If everything else is good I would ventue it's either that or the OT and that could be easily checked with an ohm meter.
                  KB

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                  • #24
                    Originally posted by w302nv View Post
                    48 and 46 Ohms respectively...this looks OK because red is the CT and Brown / Blue are the start and end of the secondary...right?
                    Brown & Blue between the Red (CT) are the primary winding.
                    The secondary goes to the speaker.
                    As to the bias: use your bias meter with only one tube installed.
                    Then check tube to tube for current draw.
                    (No, don't play the amp)

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                    • #25
                      No word from the tech that did the work, so I think I may go ahead and change the socket to see if that fixes things....I figured a dental tool and DeOxIt would have taken care of it but a tube socket is not that bad of an operation plus I can change out the 1k5 resistors that have drifted a bit...As for the tubes, I was told that it needed new tubes but he sold me the set that he used to test the amp....and as for the tech, I am pretty confident that he knows what he is doing...he came highly recommended and the other amps I took to him are 100%...so this may just be a fluke...sh1t happens.

                      Thanks again to everyone for the assistance, it is much appreciated...and thanks for taking it easy on a n00b...I appreciate the patience.

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