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Help in determing bias on Blue Jr. Mark 1

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  • #46
    I seem to recall we've touched on this before.
    Originally posted by Chuck H View Post

    LOL!!!

    I can actually imagine Matchless owners talking to each other saying things like: "I replace my tubes every two hundred hours." And the response is "Yeah, well, I replace my tubes every hundred hours!"...

    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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    • #47
      Originally posted by Helmholtz View Post
      My experience is that despite them lying about this for 25 years, the normal mains voltage in the UK has remained 240V.
      Why can’t they just be honest, ie that whilst the nominal voltage is 230V, the power distribution network aims to maintain 240V?
      Then manufacturers and importers could set equipment where this distinction mattered appropriately.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #48
        Nevertheless also the nominal 230V EU voltage has a tolerance of +/-23V (or +/-10%).
        So (at least newer) products should be designed to work from 207V to 253V.
        - Own Opinions Only -

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        • #49
          Originally posted by ArturoQ View Post

          Hello
          From what I've garnered, 70% is the general consensus. That is from what I've seen online. Thanks for the information!
          I think AB1 fixed bias idling at up to 70% of the anode dissipation limit is fine for valve types where the limit was derived under the design centre rating system. eg EL84, EL84.
          I stress ‘up to’ because less than that generally works and sounds fine. So it that case it would be counter productive to idle any hotter than that, eg thinking that 70% was a target rather than an upper limit ‘rule of thumb’.

          But for valves rated under the design max system, idling up to 60% seems a more appropriate guideline.
          As the design maximum system doesn’t include any allowance for mains voltage variance etc.

          eg 6V6 (whatever their suffix) have a 12W limit design centre, and a 14W design maximum. I suggest that idling them up to about 8.4W is appropriate.

          Last edited by pdf64; 10-14-2023, 03:31 PM.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #50
            Originally posted by pdf64 View Post
            I think AB1 fixed bias idling at up to 70% of the anode dissipation limit is fine for valve types where the limit was derived under the design centre rating system. eg EL84, EL84.
            I stress ‘up to’ because less than that generally works and sounds fine. So it that case it would be counter productive to idle any hotter than that, eg thinking that 70% was a target rather than an upper limit ‘rule of thumb’.

            But for valves rated under the design max system, idling up to 60% seems a more appropriate guideline.
            As the design maximum system doesn’t include any allowance for mains voltage variance etc.

            eg 6V6 (whatever their suffix) have a 12W limit design centre, and a 14W design maximum. I suggest that idling them up to about 8.4W is appropriate.
            This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

            And... Keeping in mind that tubes can vary a bit in their ideal operating conditions you can always do what's been done forever. Put it on a scope and bias out any crossover distortion that occurs with a clean sine wave at maximum output. Then go just a little hotter to allow for variance in AC input voltage. Make sure measurements taken with this adjustment are within safe operating conditions. Why a little hotter? Because you'll probably have room that direction and this avoids crossover distortion if the AC is a little low. Usually you can get a little less crossover distortion on moderate clipping by going a little hotter that this even. And this usually lands you in the 70% range IME. Once the amp is clipping hard no amount of hotter bias will correct crossover distortions short of incinerating the tubes.

            So why is my amp biased at 90%? Because it's cathode biased. I do also have a shunt Zener diode that limits the voltage rise at the cathode resistor with current (which cools the bias) but it's not valued to fully eliminate this voltage rise. I've never checked but I'll bet the bias on my tubes in this circumstance (esentially fixed bias at the Zener voltage) is real close to what would be 70% at idle. Though I'm not sure how I would evaluate that since it only occurs when the tubes are conducting signal.
            Last edited by Chuck H; 10-14-2023, 06:35 PM.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz

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            • #51
              Total plate dissipation Pd with signal is found from Pd = Pin - Pout.
              Pin is consumed DC power, given by DC current into OT CT times actual B+.
              Pout is output power, multiply by 1.05 to 1.1 to account for OT loss.
              Divide resultant Pd by number of power tubes.
              - Own Opinions Only -

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              • #52
                Hello again and for now I've just realized that it was pointed out that I was running a bit hot. I need to put more time into understanding the circuit and operation. This will take me some time..
                Thanks to all for the various points of view and the assistance in making measurements.
                Cheer, thanks and Danke.

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                • #53
                  Originally posted by pdf64 View Post

                  My experience is that despite them lying about this for 25 years, the normal mains voltage in the UK has remained 240V.
                  Why can’t they just be honest, ie that whilst the nominal voltage is 230V, the power distribution network aims to maintain 240V?
                  Then manufacturers and importers could set equipment where this distinction mattered appropriately.
                  History of the nominal voltage in the UK

                  In 1994, to remove a perceived barrier to trade between European countries, the
                  European Commission (EC) decided to ‘harmonise’ the standard UK mains voltage
                  of 240 V and the European standard of 220 V at 230 V.

                  However, the cost of replacing or adjusting all the electricity supply equipment across
                  Europe to actually deliver 230 V was prohibitive, there being no technical advantage
                  whatever in changing, other than ‘harmonisation’.

                  So, to avoid accusations of failure to harmonise, the European Commission simply
                  altered the legal voltage limits - nothing actually changed!

                  The law in the UK currently states that the mains supply voltage must be 230 V +10%
                  / -6%, thereby allowing the European 220 V system to stay at 220 V and the UK to
                  stay at 240 V, yet both appear to be harmonised!

                  In 2010, the voltage tolerance band may be increased to +10% but again, in reality,
                  nothing will really change!

                  Comment


                  • #54
                    Originally posted by Dave H View Post

                    History of the nominal voltage in the UK

                    In 1994, to remove a perceived barrier to trade between European countries, the
                    European Commission (EC) decided to ‘harmonise’ the standard UK mains voltage
                    of 240 V and the European standard of 220 V at 230 V.

                    However, the cost of replacing or adjusting all the electricity supply equipment across
                    Europe to actually deliver 230 V was prohibitive, there being no technical advantage
                    whatever in changing, other than ‘harmonisation’.

                    So, to avoid accusations of failure to harmonise, the European Commission simply
                    altered the legal voltage limits - nothing actually changed!

                    The law in the UK currently states that the mains supply voltage must be 230 V +10%
                    / -6%, thereby allowing the European 220 V system to stay at 220 V and the UK to
                    stay at 240 V, yet both appear to be harmonised!

                    In 2010, the voltage tolerance band may be increased to +10% but again, in reality,
                    nothing will really change!
                    Well I wont search for analogies but this sort of thing seems to happen a lot with infrastructure and planning commissions around the world. A classic case of fixing a problem by changing what qualifies as broken.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "If you're not interested in opinions and the experience of others, why even start a thread?
                    You can't just expect consent." Helmholtz

                    Comment


                    • #55
                      What matters for products are the actual line voltage limits.
                      Up until 1987 nominal voltage in Germany (and many other European countries) was 220V with a range from 198V to 242V.
                      Thereafter the nominal voltage was increased to 230V and the range was changed to 207V/244V for a transitional period.
                      Finally since 2003 the range is 207V/253V.
                      This actual voltage increase did produce costs at both utilities' and product manufactures' sides.

                      UK representatives in the IEC had agreed to lower their actual voltage for harmonized voltage limits after a transitional period of asymmetrical tolerance.
                      - Own Opinions Only -

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