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Bias help for a 1969 Laney LA60BL pleeeease!

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  • #31
    [QUOTE=SoulFetish;557641][QUOTE=nickb;557627]Yes, it should be fine. A healthy fear of electricity is not a bad thing at all. Always pull the plug from the wall and discharge ( check with meter) before touching.

    YES! do this. The last several Laney amps I've worked one do not have any resistors to bleed off the HV. In fact, it's not a bad idea to install some ballast resistors across the reservoir caps. In the schematic, it looks like they gang up a parallel combination of 2 x serially connected 33µF caps(4 in total).
    I'm not sure what the manufacturer recommends according to the leakage, but it's common to see values of around 220k in parallel across each cap (in your case, 1 resistor for each pair of caps). This helps balance the voltage across the series connected caps because of the differences in tolerances in electrolytics. But they will also serve to bleed off the high voltage on those caps as well, as an additional measure of safety, in case one might forget to bleed them off manually – which happens to the best of 'em.
    I *think* it is 50/capacitance or less.

    But usually I do not use the standby switch... usually. Faithfully do not use it for shut off. So when the amp is on the bench, I just take an alligator clip to the chassis and tap the other end to the positive side of the reservoir caps - sometimes it'll spark, like 1 out of 20 times (as in *if* for some reason I had the amp in standby). Then just clip it there.
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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    • #32
      Originally posted by Jon Snell View Post
      Yes I meant 360mV across 10R per valve, as you say 700/720mV across 10R.
      It has no effect on the bias, it is how you monitor the bias.
      Alright - so far so good... I am just shy of 700mV across the 10R resistor with the TAD EL34B-STR installed... gonna let it run for a bit, re-check and if all is good put it back in the headcase and plaaaaay.

      Thank you, Jon - and EVERYONE - for you help and input!

      Finger's crossed.
      "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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      • #33
        Just a reminder that 70% in this circuit is the maximum.
        In your 1st. post you seemed concerned about tube life. If so, running at max. (70%) is not as easy on the tubes as running cooler.
        Many amps that run high voltages like this bias the tubes more in the 50 to 60% range for longer tube life.
        A lot of people think that 70% (for class AB) is a number that they need to hit, when it is really a 'do not exceed' value.
        Nick touched on this a bit in post #5.
        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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        • #34
          ^^^^^ what G1 said.

          I find no additional tonal benefit to biasing at maximum (whether class AB or A) that can't be added via other circuit tweaks or knob twiddling.

          Plus, I have some very nice tubers in my amps. So I wanna take care of em, cuz they're a LOT to replace!

          Justin
          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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          • #35
            Yep. I'm not sure how the 70% "rule" got started. Perhaps it was a sweet spot for a popular Marshall amp model? Just a guess. At any rate it's become cumbersome in both directions. There are guys trying to hit 70% in amps like 135W Twin Reverbs or silver face Deluxe Reverbs. Too hot for either of those models. And then there are guys making cathode biased el84 amps at reasonable voltages or 5e3's that are also shooting for 70%. Which is way too cold.
            "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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            • #36
              Originally posted by Chuck H View Post
              Yep. I'm not sure how the 70% "rule" got started. Perhaps it was a sweet spot for a popular Marshall amp model? Just a guess. At any rate it's become cumbersome in both directions. There are guys trying to hit 70% in amps like 135W Twin Reverbs or silver face Deluxe Reverbs. Too hot for either of those models. And then there are guys making cathode biased el84 amps at reasonable voltages or 5e3's that are also shooting for 70%. Which is way too cold.

              If you run the a typical AB tube guitar amp at full power and it's biased at 70% then the tubes will be at about 100% dissipation. That's where I think it came from. It does provide a sensible UPPER limit. Most AB amps are way less than this.

              I usually set on the cool side and verify with scope. If too much crossover distortion I'll raise it a tad. Above, I suggested a lower setting of 30mA cathode current (600mV) not least influenced by the OP's desire to not burn up tubes.
              Experience is something you get, just after you really needed it.

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              • #37
                If you run the a typical AB tube guitar amp at full power and it's biased at 70% then the tubes will be at about 100% dissipation.
                According to Aiken max plate dissipation occurs at some intermediate output power (maybe one forth of full output) as shown here:
                https://www.aikenamps.com/index.php/...why-70-percent
                Last edited by Helmholtz; 05-03-2020, 07:46 PM.
                - Own Opinions Only -

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                • #38
                  Originally posted by Helmholtz View Post
                  According to Aiken max plate dissipation occurs at some intermediate output power (maybe one forth of full output) as shown here:
                  https://www.aikenamps.com/index.php/...why-70-percent
                  I'm well aware and I am not disputing that. But that is not what I said at all. I made a generalization about running at 100% of rated output to provide a rationale as to where the 70% rule originated. Let's not derail the thread in minutia.
                  Experience is something you get, just after you really needed it.

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                  • #39
                    I think the 70% idle rule ensures that plate dissipation stays below 100% at any intermediate output.
                    But the plate dissipation plots I have seen indicate that dissipation at full output is actually lower than at idle for a 70% biased amp.

                    Edit: My statements above only hold if the plate load is not too low for a given plate voltage. Aiken shows a dissipation plot in the link above with Vp=550V and an OT primary impedance of 4k (Marshalls typically use 3.4k, even worse), where dissipation vastly exceeds 100% with 70% biasing.
                    (A much too low plate load also results if an 8 (4) Ohm speaker is connected to the 16 (8) Ohm output.)

                    So it makes a lot of sense to stay below 70% with high plate supply voltage.
                    Last edited by Helmholtz; 05-03-2020, 10:13 PM.
                    - Own Opinions Only -

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                    • #40
                      Originally posted by Helmholtz View Post
                      I think the 70% idle rule ensures that plate dissipation stays below 100% at any intermediate output.
                      But the plate dissipation plots I have seen indicate that dissipation at full output is actually lower than at idle for a 70% biased amp.
                      Ah! Okay doke. Thx. So it happens at lower power than my recollection (which was from many years ago). Still the point is there is a rationale behind the rule.
                      Experience is something you get, just after you really needed it.

                      Comment

                      • #41
                        Originally posted by Helmholtz View Post
                        According to Aiken max plate dissipation occurs at some intermediate output power (maybe one forth of full output) as shown here:
                        https://www.aikenamps.com/index.php/...why-70-percent
                        It seems like the graphical plots on the Data sheets would support this when you look a how the load line intersect with the dissipation curve
                        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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