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Bias help for a 1969 Laney LA60BL pleeeease!

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  • #1

    Bias help for a 1969 Laney LA60BL pleeeease!

    Hi Everyone.

    Yes, back with another question pertaining to that amp.

    Like an idiot, I didn't notice this before but the 2x EL34 cathodes are tied together then to a 10 ohm resistor to ground. is this actually a cathode biased setup?

    I must admit a healthy fear working on amps in general, which I consider a good thing, but this one has 570 VDC on the plates and 558 VDC on the screens! I imagine it'll be tough to find an EL34 that will ... be happy under those conditions. ??

    But perhaps this is why the amp has no bias adjust pot? I have never seen a cathode bias amp on the inside before, so I will have to do a bit of research once you guys confirm or otherwise.

    I did put a set of TAD EL34-STR in there and they started to red plate lightly rather quickly, after a minute tops. The bias voltage is -45V at the tube pins. Putting others in though did not (Harma/Watford EL34s) - at least I could take loaded measurements power powering down.

    So, please fellow MEF'ers, let me know your thoughts :-)

    Thank you!!
    "'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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  • #2
    The 10R is to measure the voltage drop and that equates to the bias current.
    Power up your amp, with no signal and a loudspeaker connected, measure the voltage across the 10R. I would look for about 36mV for a cool output if you have a bias pot fitted.
    EL34s will run at up to 800volts on the anode so no problem there.
    Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
    If you can't fix it, I probably can.

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    • #3
      Originally posted by Jon Snell View Post
      The 10R is to measure the voltage drop and that equates to the bias current.
      Power up your amp, with no signal and a loudspeaker connected, measure the voltage across the 10R. I would look for about 36mV for a cool output if you have a bias pot fitted.
      EL34s will run at up to 800volts on the anode so no problem there.

      I was referring to the screens where some EL34 datasheets quote 450, some at 500.

      So I used to check mA across 1 ohm two watters, but never saw it with a 10 ohm... and the tubes share the same resistor... so when you say 36mA, your talking combined, right? So that means some 18mA per tube? My math tells me that would just about equate to a 50% bias. Which I think is 22mA per tube at 570VDC. Am I about right here? Or am I misunderstanding something?
      "'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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      • #4
        the 2x EL34 cathodes are tied together then to a 10 ohm resistor to ground.
        As this is a common cathode resistor, it carries both cathode currents. The voltage drop should be around 0.7V, so the influence on bias is small.
        To be able to monitor each tube's current individually, you may wire 1R resistors between each cathode and the common 10R resistor.


        Seems you need to make your grid bias supply adjustable. Do we have a schematic?
        Last edited by Helmholtz; 05-01-2020, 02:58 PM.
        - Own Opinions Only -

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        • #5
          With 570V on the plates and assuming this is a typical class AB, then the max plate current for 70% of the EL34's max of 25W would be 25/570 * 70% = 30.7mA. The cathode current will be about 10% higher i.e 34mA. Both EL34's are feeding into the 10 ohm resistor the voltage drop will therefore be 680mV maximum. Personally I would set around 600mV.

          I think the 36mV that Jon gave was a typo, I think he meant 720mV ( 10 ohm instead of 1 ohm and x2 EL34's). I'm not saying 720mV is wrong but I prefer to go cooler as it's a little easier on the tubes and the power supply.

          I'd be a bit cautious about using many current production tubes at 800V.
          Experience is something you get, just after you really needed it.

          Comment

          • #6
            Originally posted by Jon Snell View Post
            EL34s will run at up to 800volts on the anode so no problem there.
            With all due respect, this was only the case if you kept the screen voltages low. And I honestly don't think it applies to newer production tubes. As old guitar amps typically run the screens close to or virtually at the same voltage as the plates and new production tubes are not as good with the high voltages I would say that caution is in order with anything over 500V.
            Last edited by Chuck H; 05-02-2020, 12:37 AM.
            "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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            • #7
              Originally posted by Helmholtz View Post
              As this is a common cathode resistor, it carries both cathode currents. The voltage drop should be around 0.7V, so the influence on bias is small.
              To be able to monitor each tube's current individually, you may wire 1R resistors between each cathode and the common 10R resistor.


              Seems you need to make your grid bias supply adjustable. Do we have a schematic?
              Not an official one - but I recently found this and at a glance it seems to be on par with mine.

              Laney-60.pdf
              "'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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              • #8
                Also... here were my measurements for the bias as it sits currently...

                Plates: 570VDC
                CT to Plates resistance: v4 @ 65 om, v5 @ 75 ohm
                CT to Plates vDrop: v4 @ -2.6 VDC, v5 @ -3.5 VDC - this is going off of memory, so it may be slightly different but this is what I remember (I don't have the actual notes on me)

                If I remember this equates to some 50mA on one tube and 43mA on the other!!

                The bias should be 20 (cool) to 30 (70%) mA - so, that's a lot, nearly double!

                the bias circuit output to Pin 5 was -45 VDC at the pin. Again, IIRC.
                "'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

                Comment

                • #9
                  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.
                  Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                  If you can't fix it, I probably can.

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                  • #10
                    If a new production valve does not meet with the specification of older types, the identity must be change otherwise it is fraudulent.
                    Attached Files
                    Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                    If you can't fix it, I probably can.

                    Comment

                    • #11
                      Agreed, one must keep the G2 less than 450volts otherwise they turn into a volcano when they fail.
                      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                      If you can't fix it, I probably can.

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                      • #12
                        Thank you for the info.

                        So I assume that I need to take care of the screens before even powering this up again... I have read about some of these higher voltages in this model...

                        and this... https://music-electronics-forum.com/...ad.php?t=44042

                        Does this look like a solid solution?

                        Thanks!!!
                        "'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

                        Comment

                        • #13
                          Originally posted by Gtr0 View Post
                          Thank you for the info.

                          So I assume that I need to take care of the screens before even powering this up again... I have read about some of these higher voltages in this model...

                          and this... https://music-electronics-forum.com/...ad.php?t=44042

                          Does this look like a solid solution?

                          Thanks!!!
                          The screen grid resistors are 470 ohms and so something quite a bit less than 570V when operating and been working for 50 years. I don't see the need to change it. Some brands only spec 425V for the screens, others e.g. the Shuguang EL34B are 500V (and 30W plate dissipation instead of 25W). If you want a bit of peace of mind then change the screen grids to 1K.
                          Experience is something you get, just after you really needed it.

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                          • #14
                            Do you use the 250V PT primary tap?
                            - Own Opinions Only -

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                            • #15
                              Couldn't you replace the 56K resistor in the bias supply with a 100KL pot to make it adjustable?
                              It's weird, because it WAS working fine.....

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