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Ampeg B18N Plate voltage and bias current too high

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  • Ampeg B18N Plate voltage and bias current too high

    I'm finishing up an Ampeg B18N that is getting a 3 prong cord, new filter caps, new 7027A tubes and one new 6SL7 tube, plus a new speaker. I had no issues with the power cord changeout or the filter caps. I love that they include the schematics on the inside plate of the amp.

    I'm now checking the bias on it. I am getting 536V on the plate, with -56.4VDC on pin 5. I'm using a socket bias probe to measure the cathode current thru the two power tubes and each is 57mA. With the plate voltage of 536V, that's 30.5W per power tube. If I increase the 47k resistor to make it more negative on pin 5 (-62V), I get about 40mA and plate voltage increases (as expected) to 547V. Excluding the initial 600V cap on the first stage, the cap can is only rated to 525V per leg. Got any suggestions to lower the initial voltage, or how to bring the bias a little cooler while keeping the cap can happy? Thanks!
    Attached Files

  • #2
    Is the heater voltage also high? If so, a buck transformer in the primary may be a good option. It will bring all secondary voltages down.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      Heater is at 6.77VAC, so yes, it's also high. Any suggestions on a lead for a buck transformer? I'm assuming that's inserted between the wall power and the PT primary winding inside the amp.

      Would swapping out the 5AR4 (GZ34) for a 5U4GB be a viable option?

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      • #4
        Just thought of another possibility, though I don't know if this is a good idea or not. Thus why I'm here to learn and ask questions.

        What about if I add a 100ohm 5W resistor between the PT secondary red wire to Pin 4 of the rectifier tube socket and another 100 ohm 5W resistor between the other PT secondary red wire to Pin 6 of the rectifier tube socket? I have room to add a terminal strip to secure them, but would like feedback if this is a viable option or not. Perhaps a different value or wattage? That would drop the voltage to the rectifier and thus reduce all B+ and subsequent DC voltages, but it would not address the higher voltage on the filaments. While 6.77V is high for the heaters, is it too high?

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        • #5
          http://www.mh-audio.nl/Calculators/D...gResistor.html

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          • #6
            Don't forget the voltage chart is measured with a 20kOhm per volt moving coil meter and that will load the results. With a DVM at 20M Ohm/volt, the readings will be higher. 505 on the anodes with the moving coil meter is probably 515v with a DVM and if you factor in the mains voltage tollerances, I would say the voltages you are getting are acceptable.
            The maximum recommended 6.3v heater supply is 7volts.
            Here is a diagramme that is more legible. Click image for larger version

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            Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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            • #7
              Thanks for the clearer diagram, Jon!

              I'm still trying to figure out the proper next steps. My initial concern was the bias was 30W per tube. Lowering the bias raised the plate voltage to close to 550V, but that's still within the 600V rating of the first cap. However, the first stage of the cap can is seeing about 10V lower than the initial stage voltage, meaning it's now around 540V, which exceeds the 525V rating of the cap can. Would increasing the 1kΩ 10W resistor be the right thing to do? That would lower all the voltages after it, keeping them below the cap can's 525V rating.

              Before deciding to make any component and wiring mods, I tried a 5U4GB tube in it. It dropped the plate voltage down to 515V, which is definitely in the right range. I realize this rectifier tube pulls more heater current, but this PT seems to have been used on multiple Ampegs, some of which used this rectifier instead of the 5AR4 (GZ34).

              As this is the cleanest change, would swapping to this 5U4GB rectifier be a good solution, or will it be introducing problems?

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              • #8
                Is the mains transformer set correctly?

                http://web.archive.org/web/20110621210909/http://diyguitarist.com/GuitarAmps/PowerTubeBias.htm
                Power Tube Bias Charts

                https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator
                Biasing Tube Amplifier Calculator

                https://amprepairparts.com/bias.htm
                TUBE BIAS CALCULATOR (for push pull class AB1/AB2)
                Who does not know and knows that he does not know - teach him Confucius)
                Who knows and does not know that he knows - wake him Confucius)

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                • #9
                  The primary only has a 120V winding and there is no setting to change for it for say 115 or even the 230 or 240 volt options so I feel confident it is wired correctly. All PT wiring is original and unmolested.

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                  • #10
                    Originally posted by Jon Snell View Post
                    Don't forget the voltage chart is measured with a 20kOhm per volt moving coil meter and that will load the results. With a DVM at 20M Ohm/volt, the readings will be higher. 505 on the anodes with the moving coil meter is probably 515v with a DVM and if you factor in the mains voltage tollerances,
                    A meter input resistance of 20kOhm/V means 12M in the 600V range. That's better than most DVMs.
                    Handheld DVMs typically have a fixed input resistance of 1M (general purpose) or 10M (professional grade).

                    Apart from that, power supply nodes have low source resistance in the 100R range, so the loading by any decent meter can be neglected.

                    - Own Opinions Only -

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                    • #11
                      Click image for larger version

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                      Who does not know and knows that he does not know - teach him Confucius)
                      Who knows and does not know that he knows - wake him Confucius)

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                      • #12
                        Fitting a series resistor will stop the HT from rising so fast but will not stop the voltage unless there is a load plus it will introduce a sag when playing ... not nice!
                        The GZ34 only drops 10volts anode/cathode voltage under moderate load.
                        The 5U4 drops 33volts, 5U4-G 35volts a 5R4 drops 45volts and 5R4GYB drops a pleasant 50volts with a heater current of 1.9Amps. That is your answer.
                        Last edited by Jon Snell; 05-03-2021, 03:46 PM. Reason: Corrected type error
                        Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                        • #13
                          5AR4 is equivalent to GZ34, so same drop.
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            5AR4 is equivalent to GZ34, so same drop.
                            My Typo I meant 5R4GYB

                            Click image for larger version  Name:	Valve rect. voltage drop.png Views:	0 Size:	587.8 KB ID:	930619
                            Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                            • #15
                              Speaking of no-suffix version.

                              You wrote "The GZ34 only drops 10volts....a 5AR4 drops 45volts (and 5AR4GYB drops a pleasant 50volts)."
                              - Own Opinions Only -

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