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Marshall Super Bass low output volume

Discussion in 'Amp Tech Center' started by Joobacz, Jul 12, 2018 at 7:11 PM.

  1. Joobacz

    Joobacz TDPRI Member

    Age:
    33
    6
    Mar 23, 2018
    Czech republic
    Hi guys,

    I've just finished my handwired Marshall 1992 Super Bass (following the Metropoulos assembly instructions). When I turn it up and play my bass the output volume is very low. I know how 100 watt plexi sounds. It's f..king loud. But not this one. I double checked all connections and I switched all tubes for brand new ones. Here are my measurements:

    V1-1: 150
    V1-2: 0
    V1-3: 1,14
    V1-4: 2,71
    V1-5: 2,71
    V1-6: 163
    V1-7: 0
    V1-8: 1,14
    V1-9: 2,71

    V2-1: 151
    V2-2: 0
    V2-3: 0,69
    V2-4: 2,72
    V2-5: 2,72
    V2-6: 245
    V2-7: 151
    V2-8: 153
    V2-9: 2,72

    V3-1: 204
    V3-2: 18,3
    V3-3: 28,7
    V3-4: 2,74
    V3-5: 2,74
    V3-6: 193
    V3-7: 18,8
    V3-8: 28,7
    V3-9: 2,74

    V4-1: GND
    V4-2: 2,75
    V4-3: 394
    V4-4: 392
    V4-5: -40
    V4-6: 392
    V4-7: 2,75
    V4-8: GND

    V5-1: GND
    V5-2: 2,76
    V5-3: 394
    V5-4: 392
    V5-5: -40
    V5-6: 392
    V5-7: 2,76
    V5-8: GND

    V6-1: GND
    V6-2: 2,78
    V6-3: 394
    V6-4: 392
    V6-5: -40
    V6-6: 392
    V6-7: 2,78
    V6-8: GND

    V7-1: GND
    V7-2: 2,8
    V7-3: 394
    V7-4: 392
    V7-5: -40
    V7-6: 392
    V7-7: 2,8
    V7-8: GND

    These measurements are generally lower in comparison with Metropoulos voltage chart.
    I also measured voltage drop across 1 ohm resistor between pins 1, 8 on every output tube and ground but the value is very low - only around 5 mV. Even if I turned the bias pot all the way up to get -27 V the voltage drop is just 23 mV and output volume is still low.

    I'll be thankful for any suggestion what to do with my ill baby.
     

  2. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    A common mistake when building these Marshall circuits is accidentally using a 470k resistor in the PI where it calls for a 470r. This severely cuts output and is a very common mistake with these builds.
     

  3. Joobacz

    Joobacz TDPRI Member

    Age:
    33
    6
    Mar 23, 2018
    Czech republic
    Thank you for your quick respons, Dan. You're right this is common mistake. It is specially mentioned in the Metropoulos instruction manual. I checked it twice and the value of the resistor is 470 ohm.
     

  4. justin.ramsey

    justin.ramsey Tele-Holic

    799
    Jun 3, 2009
    Northern, KY
    Do you have a layout and schematic you can post? Also pictures of the build? I’m suspecting something is off in the bias supply.
     

  5. Joobacz

    Joobacz TDPRI Member

    Age:
    33
    6
    Mar 23, 2018
    Czech republic
    Here are some photos of its guts. If you want to see something in detail just ask. The attached schematic contains a few voltage mesurements I made on several points of the circuit.

    I used shielded cabel for the whole signal way from input to .022uF cap before PI. Also I applied the famous Larry's grounding.
     

    Attached Files:


  6. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    In your measurements you list -40vdc as the bias voltage. Can you list the voltage you see at pin 5 with your bias pot turned fully in both directions? Your voltages are low for a 100 watter but we don't yet know if you have the tubes biased extremely hot or if you used a lower voltage PT in this build. Some 50 watters measured in around 380vdc in the late 1970's, but I'm not aware of any 100 watter amps that ever measured that low with a standard PT. This alone would not cause your low volume problem that you are experiencing though. With 395v at the plates you should be biased up around 45ma...are you able to achieve a 45mv (45mv=45ma) reading across the 1 ohm biasing resisters using the bias pot?
     

  7. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    I have had tube sockets that didn't conduct to a pin on the tube before. With power tubes you can just use one and run it through the sockets to see if it sound's the same in each one. (don't run it very long on one tube) A preamp tube only working on one triode is also a possibility. Obviously a scope or signal tracer is the preferred method for this kind of thing.
    Not saying that's it here, just a thought.
     
    dan40 likes this.

  8. Joobacz

    Joobacz TDPRI Member

    Age:
    33
    6
    Mar 23, 2018
    Czech republic
    I did the measurements today.

    The maximum negative bias (the bias pot turned all the way right) is -39,7 VAC on pin 5 of all the output tubes and the voltage drops across the 1 ohm biasing resisters are

    V4: 5,3 mV
    V5: 4,4 mV
    V6: 6,0 mV
    V7: 5,9 mV

    The minimum negative bias (the bias pot turned all the way left) is -27,4 VAC on pin 5 of all the output tubes and the voltage drops are

    V4: 21,6 mV
    V5: 19,6 mV
    V6: 22,9 mV
    V7: 20,6 mV

    The power tansformer is Heyboer 1203-80-ms and its secondary winding for bias circuit gives 85 VAC.
     

  9. BobbyZ

    BobbyZ Doctor of Teleocity

    Jan 12, 2011
    Snellman MN
    If I'm looking at the numbers right the tubes are biased really cold. You might need to readjust the bias supply for less negative voltage to warm them up. If I put the numbers into the bias calculator right, and IF I'M THINKING RIGHT ! ! !

    Weber's calculator.

    http://www.tedweber.com/webervst/tubes1/calcbias.htm
     
    peteb likes this.

  10. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    5 mv translates to 5 Milla amps, way too cold.

    20 mv translates to 20 milla amps. While medium biased, should make some serious noise.


    Something else is wrong,
     

  11. Joobacz

    Joobacz TDPRI Member

    Age:
    33
    6
    Mar 23, 2018
    Czech republic
    I don't know how to lower bias voltage without losing plate voltage. I tried to switch 47k resistor right next to the bias pot to 27k. The bias voltage was -15,4 V and the voltage drop across 1ohm resistor was 44 mV but the plate voltage fell down to 244 V. Calculated plate dissipation was 10 W but the amp didn't seem to be louder.

    I think it needs rectifier to give 100 V more...
     

  12. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    Sounds like you have the wrong PT or there is a wiring issue in the power supply. Voltage should drop as the tubes conduct more current but you should not be seeing a B+ of 244vdc with the correct bias. You heater voltages are also low which would suggest a wiring issue. Does you PT have more than one primary voltage option? Are you perhaps taking these readings with a light bulb limiter plugged in?
     
    peteb and BobbyZ like this.

  13. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    What are the voltages supposed to be like on a 100 W Marshall?


    I looked at a schematic.

    The plate voltages are 560.
    The bias voltage on them is -60 to -70

    What’s the max plate dissipation for an EL34?

    Why is it listed as 20 W?



    If it is 20, how can it put out 100 W?
     

  14. elpico

    elpico Tele-Holic

    640
    Sep 14, 2011
    Vancouver BC
    It's 25W.

    The output power you get from a pair of tubes in a push-pull amp depends on the design of the amp. Put two EL34 in a class A amp and it will put out less than 25W. The same two tubes plugged into a class B design can put out nearly 100W. Guitar amps are generally class AB designs which put out something in between those extremes, getting around 50W out of a pair of EL34.
     

  15. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Thanks Elpico.


    Looking it up, typical Marshall voltages might look something more like this:

    Plate voltage is 460 V

    Bias voltage of - 50 volts

    Current draw 40 mA per tube





    Cranking the bias down to make the current right but everything else wrong shows the limited affect of biasing.

    You can’t get there from here.




    Is the PT the wrong one? Good thing to verify.

    assuming the PT is the right one, then something is sapping the power from it.
     

  16. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    100 Watts from two tubes in class B? Is that right?



    I was noticing that in class AB the plate dissipation of four tubes is the same as the power out, 100 W.
     

  17. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va
    The Superbass (1992) is a 100 watt circuit running four el34's. They will usually read 450-500vdc but some of the earlier models exceeded 500v. There is a Bass circuit (1986) that is a 50 watter running a pair of el34's. Their voltages changed over the years from 450v down to 380v and then back into the 400's in the newer models. The 100 watters ran a 175-0-175 or 190-0-190 pt with a non centertapped, voltage doubler rectification setup.
     

  18. elpico

    elpico Tele-Holic

    640
    Sep 14, 2011
    Vancouver BC
    Yep 100W from two EL34 in class B. In a class B amp one tube amplifies the positive half of the AC wave and the other handles the negative half. Each tube is completely turned off and resting while the other tube is doing it's job, so the tubes are on for 50% of the time and resting for 50% of the time. Tube power ratings are meant as an average over time, so an EL34 can safely put out 50W in class B because it's only doing it half of the time. 50W * 1/2 = 25W
     
    dan40 and peteb like this.

  19. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Thanks Elpico, that’s interesting that class B puts out the most. I see what you mean, how it does it.






    This is what I found for the really high voltage Marshall’s:



    There is one schematic that shows a table of all voltages at all pins for all tubes.



    In the Kendrick book it is on page 446 and it is called the 100W super trem amp.


    In schematic heaven the same schematic is linked as 1959T lead trem 100W


    And it is sub grouped with the


    1992 super bass 100W

    And the

    1992T super bass/trem 100W








    And I read a reason for +500 voltages on some certain amps is the use of a 3” power transformer. A switch to a 2.5” power transformers brought the voltages back below 500.


    The schematic I mention above has 560 V on four EL34 tubes.
     

  20. dan40

    dan40 Tele-Holic

    865
    Aug 19, 2015
    Richmond Va

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