Recent content by 2L man

  1. 2

    How do you increase headroom in a 50w JMP?

    Amp which has only one volume potentiometer after first stage does not allow more tweaking other than power tube bias! If you don't want to install more pots you could install trimmer pots between stages. You can install them so that only hole for screwdriver is visible outside if you want to...
  2. 2

    Strange sine wave

    Yes, resistor load should make amp produce more clean power than true reactive load and guitar loudspeaker load because it react only resistively :) There are quite a many variables which effect the amp output power but if it was good looking 38W sine wave I think it is not far what a pair of...
  3. 2

    Strange sine wave

    On first photo the drive is too high! To get an idea what amplifier amplify it is better to use non inductive resistor on output. Loudspeaker impedance react too much to frequency. Start testing how much clean power you get out using 1kHz. Tune all volumes starting from signal generator, EQ...
  4. 2

    Calculate powersupply rc

    If you know voltages and Operating Points it is possible to calculate anode currents and then add them and use this combined current to calculate dropper resistor values. If you need to define OPs it is very fast using this link loadline calculator. I think transformer output voltage is...
  5. 2

    Questions about tube biasing

    You should first test the bias range and mark trimmer(s) where 100% is. Possibly range never go to 100%? Then play and adjust the bias for the sound You like and watch that tubes do not glow too bright! Then measure again what the bias currents came.
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    That is strange if wires are same color??? If amp used Negative Feed Back OT phasing should be correct so look datasheet! IF NFB phasing is wrong amp usually oscillate or produce "chost notes" etc. If it does then two wires can be cross changed.
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    You mixed primary and secondary (output) but obviously did feed 27VAC to primary? OT primary coil has much higher resistance than secondary so measure them before connecting.
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    Edit, nevermind ;)
  9. 2


    First use ohmmeter and solve the coils and you can draw it. Then input Mains and measure outputs. Measure iron core area and compare to various tables and you get very good idea what its VA rating is. Sometimes primary resistance does not produce reliable reasing because of transformer coil...
  10. 2

    Changing Impendence on 4 x 12 cab

    Usually they do if not driving amp too hard. If player does not understand why that setup volume is lower and turns vol to 11 the power tubes and/or Output Transformer might destroy. I think it is better to use correct output load which has less speakers than wrong load which has more...
  11. 2

    SuperBass 100 Output Question

    Pure business! First it save some money on components and manufacture. Later, usually tubes last until warranty time is exceed and then when screen resistor burn repairs increase repair shop cash flow.
  12. 2

    Burnt diodes?

    Select diodes which current rating is at least double what the secondary fuse is. Like Jon wrote diode voltage rating is not important because they are parallel opposite each others so voltage does not rise above 1V. 1N4007 was used there because it is very cheap and obviously circuit was not...
  13. 2

    Burnt diodes?

    That circuit is called "ground loop hum killer" and it connects secondary Common (ground) to Chassis/Safety earth "smoothly". Those diodes current rating should be at least twice the HV fuse rating so that they do not burn before the fuse if secondary voltage leaks to Chassis. 10 ohm resistor...
  14. 2

    Mixing cabinet Impedance, yes or no?

    If your amp has 4 and 8 ohm outputs you can install 8 ohm cabinet to 4 ohm output AND 16 ohm cabinet to 8 ohm output same time and both take equal power and power tube impedance stay what it is designed to be..
  15. 2

    JTM45 and Master Volume Options – Pros and Cons?

    T Yes you can and it make the switch "better" when it disconnect both hot and neutral!

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