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Calculating VA in a Tube Preamp Circuit

  1. Apr 2, 2009 #1
    I am getting ready to make a tube preamp to drive a Leslie speaker. I want to use a 12AX7 and 12BH7. I plan on using both triodes of the 12AX7 and possibly both oh the 12BH7. I have two questions: First, any design suggestions would be greatly appreciated. Any schematics would be GREAT! and secondly, how do I calculate the VA so I know what transformer to get. I want to use a 250-0-250 transformer, that will put my B+ plate voltages right around 250 and at the optimal operating range of the tubes. So how do I know what VA rating I need when I pick out a transformer. I am attaching a link to a schematic for a Leslie 147 amplifier so you will know what I am trying to drive. I want to drive a line-level signal (keyboard) into this power amp. Thanks!

  2. jcsd
  3. Apr 3, 2009 #2
    Thank you for posting a vacuum tube circuit. I last worked on vacuum tube circuits at Hewlett Packard in the period 1954 to 58. I built an audio amplifier at home using 12AX7s (high mu triode) and EL34's (push pull output).

    I understand your output signal will go into the input of the Leslie 147 amplifier that has a 8 ohm/16 ohm single-ended input? Or are you planning on building a replacement for the Leslie 147 ? When you mention a 250 -0 - 250 transformer, are you discussing a 250-ohm push-pull output transformer, or a 250 volt - 0 - 250 volt power transformer? If the latter, will this be for a full-wave center tap rectifier? If the latter, sum up the plate current for all tubes, and multiply by 250 volts to get the VA rating. If you plan to drive the Leslie input. all you need is a single ended output with an impedance transformer.
  4. Apr 3, 2009 #3
    Thank you for replying! I am planning on using a 250V-0V-250V power transformer with a center tapped full wave rectifier. The Leslie 147 input is not actually 8 ohms. There is a "console load resistor" selector switch for when the Leslie 147 is to be connected to an organ's internal speakers. The Leslie would be directly connected to the organs self-contained power amp. I will have the switch in the "open" position causing there to be no additional load. I want to take a line level 1/4" keyboard signal and drive the Leslie 147 amplifier with it.


    The Leslie speaker was originally designed to connect to a Hammond Organ either via a self contained amplifier or directly to the organs preamp for consoles without a self contained speaker system. I will attach a schematic of the Hammond preamp of which I am trying to model. The Hammond preamp has a 340-0-340V power transformer and has an additional section of amplification and an envelope to create a percussion effect used on the organ. I will not be using that part nor will I be using the first 6AU6 gain stage. I want to use the 12AX7 and 12BH7 to drive the Leslie amp. I would build from C6 to the point where it says "output to tone cabinet." (The tone cab would be a Leslie). Would I be better off copying the Hammond preamp design or making a new design based on a 12AX7 and 12BH7? The Hammond preamp uses an output transformer to provide a balanced output which the model 122 Leslie used a balanced input. The 147, however is identical to the 122 except it uses an unbalanced input. Also, if I wanted to build a copy of the Hammond preamp, could I use a 250-0-250V transformer and have the plates at a lower voltage or would I need to use the original Hammond design with a 340-0-340V transformer? Thanks!
  5. Apr 3, 2009 #4
    When calculating the VA would I include the heater current as well as the plate current...the transformers I am looking at have a separate heater winding.
  6. Apr 3, 2009 #5
    Why arn't you interested in using bipolar transistors?
  7. Apr 3, 2009 #6
    I have tried many solid state preamps for this purpose and I am not happy with the tone.
  8. Apr 3, 2009 #7


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    Last edited by a moderator: Apr 24, 2017
  9. Apr 3, 2009 #8
    I had actually already found that circuit and considered building it. How would I go about adding an additional gain stage...maybe using a 12BH7:)?

    Also, I would want to use a linear power supply instead of a switching PS.
  10. Apr 3, 2009 #9
    One final question: would it be an issue to run that Hammond circuit with a 250V transformer and a solid state rectifier??
  11. Apr 3, 2009 #10
    Last edited: Apr 3, 2009
  12. Apr 4, 2009 #11


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    The 250 volt(ac) measurement is an Average value (Vavg=0.9Vrms). Where Vrms is the http://en.wikipedia.org/wiki/Root_mean_square" [Broken] value. So the rectified voltage will be around the peak ac value (Vp=Vrms/.707). Note that the cathode of the 6X5 is shown to be at around 320 volt(dc).
    Last edited by a moderator: May 4, 2017
  13. Apr 4, 2009 #12
    I have the power supply figured out but my next problem is the output transformer is extremely expensive and hard to find. So I am wondering if there is a way to omit the output transformer using that circuit maybe by making the 12BH7 a cathode follower? If that is not feasible, I am considering building this circuit using a linear power supply:


    How could I add a 3rd gain stage to this circuit, particularly a 12BH7 gain stage? Does anyone have or no where to find a schematic to a circuit using 3 gain stages using either 12AX7/12BH7s or 6SN7/6SJ7. Thanks for all your help!
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