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Thread: PI drive for 6 power tube amp

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    PI drive for 6 power tube amp

    I am looking at a bunch of schematics to learn about this scenario of phase inverters for 6 power tube amps. A couple things in the Hiwatt DR405 caught my eye and made me wonder

    Here are the schematics
    power amp - https://hiwatt.org/Schematics/DR_400wOutput.pdf
    power supply - https://hiwatt.org/Schematics/DR_400wPS.pdf
    preamp - https://hiwatt.org/Schematics/DR_Pre4Input405.pdf

    1. What is the purpose of the 22K/100pF in parallel that goes from the cathode of PI to the power tube control grid? is it just there to shape the frequency response of the signal going to the control grid?

    2. What is the purpose of the 100nF to ground from the power tube screen supply? is this a "power supply" cap for the screen node? Or does it have some sort of shunting high frequency purpose similar to how you would typically have a 47pF or 100pF cap between 82K and 100K PI plate leads, but you can't do that in this type of PI scenario since it's a 6 power tube style?

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    Anybody? Maybe I should have called the thread Hiwatt DR405 PI question. EVeryone likes reading about Hiwatts

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    Just a thought:

    1. The 22k/100p boost high frequency grid drive above around 70kHz.
    2. The 100nF to ground and the 100 Ohm supply resistor improve high frequency screen and PI supply filtering above 16kHz.

    At first sight both measures didn't seem to make sense to me in an instrument amplifier especially without global NFB. Then I noticed that there is actually a feedback loop.
    This makes me think that both networks are part of an advanced feedback stabilisation concept, introducing high frequency phase lead of the open loop response. I think it's called forward compensation.

    Don't think there is a direct relation to 6 tube PI drive.

    Other thoughts?

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    Last edited by Helmholtz; 05-21-2019 at 06:26 PM.
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    Click image for larger version. 

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    THis was kind of my plan. Thinking the 22K/100pF was some sort of high pass filter to the power amp I wanted to omit it. A friend has a DR405 and loves it but says for certain bands it doesn't work good because the amp doesn't handle low tunings well. It made me wonder if this parallel cap/resitors was part of the reason.

    If my NFB arrangement is different from the Hiwatt anyway, would it be good to omit 22k/100pF anyway?
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    t made me wonder if this parallel cap/resitors was part of the reason.
    Surely not. Look at the high corner frequencies.


    No good idea to omit the 100nF cap and leave the 100 Ohm supply resistor as this would produce unwanted HF coupling/feedback.

    The HF compensation measures prevent HF oscillation and won't influence bass response. Why not start with copying the tried design? I may get tricky to stop instabilities with a modified circuit.

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    Quote Originally Posted by Helmholtz View Post
    Why not start with copying the tried design? I may get tricky to stop instabilities with a modified circuit.
    I guess the only reason not to totally copy it is because my phase inverter design is slightly different and the NFB arrangement is a little different. I am copying a prototype of a bass amp I made that is 4xKT88 and 4x12ax7. So now I am copying it but converting it to a 6xKT88 amp and adding the 5th small bottle tube which would be me essentially copying what they did with that 12AT7 extra driver stage for the PI

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