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    Capacitors in tube audio

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    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Capacitors in tube audio

    Post by plexus on Thu Apr 05, 2012 5:06 pm

    Why are decoupling caps between plate and grid lower values like 0.1uF-0.33uF and the output decoupling cap higher values like 1.5-2.2uF? I understand the math behind the output decoupler where-by a lower value causes the cutoff knee to be at a higher frequency there-by rolling off the low end. I am curious as to the choice of values for caps in tube circuits and why these values are different.

    Update:
    After thinking about it, here is my guess: on the line level output, there is a impedance to ground that causes the final output cap to act like a filter, rolling off the low end, the lower value the cap. based on a formula, you can work out what the ideal cap value is here usually 1.5uF for a 20Hz knee roll-off. but in the middle of a circuit, i am guessing the impedance are such that you can use a much lower value cap before you get this roll off. i see 0.1uF and 0.22uF often used for these. presumably you can use larger values (within reason) in the same way - just don't go lower because you'll get low end roll off... something like that, right?

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Capacitors in tube audio

    Post by sailor on Thu Apr 05, 2012 8:01 pm

    I was having the same problem until 2 months ago then it hit me, it's all about the next stage and stopping DC while not rolling off the Bass. In a circuit you know the ohms resistance of the next stage. So you don't really need a fudge factor but people doing it anyway say going from .1 to .22. But in a preamp coupling cap. the amp. could have an input impedance of 25,000 to 500,000 so to have a cap that will not roll off the Bass for all possible amps. designers will use the low end say 40,000 and flat to 10HZ. What is the difference? By my calculations. 40,000 flat to 10 HZ. requires a .8 MFD. At 270,000 input impedance you only need .12mfd. Half those values if you only want it flat to 20 HZ.
    I figured this out on my own so if I'm wrong I'm sure someone will correct me.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Capacitors in tube audio

    Post by tubes4hifi on Sun Apr 08, 2012 8:27 pm

    most tube grids have an input impedance of well over several MegOhms, so coupling caps can be tiny 0.02 or even smaller and have no effect on the frequency response

    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: Capacitors in tube audio

    Post by plexus on Sun Apr 08, 2012 9:18 pm

    tubes4hifi wrote:most tube grids have an input impedance of well over several MegOhms, so coupling caps can be tiny 0.02 or even smaller and have no effect on the frequency response

    thanks Roy...
    Why is 0.22uF used in the PH14/15? Why is 0.1uF used typically?

    tubes4hifi
    Admin

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    Re: Capacitors in tube audio

    Post by tubes4hifi on Sun Apr 08, 2012 9:29 pm

    phono EQ circuits are a different story. The coupling capacitors in the PH14/15 are part of the RIAA EQ.
    If you change any component value, it has an effect on the EQ and on other parts in the circuit.

    plexus

    Posts : 94
    Join date : 2012-02-18
    Location : Toronto, Canada

    Re: Capacitors in tube audio

    Post by plexus on Sun Apr 08, 2012 9:40 pm

    That's what I suspected. and that's good too because there is a big price difference between lower values and 0.22uF when it comes to V-Caps. I wonder if I needed 0.22uF in the VTA 6SN7 MK3 drivers? could I have used a lower value even as low as 0.01uF?

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