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Thread: Switch on coupling cap?

  1. #1
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    Switch on coupling cap?

    I have an AC15 type build with an EF86 front end (set up in a typical fashion) with an .02 cap going from it (EF86) to a simple tone control and then to the PI. Most times I like this sound but there are times I feel there's too much low end. I was thinking if there's a way to put a switch on that .02 cap to go from say a .01 or .0047 to the .02.

    I am a little hesitant because there's the whole plate voltage on one side of that cap, a little switch may not like that and it seems a little unsafe.

    I have seem many rotary switches for this, is that the only way to go? Other ideas?

    Thanks for looking!

  2. #2
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    You can have 2 coupling caps in series to lower the total capacitance and then use a switch to bypass one of them. This is used in the laney gh series of amps so look over one of those for ideas.
    Justin Thomas, The Dude and frus like this.

  3. #3
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    Quote Originally Posted by HBamps View Post
    I have an AC15 type build with an EF86 front end (set up in a typical fashion) with an .02 cap going from it (EF86) to a simple tone control and then to the PI. Most times I like this sound but there are times I feel there's too much low end. I was thinking if there's a way to put a switch on that .02 cap to go from say a .01 or .0047 to the .02.

    I am a little hesitant because there's the whole plate voltage on one side of that cap, a little switch may not like that and it seems a little unsafe.

    I have seem many rotary switches for this, is that the only way to go? Other ideas?

    Thanks for looking!
    The cap drops the voltage; switch after the cap. A megohm resistor from there to ground should help keep down transients. It is hard to be more specific without seeing the circuit.

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    Quote Originally Posted by Zozobra View Post
    You can have 2 coupling caps in series to lower the total capacitance and then use a switch to bypass one of them. This is used in the laney gh series of amps so look over one of those for ideas.
    If you had the two in series you could switch in and out the second as the first is blocking all the big DC voltage. Right? Brilliant!

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    That is how it's done in the GH100TI. Pretty easy to implement. Far easier than rotary switches if you only need 2 settings and in my experience of playing Orange/Matamps with them I only ever used 1 or 2 of the setting available anywayy

  6. #6
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    Hi Guys

    A 1M to ground to damp transients also throws away 10% of the gain. 10M will do the same for transients but not reduce gain.

    If you use good quality switches like the C&K 7000-series, there is no issue with them being connected to DC as found in a preamp or PA.

    You can take the switched cap idea a step further and have it be infinitely variable as shown in the Soma-84 project in TUT5, and as I do in my Studio amp. The large-value coupling cap ties to the tube plate with the low-value following it. Across the low-value cap you wire a pot as a rheostat. At one end of the sweep, the pot resistance is high and the small cap is in play reducing bass response. At the other end of the pot sweep, the small cap is shorted out and the full bass response of the large coupling cap is available.

    Have fun

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