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JimNomad 22nd January 2013 10:14 AM

Switchable Cathode Bypass Capacitor for a Tube amp
2 Attachment(s)
Hi All,
I am building a simple tube amp using ECC83, I read that the value of the Cathode Bypass Capacitor can have an influence on gain at different frequencies and headroom. Would it be possible to build in a switch so that I can switch between different capicitors and a a consequence produce fully / partially / un - bypassed effect . If so would attached circuit do the job ?
Many thanks


Enzo 22nd January 2013 12:28 PM

Yes, but don't do it that way. In your example, wire one of the 1uf caps permanently across the resistor, then just switch the other 1uf in and out. Remember, caps add in parallel, so two 1uf caps in parallel is the same thing as a 2uf cap. SO selecting between 1uf and 2uf is simple as adding a parallel cap to the other one.

It will work but it will pop when you switch it in. SO wire a high value resistor, like 1 meg or 470k, across the switch. The resistor will keep the cap charged, but the high resistance will prevent th cap from acting in th circuit. Then the switch shorts across the resistor and that leaves the cap in the circuit.

JimNomad 22nd January 2013 01:16 PM

1 Attachment(s)
Many thanks, this is better and easier , nice one !

For the large resistor 'across' the switch - in parallel with it ? as shown in attachment.

Also do you know if it would be possible to use a three way switch to introduce 3 different capacitances to this part of the circuit - like 1/2 , 1 and 2 uF - also would it be worth doing ? Trying to use this first amp to test out sounds ..

Thanks again...

doozerdave 22nd January 2013 03:18 PM

You can use as many switch positions as you like. In my last amp build I used a 4 position rotary switch, switching in different resistors and capacitors.

Enzo 22nd January 2013 04:16 PM


You will find some commercial guitar amps use the technique as part of their channel switching. Switch in some gain enlarging caps and maybe a voltage divider change elsewhere, and poof - channel switching.

If you are just experimenting, hey, let it pop. But you do want the large resistors in a finished product. Yes, your new drawing is exactly what I tried to describe in words. The same technique works when the caps are wide apart too, like 0.68uf and 25uf. hardwire the lower one, then just switch the larger one into parallel. In such cases, the smaller one essentially disappears into the larger one.

gingertube 22nd January 2013 09:04 PM

You will see this (switching in of extra bypass capacitance) used quite a bit in more recent designs. Most commonly the switch will be labelled "FAT" with the extra capacitance IN with the switch in the FAT position.
In a preamp with a Gain or Drive Control you may want the sound FAT at lower gain settings but with some bottom end knocked off (FAT switch OFF) at high gain settings to keep the sound from getting too muddy.

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