Switchable Cathode Bypass Capacitor for a Tube amp

[Jim Nomad]

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

Jim

[G8HQP Dave]

You will get clicks as you switch. To reduce this add a high value resistor from each capacitor to ground. 100k-220k should be OK.

You won't notice much difference between 1uF and 2uF. You will move an LF shelf from starting at 360Hz to 180Hz.

To change the gain (and the amount of negative feedback) without altering the frequency response put a resistor in series with each capacitor. To stop the 'click' refered to in post 2 put the bleed resistor to the cathode. Caps about 10 to 20uF. You could do a 'binary' thing with 1/2 of 1K5 then 1/4 then 1/8th etc. giving a large number of settings with but a few extra components.

[ColinTheAmpMan1]

Jim,

Your suggestion of having different value cathode bypass capacitors is quite sensible. While most of these capacitors are likely to be 25uF/25VDC, Marshall used values as widely separate as 330uF and 0.68uF (see the attached pdf of a 100 W guitar amplifier). I wouldn't necessarily recommend values much above 100uF as the potential increase in gain at low-frequencies could just cause instability and "motor-boating". Of course, having no cathode bypass capacitor at all is a possibliity, too.

If you aren't likely to switch between different capacitors with the amplifier turned on, then the "popping" will not be evident and you don't need to worry about suppressing it. The simple way to suppress "popping" while switching, if you think you need to, is to connect a resistor going to ground in series with the bypass capacitor and connect the switch across this resistor. It should have a value of at least ten times the cathode resistor, but 47k is probably a good starting point. with this resistor in circuit, the effect of the capacitor is negligible, but when shorted out by the switch, the capacitor is in circuit in all its glory.

You could replace a volume or tone control with a pot having a push-pull switch on it for this switch, thereby making no obviously visible change to the escutcheon. Bear in mind that these push-pull pots are not cheap, though.

I would be interested in the results, if you try this out.

Regards, Colin.