Switchable Cathode Bypass Cap help

[Pryde]

Hello friends

Just built a little Ampmaker SE5a amp (got it up and running with help on this forum)

The first gain stage has a 1uf cap with 1.5k resistor on the cathode. I want to have a switchable larger cap with this to get a bass frequency boost for my single-coils. The amp sounds great with HB but is to thin/bright with my strat.

Researching this gave a lot of different/conflicting ideas on implementing the switch with no/minimal pop so I made a diagram for switching in a 22uf cap for Bass and want to know if this looks correct.

Also, not sure about the 33k resistor value on the switch, is that ok or do I need a different value?

Thanks

[Enzo]

You do need the resistor, it keeps the cap charged up. Is 33k large enough? I don't know. I usually use 1 meg or 470k in such places.

[Mick Bailey]

The first thing I'd do before wiring in a switched cap is to parallel the 22uf cap across the 1uf to be sure that with your Strat sounds ok.

That circuit looks ok to me. The 33k resistor is small enough to maintain the larger cap at roughly the same potential as the smaller one, but large enough not to affect the circuit too much when the cap is switched out. You'll still probably get a little bit of noise when switching.

You may want to try the amp just with the larger cap and roll the bass control back a little for your humbuckers. If it sounds ok like that there's no need for the switch.

[Pryde]

Thanks, I will do some experimenting.

would I hear any real audible difference between say a 22uf or a 10uf cap with a guitar?

[Enzo]

Maybe, it all happens within the context of the entire amp.

When I putz around with values, my general rule of thumb is to use at least a factor of 2 in value. Your example meets that minimum. In other words select between 10uf and 20uf instead of 10uf and 11uf.


it is a cathode cap, so there is little voltage across it. You can experiment with almost any cap in your drawer. A quick tack solder in place and give a listen.

[Pryde]

Well I tried a couple values on a temporary switch (10uf and 22uf) to have a listen and honestly don't hear no difference between the 10 and 22 and very minimal difference with both compared to the stock 1uf cap.

Could be that I am testing into a couple junkie/cheap speakers in a 2x12. But the difference is not enough to drill a hole or buy a push/pull pot.

Might just replace the stock 1uf for a 4.7uf to boost a bit more bass for a happy medium and call it good?

[Gingertube]

See the graph at the top of this link for bypass cap value vs frequency response.
The Valve Wizard - Other Stuff
Then look at the chart just under it. The open bottom E string is 82Hz.
Not much use in cap values above 2u2 EXCEPT that bigger caps will shunt Mains Frequency (from AC heaters) noise better. I generally use 10uF as the maxcimum cap value for Clean Channel. That is adequate for even Bass Guitar.
If you choose to use 4u7 then polyester caps are readily available and you don't need to use electrolytics.
Cheers,
Ian

[Pryde]

Thanks gingertube.

I have seen the frequency response chart before and always wondered why a larger cathode bypass cap would ever be used since there is no audible difference (especially with guitar). Not sure how a 25uf would make the bass "more mushy" than a 4u7 since you can't hear the boosted frequencies past a certain value?

In any case, my experiment did not yield any real dramatic difference even between the 1uf and 10uf. There was a very slight bass freq gain I could hear but not much.

I still want to experiment with a better speaker/cab though to get a better picture.