Cathode bypass capacitor blend pot

[sgbrewing]

Hello,
Just wondering if a pot blending 2 cathode bypass caps, one larger and one smaller on the first stage would work?
I’ve tried searching but haven’t found anything on it so I’m just wondering if it actually would work.
I’m worried that the pots resistance would be in parallel to the cathode resistor and cause problems.

Thinking of using it as a bass control to fine tune the bass going into the circuit.

Cheers!

[Fresh_Start]

You might use a 1.5k potentiometer and blend in your larger capacitor similar to a Bassman presence control. However, the only panel mount 1.5k pots are really expensive. A trim pot would be more practical price wise.

Chip

[sluckey]

I've done something similar. Take a look...

     http://sluckeyamps.com/smoky/smoky.pdf

[Williamblake]

Cathode bypass cap is a shelving filter, it will only give so much frequency boost anyway. Do you really need a pot to adjust that amount or would you rather have a switch to select different caps for different cutoff/boost frequencies? To each his own, beauty is in the eye of the beholder.

[sgbrewing]

Was more thinking of using it as a bass cut. With a pot there would be more tweak-ability.
Was just wondering if the resistance of the pot would effect things?

[sgbrewing]

I've done something similar. Take a look...

     http://sluckeyamps.com/smoky/smoky.pdf

Cheers! Did you find that the resistance from the pot effected the bias at all?

[sluckey]

The cathode resistance remains constant so the bias is unaffected. However, the most noticeable effect is gain, not tone.

[sgbrewing]

The cathode resistance remains constant so the bias is unaffected. However, the most noticeable effect is gain, not tone.

Ok thank you, so I would need to calculate the value of the resistor and the pot in parallel to get the required cathode resistance?
So a 2k2 resistor with a 5k pot for 1.5k cathode resistance? I am I thinking about this wrong?

I was hoping to blend a 25uf and a 0.1uf on the input stage to affect the bass entering the circuit?

[T Wilcox]

Rather than 2 caps you can put something like a 22uF and a 50K potentiometer in series for variable shelving
The circuit I know that does this calls it the FAT circuit

[tubeswell]

For ‘maximum’ effect, make the variable bypass a ‘full bypass’ and the permanent bypass cap quite small, e.g, a 25uF e-cap in series with a 5kA or 10kA pot (wired as a variable resistor) to ground - in parallel with a 0.01uF (10nF) or even smaller permanent bypass cap (in parallel with your 1k5* cathode resistor). As Steve said, the perceived difference is more of a gain boost than a tone control.


*or larger e.g. 2k7 or 4k7 Rk. This will ‘centre bias’ the stage more (helping to keep the signal cleaner) enhancing the ‘tonal’ aspect of the variable full bypass control (but the perceived effect will still be more about gain boost)

[HotBluePlates]

Rather than 2 caps you can put something like a 22uF and a 50K potentiometer in series for variable shelving
The circuit I know that does this calls it the FAT circuit

Funny, that is the exact schematic fragment I was about to post!

[T Wilcox]

Funny, that is the exact schematic fragment I was about to post!
[/quote]

Its not perfect as shown but works. Iirc I tried both Log and Linear pots and both had a very small portion of the dial be useable. Never tried a Rev Log though.
The relay in that schem is wrong as well and should be in series between the Pot and ground

[PRR]

> a very small portion of the dial be useable.

A 50k pot against a roughly 2.5k resistor, I would expect like 5% of the turn to do anything. Values like 1.5k and 5k seem more likely.

But the maximum effect of a typical Ck is like 6dB, audible but hardly WOW. Use 0.68uFd to "lift the mids-highs", 10uFd for full bass, 25uFd for full bass and deep mud.