Pentode Bass control

[ChrisM]

Anyone have a good circuit for a bass control coming off of a pentode's plate?

The pentode has a high output impedance so that is the big consideration...

Thanks

[matt h]

(deleted)

[Dingleberry]

Anyone have a good circuit for a bass control coming off of a pentode's plate?

The pentode has a high output impedance so that is the big consideration...

Thanks

I've been thinking the exact same thing. Planning to build a GA-40 Les Paul amp variant and it would be nice to have a little more control over the sound. I have one C3M pot on hand so when I came home from gigs on sunday I will just probably try that "Dongbiter" approach as it is probably the simplest form of "bass control" and see how it sounds.
-T

[matt h]

(deleted)

[ChrisM]

Before the Dongbiter was a thing, a very similar circuit specifically for pentodes floated around the less popular corners of the DIY guitar amp world. If you think of it like a variable-bass-bypass coupling cap, you can see both the simplicity and the effectiveness of it. It works a lot like the rotary switch controls that amps like matchless (and all that followed) use, but is infinitely adjustable and doesn't require six caps and a rotary switch.

With pentodes, you're typically looking at a much higher internal ra, a much larger value Rp (plate resistor), and a much higher output impedance. As much as the 3M pot was used for the dongbiter, I suspect satisfactory results could be had with a 1M if its onhand. You'd be using much lower value caps for the pentode anyway.

So what is this variable coupling cap circuit you are talking about? A 6 throw rotary switch isn't bad but I would prefer a pot.

[matt h]

(deleted)

[ChrisM]

Forum search Dongbiter.

There is 5 threads on it. No schematics

[matt h]

(deleted)

[ChrisM]

Right, just words and pictures. (Edit: pictures as both gut shots, gut shots of clones, and a layout)


It's a cap of a smaller value that's always in the circuit. If you need a starting point, take a small value from the matchless 6 cap design.

in parallel with another thing. This other thing is a larger cap in series with a pot-wired-as-a-variable-resistor.

As the value of the pot (as variable resistor) decreases, it lets more of the big cap into the circuit.

Since two caps in parallel results ina larger effective capacitance, there's your bass control.

Cool
I've seen this done in pedals before. I will give it a shot!

[ChrisM]

So based on my design a 1nF cap off the plate will give me a cutoff frequency of ~200Hz. A 100nF cap will provide a full frequency response all the way down to ~5Hz.

Now on the breadboard I set the 1nF cap in parallel with a 1M(B) pot in series with the 100nF cap. I can tune in 1.1nF to 101nF. So there is a little leakage as noted by the 1.1nF value (note 1nF would be total isolation). Overall I really like this idea for a variable coupling cap.

Why 3M(C) pot though? Such a high resistance is not needed it seems and why reverse audio taper? I am thinking a linear taper is what is needed here.

[Structo]

I have wondered about using a variable capacitor for tuning in a cap.

Such as we do with pots.

What is the problem with using a variable cap?

Power?

Range?

[matt h]

(deleted)

[ChrisM]

And also very small in actual capacitance

They are used in RF equipment where you can get away with the tiny capacitance since the frequency is so high.

[matt h]

(deleted)

[ChrisM]

Anyway, Chris, I am glad to hear you had success and my SWAG of a 1M pot hit the mark. Any additional writeup for it? Impressions? A report for future generations of TAGsearchers?

SWAG lol I like that one!

Probably won't be at least a 6 months before I actually build an amp with this circuit. Lots of other design (and life) work to do beforehand