Removing tone control on Trace-Elliot Velocette amp to use it with bass

Question

I have a Trace-Elliot Velocette that I'm very satisfied with. This amp works very good also for recording bass, but the low notes will be much less sturdy when recording through this amp than if I record through line. Of course I can use a line box and mix the different sounds, but since I also practice bass with the amp I've been figuring to put a switch which would disable the tone control. I could then switch the tone control on and off regarding if it's needed.

1. Would this be a good idea?
2. Is it usual that guitar amps do filter a lot in the lower registry, or is this due to the speaker?
3. What would the adverse effects be when the filter is switched out? Would there be low frequency (50/60 Hz) hum?

Here's a service manual for the amp. Looks like I should make a switch that changes C3 (input) and a switch that disconnects the tone control altogether.

Looking forward to your thoughts on this.

Answer 1

You could add a switch which adds a higher capacitance in parallel with C3. Label its off position "low cut". The low value of C3 is what is rolling off bass response. Elsewhere, note how 10X bigger 22 nF couplers are used.

It is usual for guitar amps to roll off bass. Fairly aggressive bass roll off is needed, in particular, to achieve "chunky" distortion in high gain channels. If bass is not rolled off, you get a "woody" kind of sound on the low notes. Though useful for some styles, it doesn't respond to palm muting.

The manual even talks about C3; it's a slight roll-off so the sound is not too muddy (exactly). But since bass goes an octave lower, what is slight in guitar is no quite so slight in bass!

Don't worry, the tone control section isn't what filters out 60 cycle hum; that's the job of the power supply reservoir capacitors like C22 and C23. You can easily bypass the tone section with a double-throw switch. The volume level will be louder with the bypassed tone control, since it is a passive block which costs you some decibels.

I think you need some tone control for the bass though. Google for schematics for some Fender Bassman schematics. The tone section topology is identical to yours, and the part values are not that far off. There is some software out there for simulating these kinds of tone controls; might be a good idea to try. I see that in your tone stack, the bass and treble control are linked together as a 250K dual-ganged pot, and there is just a fixed resistor where the middle control would be. You could easily change this to a three pot tone control with separate treble, middle and bass, with a DPDT switch to flip a capacitor or two between different voicings.

Answer 2

In simplest terms, guitars and basses have specific frequency ranges defined by the typical lowest and highest notes on the fretboards.

Amps AND speakers tend to be designed and/or chosen for their performance in these frequency ranges.

So yes, "guitar [combo] amps filter the lower registry" in that they are not generally intended to be as responsive in these ranges.

I do not think that your tone control is going to make much of a difference. The real answer is to get a "line box" as you describe, or get a bass amplifier. If you are going to purchase the line box, a cheap low watt practice amp for bass is probably not much more money.