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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.

UPDATE:

It turned out that the schematic linked above is not exactly corresponding to the Velocette I own. It seems it's more like the one below, in which the input capacitor C3 is rated at 100nF with a 1MOhm resistor after it, which I guess will give a cut off frequency of 1,6 Hz compared to the 723 Hz cut off with the 0.22nF in the schematic above (calculations done at this page).

Trace Elliot Velocette schematic Trace Elliot Velocette schematic source page

I still think the amp has bad response in the bass spectrum, but that doesn't seem to relate to the C3 cap.

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1  
Yes, with that 100nF, the input bass response is very good. The next thing that causes bass roll-off are cathode resistors bypasses with capacitors. Both triode stages in the input have this arrangement. By itself, the cathode resistor degenerates gain, for DC and all frequencies. The capacitor which bypasses it increases gain, but not all the way down to DC. The result is a bass roll-off. I'm not enough of a tube freak to know how to calculate this from the top of my head, but check this out: ampbooks.com/home/amplifier-calculators/cathode-capacitor –  Kaz Jun 11 '13 at 1:00

2 Answers 2

up vote 1 down vote accepted

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.

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Thank you so much! This made things much clearer! I'll post a link to info about the mod here when it's done. –  PetaspeedBeaver May 22 '13 at 7:35
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Be careful around the high voltages surrounding the tubes, and use capacitors with appropriate voltage ratings in tube circuits! Good luck. –  Kaz May 22 '13 at 23:26
    
There are other variables at play here as well, the grid leak resistor (I assume it's a 1meg connected from ground to input grid on both halves of a 12ax7... not that I could tell by the schematic), anyways, raising that and or making the coupling caps (.022uF probably) bigger will increase your bass response. You'll probably still be limited by a non-full-range traditional C-core output transformer, and while I'm not familiar with this amp at all... when there's a speaker in the schematic, I'd guess it's a combo, so you probably don't have much bass response from the speaker either. –  David Axtell Moore II Jun 24 '13 at 10:43

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.

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