Author Topic: Modifying a practice bass amp for guitar  (Read 2773 times)

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Modifying a practice bass amp for guitar
« on: July 10, 2013, 11:29:59 PM »
Hey everyone,

I know we talk stompboxes here, but I respect the vast amount of knowledge on this forum and thought I'd try my luck here with these questions....feel free to hit me with your feedback if it's not the right place.

I've got an old bass practice amp, a 10 watt Kustom KBA10.

I no longer play bass, but I'd love to have a small amp to lug around, test pedals on, etc. Unfortunately, it sounds pretty crappy - trebley, lifeless, clearly not made for guitar. I have some thoughts, but more questions, about how to make it more guitar-friendly:

1. I know that some stompbox circuits can be adapted for guitar or bass by changing components like input capacitors. Are there any basic components to a solid state amplifier that I can look for to change it into a more guitar-friendly amplifier? I know my way around a simple effect circuit, but not so much about amps....and unfortunately I can't find a schematic for this. Any thoughts to point me in the right direction would be great (new speaker is the obvious thought, I'm wondering what else might be done).

2. Ultimately, I may just gut the thing and turn it into a battery powered ruby or little gem. However, I also really like the idea of the amp-simulator pedals...anyone have experience with building one of these straight into an amp? Any advice for going down that road would be much appreciated.

I thought I had more questions when I started this post....guess I'll ask as they come. Thanks for the help!



Re: Modifying a practice bass amp for guitar
« Reply #1 on: July 10, 2013, 11:51:26 PM »
The solid state power amp section is probably good, no need to power with a battery... You just need to modify or build a new preamp section and run it into the power amp portion of the circuit. I would take it a part and get some pics from both sides of the board... If it were me I would trade or sell it and buy a 10 or 20 watt practice amp for guitar... They are so dirt cheap these days. But, if you are doing it because you want to build something with your own hands that's cool! My guess is that it is very simple inside...
Doesn't matter what you did to get it... If it sounds good, then it is good!


Re: Modifying a practice bass amp for guitar
« Reply #2 on: July 11, 2013, 01:22:59 AM »
> it sounds pretty crappy

#1, change the speaker (or hot-wire to a known-good guitar-amp speaker). It was apparently a low-price amp, wouldn't have a great speaker, and being for-bass it could even have a speaker they couldn't sell to guitarists.

The amp itself is *probably* fine, inoffensive, maybe unexciting. For guitar (with a good guitar speaker) you normally want a little top-boost, maybe mid-dip and bass less/more. A simple (external) preamp with a Fender-style 2/3-knob tone control will do this. A touch of overdrive is often useful. All this (except good speaker) can be done without opening it up or understanding what is inside.


Re: Modifying a practice bass amp for guitar
« Reply #3 on: July 11, 2013, 02:31:52 AM »
I''ll concur on the speaker thing. Its makes a world of difference. If you're just playing the amp clean the stock EQ probable has enough range to cover typical guitar use too.


Re: Modifying a practice bass amp for guitar
« Reply #4 on: December 20, 2015, 02:07:05 PM »
Sorry to raise the dead, but I'm hoping someone else out there may also be interested in this as a learning exercise.

So I changed the speaker in this thing, but I feel like it could still sound way better (it frankly never sounded any good on a bass either). As I sit here on a rainy day, I couldn't help but take the amp apart to look at the circuit. I've traced out the preamp section to the best of my ability, and I'm curious if there is anything glaring in here that may be really detrimental to a guitar tone. I'm pretty inexperienced with tone stacks, but I notice this one looks a bit different than the standard fender/Marshall. Is there anything that could be adjusted here to make it sound better for guitar?

Look, I know it's a cheap practice amp, but people around here also rave about their battery powered Ruby amps. I figure this already is set up for a few watts of clean power, so if there are some simple tweaks on the preamp to make it more friendly for guitar frequencies and add an easy control for some dirt (pot in the feedback loop of the second op amp??), why not go for it?

Thanks for any ideas!
« Last Edit: December 20, 2015, 02:11:01 PM by drummer4gc »


Re: Modifying a practice bass amp for guitar
« Reply #5 on: December 20, 2015, 07:42:02 PM »
Hi, neat project.

You could try making the 1n treble cap 470p...change the bass pot to 1M.  Play by ear, and if anything still sounds off, try halving the other caps (the 2 100n caps, first).   That should make a pretty big difference, IMO. 

You can also make the output cap (33u now) smaller, try different values.  That will make the amp brighter.   You can play with the 1u feedback cap in the 2nd opamp section, and the 22uF to ground there, too.    They seem big.

For reference, you can play around with this free software (really nice piece of software!) to get a sense of what is going on, and what SHOULD be going on:

Look at the tab for "Marshall" in the program...
« Last Edit: December 21, 2015, 09:01:26 AM by GibsonGM »
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Re: Modifying a practice bass amp for guitar
« Reply #6 on: December 21, 2015, 09:05:13 PM »
> anything glaring in here that may be really detrimental to a guitar tone.

I don't see a stinker.

The first stage is a standard on many guitar amps. It goes clean, or can turn-up until it clips.

The '4558 is perhaps not the shiniest opamp in the world. If socketed, try TL072 there. But it may not be a big difference.

The tone-stack is, as Mike hints, ripped-off from the Fender and Marshall classics. Except scaled-down in impedance to better suit opamps. And a difference how the MID is connected, which probably makes little real difference.

I don't think changing pot values is the way to go: they were picked to suit the opamps. The caps can be scaled-up the same ratio as the pots were scaled-down, to get same EQ at a different impedance. However I don't see any real problem here. EQ shapes are very similar to Fender/Marshal shapes.

The post-EQ booster is quite ordinary and inoffensive.

Guitar *might* favor another gain/volume pot between 2nd opamp and the TDA2040. Then turn-up the first Vol control to distort the opamps, turn-down the second Vol control so the TDA2040 does not distort. This may not be much use; don't drill the box until you hay-wire a pot out the back and play with it a while.

Did you take the back panel off? (If it comes off without great harm.) This may just make it gutless, but may give a brighter sound. Overall an Eight is too darn small for bass and not really big enough for guitar; IMHO you should always aim for 10" or 12". An Eight won't survive bass without a sealed box. On guitar it should survive. An Eight in a not-too-small open-back box will give more "throw" than a closed-box Eight. But this won't polish a turd.

It may be an option to post it on CraigsList and put whatever value it has toward some more suitable amp.


Re: Modifying a practice bass amp for guitar
« Reply #7 on: December 03, 2017, 05:39:05 PM »
I put an opamp socket in my KBA10.  Clean, TL072 sounded no better, OPA2132 sounded much better.  I don't like the overdriven opamp sound so I tend not to overdrive this.
Cheap mod for a cheap amp which you can play around with easily.
I suspect Blue LED will also make it sound better.

Rob Strand

Re: Modifying a practice bass amp for guitar
« Reply #8 on: December 04, 2017, 04:05:39 PM »
The biggest difference between bass amps and guitar amps is guitar amps have power amplifier with low damping factors.   It emulates the damping of a tube amp.  Look at the added resistor R95 near the speaker.  The correct damping depends on the speaker impedance in the amp.

What is does is add brightness (and a bit of bass).

A way to emulate this is to add a series R + C network across the existing 2k2 + 10uF network on the second opamp.   Maybe start with 2k2 + 56n.   Decrease the 2k2 for more high-end.  Decrease the 56n to change the voicing for more sparkle less midrange.    You will see this on SS amplifiers also.   IMHO it never sounds quite as good as the damping method.

You will find amps with small speakers say lest than 6" need a different recipe as the high-frequencies are much different to a 12".

For either methods you can add a switch for guitar/bass.

Beyond that rolling off the lows like others mentioned.

The mind often distorts without gain.


Re: Modifying a practice bass amp for guitar
« Reply #9 on: December 05, 2017, 02:05:11 PM »

If it's a more clean with acoustic sheen you're after...

Raise the input impedance. The 220k on the input can be higher. 1M is good. That should right away get you more "air", especially with humbuckers.

The input is DC coupled. Bi-polar Op-amps put out input bias current. This can make the guitar volume a little scratchy. Together with the change to the input resistor, insert a 4n7 film type cap between the input and socket tip contact. This will help keep out low-end rumble and hum that may get you a bit more clean headroom.

You can probably use some of the back board. I think there may be some kind of magic ratio to how much of the back is closed and you'll find many guitar combo's with only the lower part backed -  it at least makes a cable/junk stash area. I've looked up what that magic ratio might be, but every source seemed to differ on what it is.
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Re: Modifying a practice bass amp for guitar
« Reply #10 on: December 07, 2017, 06:12:36 AM »
Secondary issue of interest, but the use of relative big electros (like 33/22μF) isn't (or shouldn't be) advisable..
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