Category: Uncategorized

Modifying Kustom Tube 12A for Enhanced Tone and Overdrive

manolonte13 Comments

I adquired recently a second-hand Kustom Tube 12A little combo, very cheap. This is a hybrid solid state / tube amplifier, with a 12AX7 tube in the preamplifier section. It has two modes, clean and gain. In the clean mode, the two sections of the tube are pushed not too hard, so they do not compress too much and the sound is very clean. It has a gain potentiomenter that acts only in the gain mode. In this “gain” mode, the tube is pushed harder, but there are a quartet of clipping diodes between the two sections of the tube that distort the sound in a solid state style (hard clipping).

It has a “shift” button that does a noticeable change in the tone, perceived as a mid range boost (in fact it is a shift in the scoop frequency).

Out of the box, its volume and tone are surprisingly good for its size and price. I am almost sure that this amplifier is the same as the Bugera BC15, announced as 15W, while the Kustom is announced as 12W. The features are almost the same, although the Kustom has some additional speaker and line outputs. The declared power difference makes me wonder if they are really the same thing, maybe the circuit is slightly different, but I would try the mods I propose here on the Bugera.

Why modding it? There are one thing that annoyed me from the first minute. If you want a really compressed and overdriven sound, but no too distorted, you have to push the gain button and turn the gain potentiometer, but then the clipping diodes start working and distort the sound. There is no overdriven but not distorted sound, something in between, coming only from the tube. You can see here the schematic (clipping diodes were wrong in the original):

When you push the gain switch, the first stage of the operational amplifier is controlled by the gain potentiometer, allowing to increase the gain of the circuit. That potentiomenter acts as a gain and volumen control for that stage at the same time. The gain switch also shorts R15 resistor, making the clipping diodes and R14 go to ground and start clipping the signal.

Making the gain potentiometer work without hard clipping

The first thing to do is shorting the first part of the gain switch to ground, so the gain potentiometer is always working, even if the clipping diodes are not connected to ground. I did this as you can see in this picture:

Pin 2 of the gain potentiometer is connected to a ground pin visible in the input jack. The effect is the same as if you connect the section one of the gain switch.

Controlling the clipping section

I replaced also the clipping diodes with led diodes, to get a higher clipping voltage and more control over the distortion, by trying different colors. Since the diodes are coupled in pairs, you can put a led diode for every two diodes, getting a similar but slightly different effect for the ear. Using different led colors you can get different voltages. One red diode has almost the same Vf as two signal diodes, but the sound is not the same, because they do not clip in the same way. Trying different diodes is very satisfying. I made all the modifications by using component sockets, making it easier to test different values and diode types.

Enhancing the tone

The first thing I perceived after doing this first modifications was that with the gain switch off, the tone was too dark, even with the “shift” button pressed. So, after some LTSpice simulation and testing, I modified the tone section, changing C11, C12 and C13.

More gain, please

Another thing I noticed was that the overall gain was helping not too much for getting a really compressed sound with the gain switch off. I did not like the range I got from turning the gain potentiometer. I tried different values for R4 resistor (feedback resistor), which together with the gain potentiometer, controls the gain of the first opamp stage. Raising its value, you get more gain (basic opamp theory).

All mods in one place

In summary, I made this modifications:

componentoriginal valuemodded value
gain switchsection 1 open with switch upsection 1 closed (see above)
D31N4148red led
D41N4148closed
D51N4148red led
D61N4148closed
C1115nF100nF
C1239nF100nF
C13270pF100pF
R4220K470K

Conclusions

After this changes, the amplifier behaves more like a tube amplifier with two channels, controlled by the “gain”switch. The mid scoop is shifted to higher frequencies, making it more mid range focused and the high frequencies are more pronounced. With the gain switch off, turning the gain potentiometer, the sound goes from clean to tube overdriven, and with the gain switch on, you get all the range from overdriven to distorted/fuzzy.

For its size and price, altough it does not sound like a 12W tube amplifier, it does the job for home practice and gigging in little venues (still to try). It did before the mods, but now it is much more usable for my taste.

BOSS OS-2 Frequency response in different settings

manolonteLeave a comment

BOSS OS-2 is a good platform for illustrating the difference in frequency response between overdrive and distortion. I have tested my modified OS-2 (see BOSS OS-2 Overdrive/ Distortion MOD) with REW software and my laptop soundcard. It is very simple, you just have to connect the speakers jack to the pedal input and the mic jack to the output. And power the pedal, of course. To make it a little more complex and accurate, I made an adaptor that connects the left channel directly from speaker jack to mic jack and the right channel passes through the pedal. You can configure the software so that the left channel acts as a reference and discount the effect of the frequency response of the sound card itself. Output level has to be kept below some level, in order not to saturate the BOSS buffer, check it with an amplifier connected to the output of the pedal or with an oscilloscope or software tool (more on this in a future post).

REW stands for Room EQ Wizard, and is intended to measure the acoustics of a room and help in equalizing the sound in it. But one of its features is generating a sweep of sine signals and measure the frequency response of a system in front of those signals. While measuring you can see the harmonics, the average 2nd and 3rd harmonic level, and at the end you see the average frequency response for the whole sweep. The software is “donationware” and works in Windows, Linux and MacOS.

Below you can see the different settings I put under measurement:

os-2_blue
Overdrive at max gain, blue

os-2_purple
Distortion at max gain, purple

os-2_green
Distortion at medium gain, red

os-2_red
Blended overdrive and distortion, medium gain, green

And the frequency response of each of the settings:

allfr

As can be seen, distortion settings have a very characteristic “scoop” at middle frequencies (800Hz) that make them more flat sounding. A peak around 100Hz can be seen (and heard as I mentioned in a previous post).

In overdrive settings, on the opposite side, a prevalence of middle frequencies around 500Hz can be seen, what makes it more pleasant and cutting through the mix, as they say.

Combining both (green curve), you have less scoop and the prevalent frequency can be higher, giving more presence to the tone.

 

POP: Autoovation feature

manolonteLeave a comment

Now POP has a new feature, that I call “autoovation”. You start playing with your favorite effect, and when you stop playing, sometimes, if the pedal likes your performance, you hear an ovation from a digital audience.

Autoovation is made basically by detecting audio activity in input, then waiting for silence and playing a file randomly from a list of mp3 recorded ovations. For audio detection, silentjack (https://www.aelius.com/njh/silentjack/) was used (thanks Nicholas J Humfrey for your work and for letting me add a feature to your code).

This feature is just a joke, of course, but gives an idea of the kind of things that can be done with a device like this one. Think about things like backing tracks, automated rythm boxes, etc.