Kustom 12 Gauge amplifier mod!

 The Kustom 12 Gauge amplifier is an affordable 16Watt solid state combo with a 12" on board guitar speaker! The 12" speaker is what makes this practice amp so special.

Here are its original specs:
RMS Wattage: 16 Watts
Speaker: 12"
Channels: Lead & Rhythm
Tone Controls: Bass, Middle and Treble
Effects: Analog Reverb and Echo
External Speaker Jack: Yes
Headphone Jack: Yes
C D/Tape/iPod Input: Yes
Dimensions(inch): 15.9H x 17.1W x 9.8D
Dimensions (mm): 405H x 450W x 250D
Weight: 25 lbs/ 11.3 kg 
And if you think 16Watts of solid state power are not loud enough, think again! The 12" high sensitivity speaker can make this amp scream like a much more powerful combo. Why's that? Let's find out and debunk a few myths.

Watts V.S. deciBels
Power amplifier Wattage is what most guitarists look for when buying a new amp. Well, these wattage numbers could be a little overrated as you cannot tell how loud an amp will sound like just by reading its power output in Watts RMS. PMPO wattage numbers and stuff like that, are usually a big fat lie. The actual usable power output of an amp is measured in RMS(rout mean square) Watts. Nevertheless, what you should look for is the maximum deciBels produced by the speaker(s) of the amplifier.
What speaker cabs and speakers produce is sound pressure levels (SPL). The amplitude of the sound waves creates a pressure on our ears and that's how sound level is perceived by human beings. Sound pressure is measured in dBs or deciBels. The amplifier manufacturers usually measure the dBs produced by a speaker at a certain frequency (e.g. 1 kHz) with a certain power input (1Watt rms)  at a certain distance from the speaker (e.g. 1 meter). The result of this measurement is called "Speaker Sensitivity". Large 12" guitar speakers can have a sensitivity of 100dB/1W/1m or more! What that means is that you can actually hear more than 100dBs of sound pressure if you are close to your amp's speaker and its power amp produces only 1Watt of power!
Doubling the power output of the power amp can only give you an extra 3dBs of sound pressure(e.g. 100dBs @50W become 103dBs @ 100Watts). That's not much of a boost... 3dB is a barely perceptible change of sound pressure. So, a 32Watt power amp sounds only 3dBs louder than a 16Watt amp on the same speaker. It sounds almost the same to our ears is terms of SPL, "Volume" or "Level" as we call it.
This is why a16 Watt practice amp with a 12" speaker can scream like a much higher powered amplifier that also uses a 12" speaker.
So how loud are 100dBs of SPL? Find out here:
http://www.gcaudio.com/resources/howtos/loudness.html

Sounds:
 This little combo sounds big and the tone is very convincing. I 've tested quite a few solid state, digital and tube amps over the years and I can tell you that this solid state amp sounds pretty much like small high-gain tube amp. I've heard small tube amps with 3 X 12AX7s and EL84s producing the almost same amount of gain, the same OD tones and the same sound level. It's pretty hard to believe that Kustom came out with such a nice and simple solid state circuit.

The mod:
The only modification I had to do was to increase the input impedance of the circuit. Most amps have an hi-Z input impedance of more than 1Mohm. This one had about 200Kohms. Passive guitar electronics are highly affected by the input impedance of the amp ( you get less output and high frequency levels) and that's why I had to increase it.

Here 's the easy mod:  Undo the four screws on the back of the amp and there's your circuit board. Find the R33 resistor (should read about 220Kohms on a Digital Multimeter-DMM) and remove it from the pcb( Just cut one of its legs). Close the amp and test it out. If you 're happy with the result, stop here. If not, re-open the amp and place a jumper across the R1 (22K) resistor.

These mods will increase gain and treble. Also the amp will respond better to your playing dynamics. It's an easy mod that will make a nice little amp even better!

Here are a few pictures of the amp:

Cheers,

Thanos