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:
| component | original value | modded value |
| gain switch | section 1 open with switch up | section 1 closed (see above) |
| D3 | 1N4148 | red led |
| D4 | 1N4148 | closed |
| D5 | 1N4148 | red led |
| D6 | 1N4148 | closed |
| C11 | 15nF | 100nF |
| C12 | 39nF | 100nF |
| C13 | 270pF | 100pF |
| R4 | 220K | 470K |
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.
electric-safari 
can you share the LTspice schematic?
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As soon as I can, Stephan
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Hi, you can find the content of the asc file in a comment below.
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WordPress does not allow me to upload asc files for LTSpice. Try to copy the lines below to an empty .asc file:
Version 4
SHEET 1 1684 884
WIRE 384 -176 -48 -176
WIRE 208 -112 112 -112
WIRE -48 -64 -48 -176
WIRE 112 -48 112 -112
WIRE 896 -16 752 -16
WIRE 1120 -16 976 -16
WIRE 1280 -16 1120 -16
WIRE 1472 -16 1344 -16
WIRE 384 0 384 -176
WIRE 480 0 384 0
WIRE 208 16 208 -112
WIRE -320 32 -352 32
WIRE -144 32 -176 32
WIRE -48 32 -48 0
WIRE -48 32 -144 32
WIRE 176 32 -48 32
WIRE 1120 48 1120 -16
WIRE 1472 48 1472 -16
WIRE -144 64 -144 32
WIRE -48 64 -48 32
WIRE 176 64 144 64
WIRE -352 96 -352 32
WIRE 208 112 208 80
WIRE 384 112 384 0
WIRE 384 112 208 112
WIRE -144 144 -144 128
WIRE -48 144 -48 128
WIRE -48 144 -144 144
WIRE -32 144 -48 144
WIRE 1472 160 1472 128
WIRE 1680 160 1472 160
WIRE -48 176 -48 144
WIRE 144 192 144 64
WIRE 160 192 144 192
WIRE 240 192 240 48
WIRE 1472 192 1472 160
WIRE -352 240 -352 176
WIRE 144 272 144 192
WIRE 160 272 144 272
WIRE 1120 304 1120 128
WIRE 1280 304 1120 304
WIRE 1472 304 1472 272
WIRE 1472 304 1344 304
WIRE 144 352 144 272
WIRE 160 352 144 352
WIRE 240 352 240 192
WIRE 240 352 224 352
WIRE 144 400 144 352
WIRE 240 400 240 352
WIRE 1472 400 1472 304
WIRE 496 464 240 464
WIRE 1472 480 1472 464
WIRE 144 496 144 464
WIRE 1120 544 1120 304
WIRE 1280 544 1120 544
WIRE 1472 544 1472 480
WIRE 1472 544 1344 544
WIRE 144 592 144 576
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WIRE 496 592 288 592
WIRE 576 592 496 592
WIRE 752 592 752 -16
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WIRE 1200 640 1120 640
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WIRE 1472 640 1472 544
WIRE 1472 640 1424 640
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WIRE 752 704 752 672
WIRE 752 704 288 704
WIRE 1120 704 1120 640
WIRE 1328 704 1328 640
WIRE 1328 704 1120 704
WIRE 1344 704 1328 704
WIRE 1472 704 1472 640
WIRE 1120 720 1120 704
WIRE 1328 720 1328 704
WIRE 288 736 288 704
WIRE 1472 864 1472 784
FLAG -352 240 0
FLAG 288 736 0
FLAG 112 -48 0
FLAG 480 80 0
FLAG -48 176 0
FLAG 1472 864 0
SYMBOL voltage -352 80 R0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC 200mV 0
SYMATTR InstName V1
SYMATTR Value EXP(0 200mV 1KHz)
SYMBOL cap 224 336 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 150p
SYMBOL cap 128 400 R0
SYMATTR InstName C2
SYMATTR Value 220n
SYMBOL cap 224 400 R0
SYMATTR InstName C
SYMATTR Value 4.7
SYMBOL res 128 480 R0
SYMATTR InstName R2
SYMATTR Value 2200
SYMBOL res 480 448 R0
SYMATTR InstName R3
SYMATTR Value 2200
SYMBOL res 272 576 R0
SYMATTR InstName R7
SYMATTR Value 5000
SYMBOL res 128 576 R0
WINDOW 0 6 41 Left 2
SYMATTR InstName R6
SYMATTR Value 5000
SYMBOL voltage 480 -16 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 24
SYMBOL OpAmpsUniversalOpAmp2 208 48 M180
SYMATTR InstName U1
SYMBOL res 256 176 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 220k
SYMBOL res 512 688 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 220k
SYMBOL diode -32 128 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMBOL diode -32 0 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D2
SYMBOL cap -160 64 R0
SYMATTR InstName C3
SYMATTR Value 150p
SYMBOL cap -256 16 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 100n
SYMBOL res -160 16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL cap 640 576 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 22n
SYMBOL res 736 576 R0
SYMATTR InstName R8
SYMATTR Value 220k
SYMBOL res 1104 32 R0
SYMATTR InstName R9
SYMATTR Value 68k
SYMBOL res 1456 32 R0
SYMATTR InstName R10
SYMATTR Value 120k
SYMBOL res 1456 176 R0
SYMATTR InstName R11
SYMATTR Value 120k
SYMBOL res 1456 384 R0
SYMATTR InstName R12
SYMATTR Value 120k
SYMBOL res 1456 688 R0
SYMATTR InstName R14
SYMATTR Value 470
SYMBOL cap 1344 -32 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 270p
SYMBOL cap 1344 288 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C7
SYMATTR Value 22n
SYMBOL cap 1424 624 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value 39n
SYMBOL cap 1344 528 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 47n
SYMBOL res 992 -32 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R13
SYMATTR Value 22k
SYMBOL res 1296 624 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R15
SYMATTR Value 1000k
TEXT 328 208 Left 2 !.ac oct 2 20 20000
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well, is not complete, also where do I get OpAmpsUniversalOpAmp2?
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I mean is it just half the IC and no tubes?
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Yes, I did not draw and simulate the entire circuit, only the preamp. I know that is not perfect, but was enough to start adjusting the values. Regarding OpAmpsUniversalOpAmp2, I have it already with the standard version, I think. Do you have the latest version?
You can also download it from: https://github.com/evenator/LTSpice-Libraries/blob/master/sub/UniversalOpamps2.sub
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yes, I have the latest version and it complains about that SYMBOL
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latest version still having trouble with that SYMBOL, what component is it?
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Ok, there is a mistake in the text above, when it says:
OpAmpsUniversalOpAmp2
It should say:
OpAmps\UniversalOpAmp2
WordPress has eaten “” characters when pasting the text, sorry for the inconvenience.
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ok thank you very much indeed, I will complete the schematic and put an opa2134.
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Greetings , I also have this amp and have the same thoughts on the sound. Ok sounding with a Les Paul but not so great with a Strat. I have simulated the pre-amp as well and was surprised at the amount of roll off. I went ahead and changes some values but feel I need to adjust tone some more so I will try your mods as you have described. Wondering what your reasons were for changing the op amp?
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Replacing the op amp in the simulation? I simply did not find the right one, and op amps don’t change too much. Tolerance of components contribute much more to the differences between a simulation and the real thing.
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