Number 5 is alive

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Modding - Stage I

In this first step I just want to optimize the simple structure of the amp and add some small extensions as a warmup:

• Tuning of parts
• Divide the controls into gain and master control
• Add a 3 band tonestack (Marshall Style)
• Power reduction and clipping

Here´s the schematic in which extensions and changes are marked in different colors.
This schematic is quite a bit different than the original one

The changes in detail:
The resistors R1,R8 and R9 were tuned to give it a more “normal” style and serve as a better basis for the follwing mods. Changing R8 and R9 decreases the operating point of the preamp slightly

Capacitor C1 has been changed to a Mallory, and changing its value reduces the bass response. You can use other values here, but keep in mind to use a high quality capacitor, because this capacitor has a fundamental effect on the sound, even if you keep the same value. You can also use Orange Drops instead of Mallorys.

The first major change comes with the change of the former master pot to a gain pot. You´ll need to do some metalwork on the chassis. Since other pots will be added, you should drill all the holes now. Where to put those holes is your decision. They should be positioned with some consideration. How you could position them is shown in the pictures below. After the drilling is done you can assemble the new pots and warm up the soldering iron. ;)

The old master volume and its PCB are to be removed, as is resistor R7. The connection between the PCB and potentiometer is done via plugs, labeled as „IN“ - „G“ and „SIO“ which is left there as it is,- desolder the wires directly at the potentiometer. After this is done, the wire labeled as „IN“ is soldered at the input of the pot and „G“ to the output. „G“ provides ground for the potentiometer. Solder two two new resistors directly to the wiper. Resistor R17 can be soldered to the „G“ point for ground and together with R16 to the „SIO“ Wire on the other side.
Before soldering, take some time to look at the PCB and the parts to see where the connections are, because it´s important to avoid wiring failures here.
This is important for the next step, the assembly of the new master and tonestack, because the signal has to go from the PCB to the new master and tonestack and back to the PCB.

By removing of the resistor R5 a solder pad is freed. You can get the input signal for the tone stack from here. The resistor R15 must be removed at one side (must be removed and replaced by a 5,6k) so that it has no connection to C2. It´s pointing somewhere in the air as you can see in the pictures. It will be connected to the output of the master potentiometer. At this point the signal comes back from the tonestack into the master pot of the amp.

The wiring of the tonestack and the positioning of the parts seems to be quite difficult and needs some practise. Take your time to think about it before you start. The important part is the output of mid potentiometer. It must be wired to ground or the complete tonestack will not function properly.

The easiest way to do so is to connect it to the output of the gain potentiometer which is already grounded („G“-Point). This completes the second stage, if everything is wired right. You should now have a properly working amp with gain, master and a tonestack and in the end it should sound a little bit like an old plexi.

Tone-Stack	Gain und Master Poti
Gain-Master-Tonestack	Signal OUT - IN

If the amp doesen't work after the mods, keep calm, get some coffee. Recheck the wiring as often as may be required to find the problem.
At the end of this chapter another topic I like to talk about, because it´s simple and yet effective. Diode-clipping... done right.

The amp in its stock state is rather loud, more than normal bedroom level and doesn't begin to distort until the powerstage is driven rather hard. To get some more distortion at lower levels you can use the diode-clipping that has been used in the TT-SAM. This is accomplished with LEDs (3mm red are good), a capacitor and a switch (if you want to make it switchable).

The result is a reduction of volume of about 50% with more distortion (by the diodes).
A possible point for the diodes is between capacitor C2 and the input of the tonestack.
You can test the distortion by simply soldering it in and testing it. If you´re happy with it assemble it the way you want it (switchable or not).

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