Stack-in-A-Box Modification, Tone

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Stack-in-A-Box Modification, Tone

Post by PAiA-Scott »

Don Kibbey wrote:

> Hello, I have a stack in a box that I recently built. The unit works
> fine, just wonder if there are some instructions for changing the
> tone. I've seen references to changing a resistor value but the
> instructions on the site did not specify which one. I fyou are aware
> of any such mods and can point me to a source for them I would
> appreciate it.

Hello Don.

It is good hearing your Stack-in-a-Box is working. I am not sure which site you are referencing but, linked below are details for a modification to get the Drive control operation in a range to complement the Crunch. The TubeHead operates much like the Stack-in-a-Box with one tube section into the other. Each have a gain control to vary the signal level into the first tube section (Drive on TubeHead and Crunch on SiaB). Each has a second control to vary the amount of signal into the second tube section (the symmetry trim on TubeHead and the Drive control on SiaB). With TubeHead, the Symmetry trim can be 'calibrated' so the overdrive of the tube section produces even amounts of clipping on the upper and lower peaks of the audio waveform. The modification to SiaB enables this same capability for SiaB. The resulting sound is with better fidelity, even if with clipping/overdrive distortion, and maximized signal throughput.

In the tips/suggestions for Stack-in-Box that follow, there is mention of substituting a greater valued potentiometer for the Crunch (input gain) control so guitar pickups easily provide enough signal to overdrive the tube. With extreme gain settings the op-amp even clips and this clipped signal overdriving the tube really makes for a nice sustaining, overdriven tube sound (the two channels of TubeHead series-connected do this too, but even better :-) , with four-tube sections and output gain adjust for lower final output volume).

About Tone change, the settings for the mic'ed-speaker-cabinet simulators are a sort of fixed tone control as are the Bright and Fat selections. The extra harmonics with the clipping/overdrive will make these more noticeable too. Try the various selections playing in different areas of the neck, with different pick-up selector settings, and pick-up tone control settings. You might find something you like, as is. If not, adding a kit such as the 6760 Parametric EQ or 9303 Four-Band EQ at the Fx patch points would certainly do the trick.

In this example, Stack-in-a-Box was used as a clean gain stage and connected to the high-level input on Quadrafuzz with settings to show distinct tone changes. Many other combinations are possible though.

Stack-in-a-Box, tips & suggestions:

The following checks and tests for the 9210 Stack-in-a-Box are for the power areas, then sections related to the sound or audio signal through the unit or lack thereof.

If the power LED is not lighting double-check to be sure the wall mount power supply is indeed the specified 12v AC type (not 12v dc) and that the power switch is on and the outlet live.

The dc voltages derived from the ac supply may be less than noted. The positive and negative dc supplies can be measured on pins 8 and 4 of the 5532 op-amp ICs. Reference the ground/common circuit that originates at wiring point B, the neutral end of the power supply transformer. I have notes for +10.19 and -8.98 v dc readings on these pins.

IC3 connects to ground and the positive dc supply on pins 8 and 1. Pin 1 should have about 10.19v and pin 8, zero. It is set up as a multivibrator that toggles from zero to about 10.19v and three of its sections are parallel connected to drive the following diode and capacitor section. Pins 2, 4, and 6 should each be the post audio rate toggling and with a dc check should read at about 6v average dc. This voltage, or not, switching causes the dc voltages on the plus legs of capacitor C27, then C11, and finally C5 to step up along the way. Check for about 9.6, 18.6, and 27.7, v dc on these points.

This higher dc voltage from this section goes to two 270k resistors Rs 4 and 5 which connect it to the plates of the tube sections. The color coding of these resistors is the reverse of 4700ohms used elsewhere in the kit. Check to be sure there wasn't a mix-up in the installation of the red-violet-yellow-gold 270k and the yellow-violet-red-gold 4700ohm.

A mis-wire, bad connection, or malfunction of the c.c. (closed circuit) phone jacks can lead to trouble. These connectors have a switch (shunt) contact that should touch the tip contact when a plug is not inserted. Look to be sure these two contacts are touching and if not it might be possible to bend one or the other so they do contact.

The order of the readout on these connectors can vary according to our source. If the Ground (sleeve), Hot (tip), and Switch (shunt) terminals get connected in place of each other, it can cause the signal to be grounded or have an open circuit when plugging into this connector on the Guitar Input position. You can look at the side of the connector and follow the bends in the contact/terminal metal to confirm that the tip which carries the audio signal in from the guitar is getting into the circuit board input, wiring point C.

Inserting the tube into the socket requires enough force that it might lead you to believe the spring in the top of the socket cover holds it against the socket contacts, but this is not the case. The bottom of the tube should meet the surface of the socket.

For a while we had builders modify a solder eyelet socket for installation through holes in pc board and if it wasn't done just right, the legs could be too short to reach all the way through into the solder joint and this was an easy way to get an open circuit. We've since revised the board and the entire eyelet reaches on through without any trimming. If the socket installation is with the trimmed eyelets, inspect the joints for evidence of a pip in the solder that is the pin extending into the joint. If there is just a smooth dome of solder, it could be an indication the pin hasn't reached through and into the joint and is an open circuit. Desolder and use a piece of wire to extend the pin length by soldering it to the pin and the solder pad.


If you get a lot of boost in the guitar signal, but find that there just isn't good overdrive and distortion effects through the unit, it is probably OK. Normally, the sound through the unit leans towards a clean sound.

The following modifications to the unit that make it work more as a tube distortion effect.

It sounds good to have some solid-state fuzz/overdrive/distortion and then run this through the SiaB for the tube effect. A fuzz/overdrive/distortion unit patched in series between the guitar and the SiaB can do this for you, or, as in the mod I have, the guitar input gain stage can be altered so as to allow for clipping through this section giving the tube a square-wave signal to 'treat'. The other part of the mod I have for the unit adds capacitive coupling and alters the biasing into the second, 'Drive', tube stage to remove some scratchiness in the sound associated with adjustment of the control and provide for symmetrical clip points at the upper and lower waveform extremes. Note this is as pertains to the kit as we supply it with, the Sovtek 12AX7WA.

The increased gain on the guitar input stage is obtained by substituting a greater value potentiometer for the 10k at the Crunch control. I put a 250k on the one I have here. This yielded such extreme gain that it helps to add a 0.001 at points O and P on the bottom of the board to help control oscillation. Just keeping the setting lower reduces the amount of gain though and a lower value pot could give more useable adjustment range (say, 50k or 100k).

The Drive part of the mod subs an 82k for the 33k at R8, inserts a 0.1uF between the wiper of the Drive control and the tube grid, and adds a 270k to ground at the grid end of the added 0.1uF.


sl 1/28/2004

Drive modification notes/illustrations:
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Scott Lee