The VOX Valvetronix speaker, or amplifier, features sophisticated modelling technology -- meaning the amp is designed to emulate the sounds of popular guitar amplifiers, such as the VOX AC30 -- that uses solid state circuitry along with a vacuum tube to help create the warmth of a classic tube amp. While the VT15, VT30, VT50 and VT100 feature various effects and amp models, they do have one common feature: noise reduction, which is useful for reducing hum from a given instrument plugged into the amplifier. If your Valvetronix is cutting out, it is most likely to due to excessive noise reduction, which will not only cut the hum, but cut the sound coming from your instrument.
Plug the guitar cable snugly into the input jack on the top left of the amplifier (make certain you are using a cable that is in good working condition). Turn up the "Power Level" knob on the back of the amplifier to the halfway point. Turn the amplifier on. Turn up the "Gain," "Volume" and "Master" knobs to about a quarter of the way. Turn the "factory settings" knob to one of presets (it doesn't matter which). Strum a few bars. If your guitar is still cutting out, then you will need adjust the noise reduction or restore the amp to factory settings.
Press the "BYPASS" switch. The bypass lead light will turn red. Hold down the "TAP" button and turn the "EDIT" knob to the left to lower the noise reduction level. Turn the knob to the right to increase the noise reduction; if you turn it too far to the right, you will most likely hear the sound cutting out --- which will be especially evident when using a particular effect, such as delay. Strum the guitar to hear how the adjustments are affecting the sound. Press the "BYPASS" switch again once you have found a level of noise reduction where the sound is no longer cutting out. You still might hear a little guitar feedback hum, but at least the primary sound you want coming from the speaker won't be cutting out.
Restore the amplifier back to the original factory settings, if needed. Hold down the CH1 and CH4 switches. Turn the amplifier back on --- the "Bank" and channel LEDs will start blinking. Release the CH1 and CH4 switches. Press the "Bypass" switch, and the "Bank" and channel LEDs will stop blinking and remain lit. The amp will begin restoring itself to the original factory settings, which will take about 1 or 2 seconds. Do not turn off the power while this procedure is going on.
If your VOX is still cutting out after making these changes, you most likely have a faulty amplifier. VOX will repair the amp if it still under warranty; otherwise, you will have to bring the amplifier into a shop for professional repair.
Keep in mind that restoring the VOX to factory settings will delete all of your own presets.