I play a epiphone les paul through a Vox Valvetronix AD50VT.
When i play solos and such i really like to take advantage of the les paul sustain and tone.
Although when playing through my Vox Valvetronix AD50VT, the amp has this feature that turns on the ability to hear when im playing the guitar. And when it doesnt regonize loud output from my pickups, it slowly turns the amp down and turns it up again when i start to play.
This is a great idea, although its not nearly enough precise.
That means that when im playing my sustain only lasts for a few seconds because the amp slowly turns it down.

Is there any way to turn this feature off? Its really annoying.

It's the noise gate. Hold down tap and turn Edit 1 all the way down.
Schecter C1 Classic Left Handed
Line 6 POD HD500
Peavey Valveking 112

On the Vox, the Noise Reduction us up really high by default.

First, press down the effect bypass....so the effect is off.
Now, with the tap button pressed, turn the EDIT 1 knob to the left.
Adjust as needed.
check the book??
Belief is a beautiful armour but makes for the heaviest sword.
Turn Bypass on.
Hold down tap and turn the knob fully to the left.

¦ Epiphone Sheraton II ¦ GFS Mean 90s ¦ Ampeg J-20 ¦
¦ Fulltone OCD ¦ MXR 6-Band EQ ¦ Behringer Chorus ¦ Artec Analog Delay ¦ EHX Holy Grail ¦
I used to get that problem too - try to turn it down...
Honestly I don't think it's the best amp for sustain...
funnily tho I actually find that it acts a bit liek a tube amp - to sound good u have to crank it!
The gear:

* Epiphone Les Paul Custom
* Schecter Jeff Loomis Signature 7 String
* Fender Squier P Bass
* Blackstar Stage HT-60
* Original Crybaby
* EHX Small Clone
* Boss DD-3