It must be an issue with the stat, not the bulb or ceramic, 150w is plenty enough for a viv this size, if anything too much. If you have the bulb plugged straight in to the wall, what temp does it get to? This is effectively the highest that bulb is capable of heating the viv to. If it gets hot enough then you just need to sort the probe placement so it isn't dimming the bulb too much.
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