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Anthony,
If you are having a dew problem with your dob and in the interests of simple solutions first, FWIW, you might wish to consider the following before investing in a heated secondary:
1. A general rule of ATM thumb for solid tube dobs is the tube should extend beyond the secondary for at least the diameter of the tube. In your case, assuming the tube is in the order of 330mm diameter you might like to consider making a dew shield from corrugated cardboard, closed cell high density foam etc to ensure the secondary has adequate dew shielding. The advertising photos for the 300mm GSO dobs look like the tube would be lucky to extend more than 150mm past the secondary. I suspect a simple (and cheap) dew shield will probably fix your problem in all but extreme conditions.
2. I understand the GSO dobs have steel tubes. I have read reports of owners lining the inside of these tubes with a thin layer of cork or felt to provide some insulation and assist with the prevention of "cold air" tube currents and resultant " potential overcooling and dewing of the optics. I have experienced dewing of both primary and secondary mirrors in a steel tubed Newtonian during winter in the Central Highlands on Victoria but have never seen dew on the optics of my Sonotube/Formatube dob in very similar conditions here during Canberra winters. This dob has a corrugated cardboard dew shield that extends the tube 450mm in front of the secondary, and this is for a 250mm diameter tube.
3. Heated secondaries have to be carefully managed as they have the potential to introduce significant heat plumes into the optical path. The Protostar web site has some interesting video footage of this effect.
4. Getting electrical power to the secondary can be an interesting exercise as well. To avoid increasing the diffraction from the secondary support structure by running wires along the secondary spider arms, it is preferable to use two of the secondary vanes as conductors. However for this to work, at least one will have to be electrically isolated from the steel tube and from the other spider arms.
Cheers,
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