I want to make a small battery powered amp using some 6418 pentode tubes. They run on 9v batteries and are the size of small christmas lights. I don't fully understand the power amplifier, so what all would I need. Must I absolutely have an output transformer? what could two of these put out in a class A-B push-pull config.?

6418 pentode tubesdiyaudioprojects.com/Tubes/6418-Tube-Preamp-Headphone-Kit/

Wish I had some spare cash

sounds like a cool project...been thinking of building a triode based mini-amp for a while. The pentode route looks a LOT cheaper and prolly could be just as good...with some very efficient speakers (which I happen to be fortunate enough to have).