KA7OEI blogged about a simple push-pull audio amplifier using russian rod tubes and power transformers:
Many years ago I’d read about the type of tube that is now often referred to as a “Gammatron” – a “gridless” amplifier tube of the 1920s, so-designed to get around patents that included what would seem to be fundamental aspects of any tube such as the control grid. Instead of a grid, the “third” control element was located near the “cathode” and “anode”. As you might expect, the effective gain of this type of tube was rather low, but it did work, even though it really didn’t catch on. It was the similarity between the description of the “Gammatron” and these “rod” tubes that intrigued me.
More details at KA7OEI’s blog.