Campbell Goodtime Hour on CBS, and in the following year, 1969, he became one of Ovation's first endorsers. The Partridge Family, 1970 Ovation guitars and amplifiers, together with musical instruments distributed by another Kaman company, Coast Wholesale Musical Co., were featured on The Partridge Family musical sitcom TV series. Design innovations Ovation guitar design reflects its founder's engineering training and development of Kaman helicopters. Ovation guitars replace the instrument's conventional back and sides with composite synthetic bowls. Kaman felt there were structural weaknesses in the orthogonal joining of the sides, and that a composite material could provide a smooth body. Ovation claims the parabolic bowls dramatically reduce feedback, allowing greater amplification. Improved synthetics techniques from helicopter engineering control vibrations in the bowl. Ovation developed a thin neck, striving for the feel of an electric guitar's neck, but with additional strength from layers of mahogany and maple reinforced by a steel rod in an aluminum channel. The composite materials and thin necks reduced weight. For its soundboards, Ovation uses Sitka spruce, a wood that Kaman engineers used in helicopter blades. In the 1970s, Ovation developed thinner soundboards with carbon-based composites laminating a thin layer of birch in its Adamas model. The Adamas model dispersed the sound-hole of the traditional soundboard among 22 small sound holes in the upper chamber of the guitar, which Ovation says yields greater volume and further reduces feedback during amplification (pioneered in the Adamas model in 1977). Although the area of the multiple sound holes is equal to the area of a single-soundhole, the altered position allows a new style of guitar bracing (e.g. Adamas Bracing). The design strengthens the soundboard, reducing the traditional design's bracing and hence weight. In the 1980s, Ovation introduced shallow-bowl guitars to appeal to electric guitarists. In 1977-1978, Gypsy, an Ovation performing and recording artist, designed the first stereo pre-amplifier for the Adamas 12 string and used it on his album "Ladies Love Outlaws." At the same time, Ovation provided small doors that blocked the sound holes from the inside in order to dampen feedback in the presence of loud stage monitors. Gypsy had also requested the addition of a round hatch in the back of the body of pre-Adamas guitars to facilitate changing the on-board battery, a feature that was then adopted for all the "round holes." Before this time, the strings on the round-hole guitars had to be removed to