People often ask me what is the difference between a spinet piano and an upright piano. The following picture is an upright piano, or direct blow. If you look closely you can see that the action assembly sits directly on top of the keys. This is a big advantage because there is no linkage and when you press down on the keys it immediately lifts the action which in turn drives the hammer forward to strike the strings. This results in a smooth and even motion with less pivot points decreasing the possibility of sticky keys.
Another advantage to this type of action is it’s easy to remove. Just disconnect three screws and the action just pops right out facilitating repairs and decreasing expensive repair bills.
The following picture is the action of a spinet piano appropriately named a drop action. As you can see, the entire action is dropped down inside the piano. Attached to each key is a long rod called a sticker which attaches to the action. When the key is depressed it pulls up the sticker which is attached to another lever which in turn moves the action up causing the hammer to move forward and strike the string. The disadvantages for this type of action are numerous. With all the added linkage there are many more center pins and pivot points. Every pivot must be pinned with a bushing to eliminate noise. All center pins go through the bushings with just the right tolerances. Over the years with numerous climate changes the wood around the felt bushings tightens and loosens causing sticky keys with high humidity, and noisy keys with low humidity. The action is extremely difficult to remove from the piano adding to labor cost for repairs. When playing a piano of this type you’ll notice a lot of uneven tension, sticky, and noisy keys!
The sole purpose for this type of action, despite its limitations, is to enable the manufacture to design a piano that is much smaller for people who have limited space. Personally, I’m not fond of these pianos and I’m glad they no longer make them, but there is a considerable used market for them and they’re reasonably priced.