What are Harpsichord, Virginals and Spinets?
These are David's words.
A harpsichord is a plucked instrument shaped like a slim grand piano. It may have one or two keyboards. It usually has two or three strings per note, controlled by stops giving some variety of tone and volume. Even at its loudest it will scarcely be heard by neighbours as the sound is non-carrying. One set of strings may play an octave higher than normal, in which case it is called a 4' register (the other being 8'). There are other possibilities of tone colour such as the buff stop, which makes felt touch the strings to give a harp-like tone.
It is not fashionable nowadays to make a lot of stop changes on the harpsichord, because early instruments (before the later 18th century) did not have many stops nor means to change them quickly such as pedals.
The virginal is like the harpsichord with only a single set of strings and generally can play only at a fixed volume. The keyboard is in the long side of the instrument. Both bridges are on "live" soundboard, giving a powerful tone.
The spinet has only one bridge on live soundboard and is shaped differently.
The harpsichord, virginal and spinet were the principal domestic keyboard instruments from about 1500 to 1800. Harpsichord music of this period sounds much better on these instruments than on the piano. There is an inexhaustible wealth of such music for the pianist to discover, or rediscover, both for solo harpsichord and "continuo" (accompaniment).
How easy is it for a pianist/organist to play them?
A pianist can play the harpsichord straight away, but it may take some time and retraining to adapt one's playing fully to the new instrument. This tends to be less of a problem for an organist.
Facts about the Clavichord
The clavichord is of very early origin and existed for centuries alongside the harpsichord. Its tone is too soft to be used to accompany any but the softest instruments, but is much more sensitive to the touch than that of the harpsichord. For this reason it has been favoured by many musicians, (the most famous being C.P.E Bach) for playing alone or to a few friends – complete quiet being necessary for this enjoyment.
Besides the advantages of simplicity and low cost mentioned earlier, the clavichord normally requires no maintenance other than tuning.
To make the clavichord still simpler, "fretting" was adopted, that is, one string had to serve for more than one note. The different keys strike the string at different places. (In the clavichord, unlike the piano, the striker or "tangent" remains in contact with the string until released; the sounding length of the string stretches from the tangent to the bridge.) The disadvantage is that two such notes cannot be sounded simultaneously, but as they were only a semitone or tone apart this was not necessary in early music. Some say fretted clavichords sound better because of reduced string pressure on the soundboard.
What about tuning and maintenance?
Harpsichords need tuning fairly frequently (say every 2 - 6 weeks), more frequently when they are new. The owner therefore usually tunes his/her own. If properly made and kept in a sensible place (not over- or under-heated) your instrument should not normally need much maintenance. Anyone who has built a harpsichord is of course well placed to maintain it.