The M300 was created in order to reduce the Mark II's size and weight.
It was the first Mellotron not to have built-in speakers. However, an amplifier unit, the LSA 300, was available as an option.
The M300 had an Hammond spring reverb, which could be used for the lead instruments sounds only, or with the whole keyboard. It also had a preamp and a transistor amp. The motor controller cards were CMC-4.
The M300 kept the multi station system of the Mark II but with a 52 note keyboard.
The keyboard was divided in 4 parts : 3 keys for diminished chords, an octave for accompaniments, 3 keys for rhythms, and the other keys for lead instrument.
The M300 was the only Mellotron to use 1/4" tapes with 2 tracks. A selector allowed the player to chose the A or B track, but not the two tracks at once. There was a total of 6 banks of accompaniment sounds and 6 banks of lead instrument sounds.
The first models (s/n 01 to 25) had the following controls : Rhythm volume, Pitch control, Reverberation, Lead volume. From s/n 26, the Pitch Control disappeared and was replaced by a Tone Control for the rhythm and lead instruments sections. The controls were placed this way : Rhythm tone, Rhythm volume, Reverberation, Lead volume, Lead tone. The Pitch Control was available as an option.
The sounds were totally different from those of the Mark II. Brand new recordings of better quality had been made for this model. A first set of sounds (M300A) has been released in 1967, and was replaced by a second set (M300B) in 1969. The first strings sound (M300A) was made of two violins playing in unison. This sound has been used by Gentle Giant and Barclay James Harvest. The second strings sound (M300B) was a solo violin, that you can hear on The Moody Blues' Watching and Waiting (from the album To Our Childrens' Childrens' Children - 1969).
The M300 has been essentially used by Barclay James Harvest (on their first albums), Earth and Fire (Song of the Marching Children, Atlantis) and Gentle Giant (Gentle Giant, Acquiring the Taste, Three Friends). Aphrodite's Child also used it on their first album (End of the World), which included the hit single Rain and Tears.
The M300 had a bad reputation because of its reliability problems. These were due to a poor lubrication of the tape guides, resulting in static electricity problems and tapes breaking during bank changes. Now, these problems are solved after a full restoration by Streetly Electronics (see Links).