The accordion bass being proposed here is something in between a free-bass and the usual Stradella bass. The complete lack of preset chords puts an instrument equipped with this kind of bass in a different category from nearly all other accordions, including ordinary accordions and convertor accordions.
A convertor accordion that features both Stradella and free-bass requires that you change your mindset when switching between its two bass modes; but if the accordion only sports this new type of layout, then you only have one layout to adapt to.
It is intended for those who would like to have the freedom to create their own chords on an accordion bass that has no preset chords in any of its working modes. You can basically get the same full sound that a Stradella provides, but you'll find that there are more possibilities. It may require a little more skill to operate than the usual Stradella bass – and fast chord changes will be somewhat more challenging – but it will be rewarding once fully mastered.
The layout is fingering consistent/invariant. It basically consists of the usual two bass rows near the bellows, and an additional four rows of what is henceforth referred to as harmony buttons. Unlike a converter bass with chromatic free basses, all buttons in the bass belong to a consistent pattern – it's not an arbitrary combination of two layouts. Although the proposed layout is quite sufficient as it is, it could nevertheless also be combined with a chromatic layout in a converter bass.
The harmony buttons could provide tones that are sufficiently high-pitched so that most combinations of harmony notes will give a nice sound. Combining a low bass note with higher pitched notes is what the new bass does really well, in a way that is much more difficult to achieve on a chromatic free-bass.
The PDF (nydanabass.pdf), that you find further down on this page, shows the new layout.
The fortunate thing about the proposed bass layout is that you can actually have it by modifying an existing accordion. The register combinations that you would get, however, depend on the accordion you choose to modify. The whole procedure is somewhat laborious and risky, so you could also consider having a professional do the work for you.
How to modify a conventional accordion is explained here:
DIY accordion bass modification
High and low notes can be played simultaneously, using only your left hand. You can, for instance, play higher notes over a pedal point bass note; or keep a higher note sounding continuously while a melody is being played in the bass register. Compared to a chromatic free-bass layout, it will be much more convenient to combine high and low notes.
There are many possible ways to play scales on the harmony buttons. You can play a scale by just using a single row (a harmony row has the same sequence of perfect fifths found in the bass rows). Although you will not find the same relationship between rows among the harmony buttons, compared to the bass rows, a more efficient way to play scales comfortably is perhaps to focus mostly on the second and third harmony row.
All notes are usually within easy reach. The hand can often be kept comparatively still under the bass strap. For instance, going back and forth between a C major chord and an E minor chord does not necessarily have to involve sliding your hand up and down quite as much as you would on a Stradella. Also note that the minor third bass-note is not necessarily the same stretch as it often is on the Stradella.
The alternating root/fifth bass technique, ordinarily associated with the Stradella bass, can be mimicked on the proposed layout quite well. A nice thing is that your index finger no longer crosses over the middle finger the way you typically do on a Stradella when reaching for a major chord's fifth.
Since you now have to play each note in a chord manually, the advantage is that you can depress each button independently. This allows each note's attack and release to be precisely controlled. It also allows notes to be arpeggiated in various ways (holding down a chord button too long on a Stradella can make the sound a bit thick and static).
You will most likely be able to play just about any possible type of chord, provided that it doesn't contain more notes than you have fingers available. The following are some examples of chords (bass note included) that will be possible to play on the proposed layout (but not on a Stradella): Csus4, C6sus4, C7sus4, Csus2, C+, C7+, C, C6. Furthermore, compared to a Stradella, it will be easier to reach chords like Cmaj7 and C6, making these chords more accessible than before. You can also play major/minor chords without the third, or a major or minor chord without the fifth. In other words, there is plenty of room for variation in the left hand.
One way to find alternate ways to finger things, is to play the notes you want, and then lean backwards and identify the buttons that collapse. Usually, however, one will only have to learn one or two shapes for any particular chord type. Although it may seem somewhat difficult to have to depress all the buttons that make part of, for instance, a C9sus4, it will usually not be necessary to do this. It will actually sound better if you play, for example, C in the bass and add only D and Bb as harmony notes.
An interesting thing about the proposed layout is that someone who is used to playing the Stradella, can play the new layout as if it were a Stradella, without experiencing any unwanted notes: some notes will be left out, but then you discover that it is relatively easy to add more notes as you become ever more proficient. Soon you will find ways to express yourself that are simply not possible to do effectively on any other layout.
The layout is the result of trying out a great number of different button arrangements throughout the years. There are so many considerations when designing a layout that is supposed to be able to support all the things that you may want to do with it. One such aspect is being able to comfortably play a major chord in its different inversions by alternately using either the major third or the perfect fifth in the bass. This is something that the proposed layout handles quite well.
There are countless ways to design a button layout. If we just focus on removing two notes from every chord button on the Stradella, then there are 81 possible layouts. If we could choose any specific note for each harmony button (while still using the same sequence of perfect fifths), we would have 20736 possible layouts. So, is the proposed layout really the best possible solution? Well, it might very well be, because there are certain design criteria that have to be met, and the proposed layout appears to be the best possible solution. The layout is not a compromise in the sense that it was the best option that could be achieved with an ordinary Stradella – it's simply a lucky coincidence that the layout is possible to have by modifying the standard bass layout.
The notation that will be used for describing how to finger various chords is illustrated by the following example of a C major chord:
C [C G]  [E] 
What it means is that C is the bass note; then C and G inside brackets are harmony notes to be picked from the first row of harmony buttons (the first row is the one that is closest to the bass rows). E should be picked from the third harmony row. An empty pair of brackets represents a harmony row that is not used. Please note that it is usually not necessary to play the C harmony note unless you alternate between bass note and chord. Actually, many times it will suffice to play fewer notes than suggested below - and often the result will be better, too.
This is the general format:
Bass [row #1] [row #2] [row #3] [row #4]
The bass note is picked from the fundamental bass row. If the bass note belongs to the counterbass row, an underscore sign would be added:
C_ [...] [...] [...] [...]
Using this notation, here are some chord fingering suggestions:
C: C [C]  [G E]  or C [C G]  [E]  or C [G] [C] [E] 
C6: C [C]  [G A E]  (G can be left out)
C7: C [C G] [Bb] [E]  (G can be left out; play Bb with your little finger)
C9: C [C D E] [Bb]   or C [C D] [Bb]  [E]
C11: C [Bb F C D]   
C13: C  [Bb] [A E]  etc.
Csus4: C [F C G]   
C6sus4: C [F C G A]   
C7sus4: C [Bb F C G]   
C9sus4: C [F C G D]   [Bb] (play Bb with your thumb)
C+: C [C] [G#] [E]  or C [C]  [E] [G#] (play G# with your thumb)
C7+: C [C] [Bb]  [E G#] or C [C] [Bb] [E] [G#] (play G# with your thumb)
Cmaj7: C [C]  [G E B]  (G can be left out) or C [G]  [E] [B] (play B with your little finger)
Cm: C [C G] [Eb]   or C [C G]   [Eb] or C [G] [C]  [Eb] or C  [Eb C] [G] 
Cm6: C [C]  [G A] [Eb] (G can be left out) or C [C] [Eb] [A] 
Cm7: C [C G] [Eb Bb]   or C  [Bb C] [G] [Eb] (G can be left out) or C  [Eb Bb C]  
Cm7-: C  [Bb]  [Gb Eb]
Cdim: C   [A] [Eb Gb] or C  [Eb]  [A Gb]
If the prospect of having to learn these fingerings seems daunting, then start off with learning just these two chord shapes:
C7: C [C G] [Bb] [E]  (play Bb with your little finger)
Cm7: C [C G] [Eb Bb]  
From those, you can omit Bb to get ordinary major and minor chords. Thus you know how to play the four most common chords, and that alone will take you a long way. Then you can experiment and see what happens to the sound if you try other buttons.
Even without any register switches, one will effectively have a bass system that corresponds to a Stradella with two register switches since the proposed bass layout already allows you to play single notes in two separate registers; and you can do that without having to flick any switch.
The new bass would also be very useful on a digital accordion since it would be possible to assign one instrument voice to the bass buttons, and a different instrument voice to the harmony buttons.
For an acoustic accordion that would be made specifically for the proposed layout, one could imagine just about any register combinations. However, a bass like the one shown in nydanabass.pdf below, features 32 really useful combinations. Some of those combinations feature a continuous range within two octaves. Such register options can be used for playing melody lines without experiencing octave breaks in the bass - and you can play arpeggiated bass lines, too. There would be four register switches that control the bass reeds, and another four (independent) register switches that control the harmony reeds. A so-called bass decoupler switch (usually situated near the air release button) allows the two sets of pallets to be mechanically disconnected from each other.
An accordion that would be made specifically for the proposed layout should preferably have a bass layout with a rectangular outline. A total of 105 buttons seems reasonable for a smaller accordion.
The same PDF, but with Italian note names:
SaLaTa is an alternative note naming and interval naming system. A chromatic scale from C to C thus becomes:
Do Pa Ro Na Mo Fa Vo Sa Go La Bo Ta Do
Intervals correspond to the number of semitone steps (X = 10, Y = 11).
The SaLaTa version:
To achieve all 32 register options, the reed blocks in the bass would be half the usual length. You would need four such shorter reed blocks. Twelve pallets would operate the lowest bass tones. An additional twelve pallets will cover the harmony tones. Eight (short) sliders are required.
Currently there seems to be very little information regarding bass decouplers on the web, so the following document is a suggestion on how it could be implemented. It also shows the reed block configuration. The whole arrangement of pistons (to which the bass buttons are attached), as well as rods and linkage, are supposed to be integrated with the whole bass button board into a cassette. The frame that houses most of the mechanical parts could be made of wood. A cassette will make access to bass pallets a lot more convenient. The proposed bass decoupler design relies on latches (spring metal wire loops) that connect mechanical parts.
The PDFs below show both the LH and RH button layouts for a complete accordion that would use the proposed bass. On the RH side, C is placed on the second row. If the three rows in the middle are considered to be the primary (home) rows, then some notes will be easier to reach with the thumb or little finger on the first row, without having to reposition the other fingers.
Let's, for instance, say that you want to play a C9 chord (C E G Bb D), and you use only the first three rows on the usual C-system layout. Now, imagine how nice it would be if you could play that D with your little finger on a duplicate row further away from the grille. Or, lets say that you want to play a C minor chord (C Eb G), with C played by your thumb, and then you want to move your thumb to B to create an augmented chord. This will also be much easier if the keyboard has B on the first row. You can, of course, play these examples comfortably if you start further in on the regular layout, but it takes some planning ahead. But, what if you would instead consider the three rows in the middle of an ordinary layout your "home rows"? One possible objection to that could be that then those rows are perhaps not optimal for the far too common C major scale.
The RH buttons C and F could have tactile marks, but then only on the second and fourth rows. The RH layout now becomes a perfect match to the LH layouts proposed in the documents found below. Another advantage with having the primary rows in the middle is that the elbow will be slightly lowered on average.
The vast majority of accordionists seem to prefer some sort of "musette" register option, but here only two voices on the RH side (bassoon and clarinet), with three ordinary register switches and no master palm switch, is suggested. A decent RH range would comprise at least 44 notes, and preferably not feature any dummy buttons.
The same PDF, but with Italian note names:
The SaLaTa version:
Although the bass being presented above is quite sufficient the way it is, classically trained accordionists might still want to have a long continuous range of chromatically arranged notes in the bass. A seamless range of notes can be valuable when you want to play a lot of counterpoint melodies and do not want to rearrange the music to fit another layout.
The converter solution being proposed here is intended to be used with any type of RH layout. With the proposed arrangement, there will always be a consistent and predetermined relationship between the chromatic basses and the basses being arranged in fifths. The tactile marks on selected buttons (cross-hatched or concave) have been chosen so that they make sense with each layout. The chromatic layout being used is, by the way, the same that you would find on a Finnish converter accordion.
One reason why the Finnish chromatic bass layout has been chosen is that it's ergonomically sound to play the chromatic buttons one step closer to the bellows - and the Finnish layout invites you to do just that. Another advantage is that the three chromatic rows to be considered primary, get closer to the bass buttons laid out in fifths.
It is suggested to mark every button on the bass layout according to the prescribed patterns (also if a button is silent). A new standard would thereby be established so that one would always expect the same behavior when switching from one accordion of this type to another. This approach means that G# (above E), for instance, along the row of fundamental basses, should also be concave (which it usually isn't on traditional accordions). Concave buttons would always be primarily associated with the bass/harmony layout, while the cross-hatched buttons belong to the chromatically arranged layout.
The following PDFs shows the two main modes of the converter bass: first the bass/harmony mode, and then the bass/melody mode. A purely chromatic free bass is also provided in each PDF; it has five rows, 72 buttons, and a range of 44 notes from F to C, but could be extended to encompass any range. Note that the marked buttons of the chromatic free bass are cross-hatched, thus having a surface structure that is similar to the texture that would be expected on the corresponding buttons on the RH side.
For a digital accordion, if the bass button layout would be fully programmable, all six rows could be converted into a coherent and purely chromatic layout. In that case, any concave button on the fundamental bass row will be a C. The sixth row, however (closest to the bellows), could alternatively be programmed to do other things, such as: invoke a program change, trigger samples, yield drum sounds, etc.
The suggested layouts work well together with the CBA-C (also the Finnish version) and, of course, also with the piano accordion. For the CBA-B, the chromatic LH layout is actually very similar to what you have in the RH.
Possibly the most interesting option, however, would be to match the new bass layouts with a CBA-C that would have D on the first row in the RH. The RH layout thus becomes a perfect mirror version of the LH chromatic layout. If reference buttons with tactile marks in the RH are desired, only the second and fourth RH row should have these (for C and F). Such a RH layout has the advantage that, when focusing on the three primary rows in the middle, some notes can occasionally be comfortably operated by the thumb and little finger (on the first row) in alternate fingering situations. The only concern here may be if the accordionist likes to rest the thumb on the outer rim of the keyboard and is used to having the three rows considered primary, nearest the thumb.
The PDF below indicates various sizes of layouts, ranging from 93 buttons, up to 117. Depending on the available reed range provided by a particular accordion, dummy buttons in the chromatic layouts may, or may not, be needed. The depicted chromatic layouts show the notes corresponding to the register that would yield the lowest range of single free-bass reeds.
The depicted convertor layout has six rows, but it would also be possible to have seven rows by adding another bass row closest to the bellows. Along a diagonal row one may then, for instance, find (counting from the bellows to the bass strap) Eb E C G C E Eb (in bass/harmony mode), which yields a nice symmetry. The outline of the whole array of bass buttons could then be made to look like a rectangle with rounded corners. A bass layout with seven rows would not normally be recommended unless insisted upon, and would perhaps only be suitable for a really big concert accordion.
The same PDF, but with Italian note names:
The SaLaTa version:
Modulo Notation is a new musical notation system. In the following PDF, the new bass layout's correspondence with this notation is shown. Modulo uses the alternative note naming and interval naming system called SaLaTa.
SaLaTa and Modulo Notation are provided by Nydana