Push Reeds Sink with High Pressures

[Dissonance]

Hello Reed Geeks,

 

Instrument:

Wheatstone model 22

Problem:

I have a hand full of mid range push reeds that drop pitch a bit with high pressure.
-The problem is unchanged when reeds are exchanged with their opposites. (Problem doesn’t follow the reed).

-The problem also does not follow the reeds to a different concertina (at least on the ones I can fit in a different instrument. 
 

I think the chamber baffles are unmolested and in their original positions. Valves seem in good shape. 
 

Anyone have any idea what is going on there. 

[Rod Pearce]

Where are the reeds located - in the same area of the reed pan or spread about?

 

If they are in the same area it may be that you are losing air in that location under the higher pressure caused by a warped pan or badly fitting gaskets?

[Stephen Chambers]

4 hours ago, Rod Pearce said:

If they are in the same area it may be that you are losing air in that location under the higher pressure caused by a warped pan or badly fitting gaskets?

 

Or, even more likely, the pan has dropped at that location due to a corner support block becoming unglued.

[Dissonance]

Interesting!

You guys think that a leak like that would affect the reeds outside the chamber more than the ones inside? Certainly worth exploring. 

[Geoff Wooff]

Interesting indeed !

 

  Although  an internal  leak  caused by  a  warped  or  shrunken  reedpan  might  cause  efficiency  problems  one  might  expect  similar affects  on both  Pull and Push  but  not  a  significant  drop in pitch of the notes.

I  thought  perhaps  the  bellows  fold  nearest  the  end    might  collapse  outwards   , touching  some reeds  or  restricting the  valves  . Could  those  valves  be  flopping  away  from  the  reedpan  or  made from  too  thin or  too  flexible  a material ?

Edited November 2, 2021 by Geoff Wooff

[Dana Johnson]

A couple other possibilities are hardened valves that don’t lift enough, 

also pads that don’t  lift high enough, which will pull the pitch down when they won’t allow enough air to pass, especially if the concertina has smaller pad holes.  Also as mentioned in another thread, the pins that keep the valves from lifting to far can be set too close to the reed, which causes the same issue as pads that don’t lift enough, and affects the press reed valves which are the only ones with pins, (except for large valves on low pitched instruments which can have a wire or other device to keep the bellows side valves from flopping away from the reed pan.

[d.elliott]

If reeds are flattening under high pressure, then they have their beating slowed by the air pressure/ flow. Remember that the air flow release on bellows compression is more pulse like than on bellow suction.

 

Things that can cause this are:

• a group of reeds that are too weak to handle the pressure, possibly through over filing, metal fatigue or even perhaps some softer, steel 
• the valves being hard, or in some way their opening constrained for the air flow required, as mentioned by Dana valve pins set too low, or the valves are too close to chamber walls, check also that the chamber walls are not leaning in towards the valve position.
• the reed set's, or tip gaps are too small which can lead up to reed stalling
• check pad clearance height in case the action is set too low.

[Wally Carroll]

Some quick thoughts:

-All reeds will drop in pitch under high pressure with the lowest notes being more susceptible. Not sure how hard you are playing but generally you would need to be pushing heavily on a note to notice this effect - particularly in the middle and upper octaves.  

-If the metal has been overly thinned this could cause the flattening to occur at lower pressures.

 -If your second instrument is not as airtight as your first, you might not notice the effect as much when transferring the notes to the second instrument.  

-Leaks actually often cause the opposite effect of raising the pitch.  If you have an instrument tuned for playing in tune at medium pressures and play a note very softly, it will be slightly sharp.  Pulling at a medium pressure will bring it down into tune.  Pulling harder will make it flat.  Longer, lower notes have a much more pronounced effect in this regard.