What is it exactly that makes PVA work?

etc-etc · Heavyweight Member Joined: 19 Jan 2008 Posts: 3959

We know PVA works.
We know PVA is reversible, that is, replacing the newly setup felts with old ones will bring back the old performance (as long as no one ground down the metal parts in process).

But why does it work? Also, why is rotational alignment less of an issue compared to vertical?

Set aside the basket cases, where valves were grossly misaligned. What about the valves that were minimally misaligned?

When PVA is done, there are TWO variables that are changed.
1. Alignment of the valves (of course).
2. Felt material, usually replaced with synthetic rubber.

Do we know for certain the effects of PVA are NOT because of the felt material replacement?

To check this, one could make identically thick felts from:
a) synthetic rubber;
b) traditional felt;
c) pressed cardboard (easy to make, and easy to shim to correct thickness).

Granted, valve cap felts are more difficult to take out, but the internal felts that sit on the stem are not. For open notes, only the internal felts on the stem should matter.

If I am right about this one, it would also explain why the rotational mis-alignment is less of an issue than vertical. The major effect is due to the change of the felt material which occurs in process of vertical PVA.

When the rotational PVA is done after a vertical PVA, the felt material (major cause of improvement) is not changed/adjusted any more. Hence, effects of rotational PVA are minimal.

Why, then, would not the major manufacturers just change the felt material to synthetic rubber and be done with it?

In no particular order:
1. Some already do it, half-way: Kanstul uses rubber tops for the valve caps (or at least used to do so for the Besson line).
2. There may be undesirable effects on sales of repair kits (sets of top and bottom felts that are changed ever so often).
3. Not all trumpeters like having the synthetic rubber felts, because of:
a) tradition,
b) louder action of rubber tops/bottoms,
c) change of the sound palette of the horn,
d) perceived "cheapness" of rubber parts.

VetPsychWars

Ed Kennedy · Heavyweight Member Joined: 15 Jan 2005 Posts: 1036

Felt compresses with use, changing the valve alignment for better or for worse. The idea is to use a material which maintains a precise thickness so that the adjustment holds. Rubber isn't the only material used. I don't know what Reeves uses now but he used to use a fiber composite. Wayne Tanabe used to use what looks like a dense foam and more recently used a rubber-like washer. Synthetics can be purpose engineered to maintain thickness, felt and cork compress.

I can tell you that every horn I've had PVA'd was improved.

LittleRusty · Posted: Fri May 04, 2012 12:16 pm Post subject:

I have never had a PVA performed. But I suspect that most of the benefit is related to constrictions and turbulence at nodal or critical points in the instrument. While most instruments are very similar, I am sure that the nodal points at or near the valve block move based on design, mouthpiece, tuning slide placement and gap, among other things.

This might explain why some people feel it makes a huge difference and others claim it is snake oil.

cheiden · Posted: Fri May 04, 2012 12:20 pm Post subject:

You can align/misalign vertically by either slightly pressing the finger button or by loosening the top cap. Of course you can always shim the felts to vertically align/misalign the ports. It's my strong sense that the noticable effects of these changes parallel what you perceive with a PVA without introducing the material change.

I don't think rotational misalignment is a non-issue but it's normally not as significant and it's much harder to adjust for. Additionally vertical alignment is largely dependent on the felts and as such, always in a state of flux (with traditional felts). Rotation alignment is defined by the accuracy of the farication of the pistons and ports and doesn't change.
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baboo · Veteran Member Joined: 06 Mar 2010 Posts: 135

When I rebuilt my trumpet, the techs replaced the felt material and aligned the valves even though I didn't really ask for it. Before the alignment the second valve was noticeably lower than the other valves, yet I can't say the difference in playing is remarkable. perhaps some horns react more strongly to misaligned valves, or it is a little bit of placebo effect

bagmangood · Posted: Fri May 04, 2012 1:06 pm Post subject:

My understanding is that the felt change is usually just to ensure that the alignment stays - and stays consistent - for long periods of time.

etc-etc · Heavyweight Member Joined: 19 Jan 2008 Posts: 3959

If the corks on spitvalves get replaced at the same time as PVA, this also can have a noticeable effect.

Mark Curry · Heavyweight Member Joined: 15 Feb 2007 Posts: 1034

interesting hypothesis, etc-etc!

I feel that that physical alignment of the ports and valves has much more effect on the blow of the horn than the material change of the pad material.

Until fairly recently, maintaining precise thickness on the rubber/composite was very difficult. A few companies have really figured this out in the last few years, though.

Every one of the OEM pads I ship with my Bach XXCAPS use the Bach replacement part 30023Z (top cap rubbers). A cursory check of each dozen I've received in the last 7 or 8 years has yielded a measurement of .100" thick On Every Pad!

Now that's consistent! Whoever that supplier is has got it down pat.
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irith · Heavyweight Member Joined: 14 Sep 2008 Posts: 790

The actual alignment of the ports is what causes the effect. The pad material is what maintains the effect for a long period of time.
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oldblow · Posted: Sat May 05, 2012 1:27 pm Post subject:

I want to preface my remark with my admission that I know absolutely nothing about the technical design or adjustment of valves.

BUT..are we missing something in the concept by concentrating only on the valve movement, both vertically and laterally, and missing the possibility that the placement of the little hump inside the valve itself, withing the airstream, affects pitch?

Perhaps some of you with design skills can explain the function of this little magic spot? It seems from just looking, that it affects the direction of flow towards the exit tubing.
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VetPsychWars · Posted: Sat May 05, 2012 2:59 pm Post subject:

There isn't a whole lot of airstream. It's a resonant system. All it has to do is not reflect waves at the frequency you're interested in.

Too much valve overlap reflects too much, which is why a precision valve alignment works.

Tom
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Polyphonic · Posted: Sat May 05, 2012 3:00 pm Post subject:

Buy Threadlocker Blue, and apply it to the threads on the valve buttons. Pull the corresponding slide, and tighten until the ports perfectly align. Threadlocker Blue dries in about a minute, and if you want, you can still twist it free. This will give you an idea of perfect vertical alignment while the VALVE IS DEPRESSED. If you find an improvement with this little "trick", then it is probably worth the money to get a REAL PVA.
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uglylips · Posted: Sat May 05, 2012 3:22 pm Post subject:

I went with 50-50 on the survey. IMHO, getting the valves aligned is half the job and keeping them there is the other half. The pad material is responsible for several factors including how long the alignment lasts and the feel of the up/down stroke.

wvtrumpet · Posted: Sat May 05, 2012 5:09 pm Post subject:

As long as we are on this subject. I have thought about sending my horn away to say Reeves and getting a PVA. Guys that I know that have used him rave about how great he is and the changes that he made in their horns have always been positive! That being said, it seems to me like a lot of money for a pretty simple adjustment. What other companies are doing PVA's and what are they charging?

And why does it cost so much to have it done? Not being sarcastic, but I really want to know every ones opinion.
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VetPsychWars · Posted: Sat May 05, 2012 5:34 pm Post subject:

Charlie Melk charges $125 according to his site.

It takes some time to do all of the measurements and verify it with a bore scope.

I think Reeves machines the stems. Charlie just uses shims.

Tom
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Buescher Lightweight 400
Other Buescher horns 1939--1955
Buescher Duo-Cup 88-E mouthpiece
Humes and Berg mutes
http://mmccband.org