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Thread: An engineer's explanation of AGR and Weighted Valve Caps

  1. An engineer's explanation of AGR and Weighted Valve Caps

    Hi everyone,

    I've been seeing a lot about the AGR and how different valve caps can affect the response of an instrument.
    I'm going to try an explain from an engineer's perspective, what is going on when these devices are being used.

    As you read through the following explanation, please keep in mind the following two points:
    1. These devices absolutely make a difference to the sound and feel of the instrument - there are no doubts about that (I will explain why)
    2. Whether these changes are positive/negative, or noticeable/negligible to the player or audience is SUBJECTIVE and depends on the player and the audience (very important to remember at the end).

    I'll start by describing a guitar amp:
    1. A "pick up" on the guitar converts the pluck of a guitar string into a small electrical signal.
    2. This small electrical signal is passed through a "filter" - a configuration of resistors, inductors and capacitors which does "SOMETHING" to the signal, to produce an output signal.
    3. This output electrical signal is passed to a speaker to convert the electric signal into sound.

    On many amps there are knobs that lets you manipulate the characteristics (timbre) of the output ("EQ" knobs).
    These knobs changes the amount of resistance, inductance and capacitance at various points of the filter circuit.
    You can boost or kill the bass, or the treble or the higher frequency stuff.

    Electrical energy passing through wires in a circuit is similar to kinetic energy passing through air in the euphonium.
    Both cases we have energy waves travelling through a medium.
    Electrical Resistance, inductance and capacitance all have acoustic equivalents:
    Air resistance, air mass, compliance/stiffness are the acoustic counterparts (mechanical counterparts too).

    The AGR changes the amount of air mass right after the input of the filter (i.e. a variable inductor knob).

    Different weights changes the characteristics of the wall that surrounds the air by changing the amount of energy that is lost at various points of the circuit. (i.e. EQ knobs).

    So just like an electric amplifier, you can't fiddle around with the EQ knobs (i.e. gadgets on our euphonium) and not expect a change to the characteristics of the output (i.e. they all do SOMETHING).

    We're now onto the subjective part of my piece:
    1. The air column actually extends into our mouths, so we can override some of the hardware changes to create our signature sound. This may lead some people to think that they are working harder to get the same result (i.e. "this does nothing").
    2. These gadgets may not deliver an outcome that is desirable. Too much weight makes the instrument sound dull, and difficult to play. Too little weight and you can't centre a note so it's hard to play in tune.
    3. In acoustic filters, you always have to make compromises - every desirable outcome will introduce something negative for you to put up with.

    If your euphonium comes with knobs, you have to use them. Even the default setting is a setting.

    If your euphonium doesn't come with knobs, and you would like to buy one:
    1. The knob may not work in the way you want it to.
    2. The knob may deliver what you want, but you pay for it in other ways so ultimate you may choose it's not worth it.
    3. You get exactly what you want out of a knob.

    You do need a level of vision and knowledge to take a set of valve caps, and work out which configuration suits you the best, and what trade offs you're willing to put up with. When you buy a set, and realises that you prefer the factory defaults, you haven't wasted money. You've bought the knowledge that they're not for you. You also have the option to use them, should you ever change your mind.

    Here's the thing:
    How you set up your euphonium (valve caps, AGR) is just the start, what about mouthpieces?
    A "free" mouthpiece like the 4AL can help you play a stiff horn with all the heavy caps on.
    A "stiff" mouthpiece like a 5G megatone might help you tame a free horn like a 967 with no heavy caps.

    There are so many variables available, we really need to know what we want to feel and hear when trying out these gadgets, and then it's a lot of trial and error.

    For many people, the best option would be to let the instrument designers make those choices.


    Besson Prestige BE2052-8G-0 Euphonium
    Besson Sovereign 956 Baritone

  2. By tightening/ loosening the AGR you also change how much energy is lost to the walls at that point of the filter (so air mass isn't the only thing you're changing). This alone demonstrates how these physical characteristics can't be isolated from one another and it's hard to get precisely what you want from a filter system.

  3. #3
    Join Date
    Mar 2006
    Hidden Valley, AZ
    1966 Besson 181 highly modified New Standard
    1915 York Bb tenorhorn original
    2019 Wessex Tornister

  4. #4
    That is a very well written and enjoyable explanation of all the variables we can change on our horns. Except for mouthpieces I haven't played around with any of the other "knobs" available to us, but my imagination is now sparked thinking of all the things one might try (of which I will probably never try any of them, but still!)

  5. #5
    An enjoyable read. I have played around with the AGR and have found where it works best, but was honestly surprised by how much difference it made to me as a player - although I suspect it made no audible difference whatsoever.

    My experience of working in the trade and having new innovations which did precisely nothing sprung on us all the time has made me cynical about such things.

    In short it is most definitely worth persevering with. I also suspect I have not yet arrived at the perfect mouthpiece to go in my E2. It may yet be worth going back to a 4AL to see how that works.

  6. #6
    Join Date
    Jan 2006
    West Palm Beach, FL
    A very interesting read Ted!

    On one the thing you shared…

    ”1. The air column actually extends into our mouths, so we can override some of the hardware changes to create our signature sound.”

    This reminded me of something my private teacher told me when I was about 12 playing trumpet. He said ‘to help you produce a good tone, open your oral cavity. Imagine you have a ping pong ball in your mouth to get that feeling.’ Of course that goes along with air support.
    Rick Floyd
    Miraphone 5050 - Warburton Brandon Jones sig mpc
    YEP-641S (recently sold)
    Doug Elliott - 102 rim; I-cup; I-9 shank

    "Always play with a good tone, never louder than lovely, never softer than supported." - author unknown.
    Symphonic Band of the Palm Beaches
    El Cumbanchero (Raphael Hernandez, arr. Naohiro Iwai)
    Greensleeves (arr. Alfred Reed)

  7. #7
    Join Date
    May 2011
    Valley City, North Dakota, USA
    Quote Originally Posted by RickF View Post
    Of course that goes along with air support.
    And Rick definitely knows about good air support!
    Click image for larger version. 

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  8. #8
    Join Date
    Dec 2014
    Honestly, this explanation (especially point 2) only solodifies that I need a bit more weight on my Globe Stamp Sovereign to help with pitch centring... now only to obtain a set of British heavy Prestige valve caps

  9. #9
    Join Date
    Mar 2007
    Indianapolis area


    I'm struck by the parallels here between this discussion and similar issues in radio transmission and antennas.
    *We are the transmitter.

    *The euphonium is a resonator at various frequencies--in essence, an antenna that propagates waves. Kind of a fan dipole acoustically, with the valve tubings representing resonances.

    *We put a signal into the resonator in the form of our buzz at various frequencies, and we expect the horn to resonate, just like I turn on my transmitter and use my Morse code key or microphone to produce signals, and I expect the antenna to resonate (or I fool the transmitter into thinking it is sending a good signal to the antenna)

    *The AGR seems to be an attempt to change the feedpoint of the antenna, with the expected resulting change in resonance.

    *If the AGR is improperly set, the standing wave (reflected/lost power) ratio will be too high--playing would lose resonance and become tubby or pinched, depending on the degree and direction of the mismatch (out too far/in too far).

    *It might also be the case that the AGR is like the most misunderstood gadget in electronics--the "antenna tuner." The antenna (euphonium) is what it is--either the correct length to resonate or not. An "antenna tuner" does nothing to the antenna itself. It tricks the transmitter into thinking it is transmitting into a resonant antenna by altering inductance or capacitance--perhaps the AGR changes musical "inductance and capacitance" by being moved in or out, thus allowing player and instrument to get along.

    *I would think that some of these issues could be examined empirically by examining waveforms a euphonium produces under different settings of the AGR, all other factors held (to the extent possible) constant. Same might well be true for other gadgets/devices such as the famed LeFreque--let's turn on the oscilloscope & see what the waveforms look like with the AGR in various positions, or with/without the LeFreque in several locations.

    *'s all about transmitters, antennas, inductance, capacitance, standing waves, forward/reflected power, etc.
    *Comments from the several amateur radio operators or Professional Engineers who hang out here? These are hypotheses on my part.
    Last edited by Snorlax; 11-03-2022 at 03:44 PM.
    Jim Williams N9EJR (love 10 meter CW)
    Shires Q41, Yamaha 321
    Yamaha 621 Baritone
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    Conn 50H trombone, Blue P-bone

  10. #10
    Join Date
    Jan 2006
    West Palm Beach, FL
    Good points Jim! Being in RADAR for over 30 years (FAA and Air Force), I often thought of our horn's tubing being similar to the waveguide of ASR radar (Airport Surveillance Radar). The electronic waves propagate or reflect down the waveguide toward the antenna (wave guides are used for some higher frequencies since they have less attenuation than RF cable (2800 mhz for ASR radar)). The length of waveguide is very critical to get most of the power out to the antenna... or out the horn of the bell.

    I remember when I was in Greenland in the Air Force (1968), an inspector wrote up a final piece of our waveguide before the antenna because it had a dent in it. Looked like someone hit it with a ball-peen hammer. We ordered a replacement. The new piece came with a very similar dent. Turns out there's a technician who tunes these final pieces for low VSWR (Voltage Standing Wave Ratio) to get the most power out to the antenna. I likened that to reading where some folks added piece of lead golf tape inside their horn's bell to get the high concert B natural to speak.
    Rick Floyd
    Miraphone 5050 - Warburton Brandon Jones sig mpc
    YEP-641S (recently sold)
    Doug Elliott - 102 rim; I-cup; I-9 shank

    "Always play with a good tone, never louder than lovely, never softer than supported." - author unknown.
    Symphonic Band of the Palm Beaches
    El Cumbanchero (Raphael Hernandez, arr. Naohiro Iwai)
    Greensleeves (arr. Alfred Reed)

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