Taking a musical instrument as hand luggage?

We are flying from LHR to LAX on 1st Aug, for the first time ever taking the plunge and going Premium Economy.
First of all I would love to have any feedback from anyone who has flown PE on this route??
My main question though is that we want to take my son's Euphonium out to him and I wonder if there is any chance that we will be able to take it as hand luggage (even though it's larger than the 'allowed' size)? I'm just worried as it's quite valuable although obviously well insured. What's the best way to go about this? Phone customer service in advance or just hope that we get a sympathetic check in clerk on the day??
Open to any suggestions!! Thank you
First of all I would love to have any feedback from anyone who has flown PE on this route??
My main question though is that we want to take my son's Euphonium out to him and I wonder if there is any chance that we will be able to take it as hand luggage (even though it's larger than the 'allowed' size)? I'm just worried as it's quite valuable although obviously well insured. What's the best way to go about this? Phone customer service in advance or just hope that we get a sympathetic check in clerk on the day??
Open to any suggestions!! Thank you