[jcx]

you may want to repeat your question with assurance that blowing your headphone isn't an issue - but whether a 50 Ohm load (or no load - what if a speaker wire falls off/disconnects on the other end?) would allow peaking/ringing in the output transformer to cause arcing/damage to the amp
 
the web has poeple warning about possible unloaded tube amp arcing but in my experience we used tube amps in labs specifically because they were hard to destroy with weird loads - and they definitley ran without loads at times

 

[Dr. Roberts]

Thanks for your time, info, and effort JCX.