Amp is a Pro Jr, older USA made. Have JJ tubes (pre and power). The other night while playing at home I had been playing for maybe 10 minutes. I then turned it up to about 4/5 and within a minute there was a very loud buzz. More like the buzzer at a sporting event. I bought new power tubes and they arrived today, and the amp did the same thing.... works ok until I turn the volume up to 4/5, then BUZZ! The volume and tone controls have no bearing on the buzz. I’ve ordered some new preamp tubes, but now that doesn’t seem very likely.... they’ll be here in a few days anyway. Does this sound like a tube issue, or something I need a text to look at?