Popping from tube integrated amp

I've owned an Eastern Electric M520 for about 3 years and have it connected to Omega Hemptones. I changed the EL34 tubes about three months ago from the stock Chinese tubes to NOS Mullards.

This afternoon about a minute after powering up the amp, very loud popping noise came through the speakers - - the popping was constant - about 90 to 100 pops per minute I would guess. The volume was at a (loud) constant level, regardless of where I set the volume control. The popping continued without regard to which input I selected.

After turning off the amp, I reseated the tubes and tried again - - same popping. The tubes all look normal - - no unusual visual cues.

Anyone have any idea what the problem is likely to be? Many thanks for any suggestions.
is the popping coming from the left or right speaker, or both? also try to swap the tubes left to right, right to left........
Good luck.

Sounds like you have a bad tube already....If you still have your old tubes ( as back up ) take your new tubes out and put back in the old..I bet the problem goes away.................Not that unusual to get a bad new tube...I recently purchased a beautiful pair of Tungsram 12ax7 only to find out after 30 minutes one tube was bad........Had to send them back for a refund..................If you have a good tube supplier he should help you out with a replacement set...Good luck..
Have you tried reinserting the stock (Chinese) tubes to see if the problem continues?
So the consensus is I've got a bad power tube (?) - - I have another set I can swap in. Will try to do that in the next several days. (I've taken the Eastern Electric off-line and am using a back-up Denon SS integrated. Pain in the ass - - - will put new EL34's in the M520 next weekend and post the results.) Thanks to all.
If it's a constant popping sound I would bet it was one of the preamp tubes and not the power tubes. I'm not familiar with that amp, but do check those as well. If you swap in your other set of power tubes and the noise persists its an even better bet it's one of the smaller tubes in your unit. I hope I'm right as those NOS EL34 Mullards had to be pretty spendy.
Phd - - I pitched the Chinese tubes when I got the NOS Mullards. I've got some new (matched quad) Yugoslav Ei Fat Bottle 6CA7's I bought from Kevin Deal a couple of years ago, which I may end of using.

Tejas Tubes (a sometime-presence here on Audiogon) is located about a mile from my office, so I will give them a call tomorrow and see if I can get my tubes tested there (will, of course, by replacements from Tejas).
You may be having the same problem I'm having with my integrated tube amp in which case, tube swapping won't make a difference. At the worst one or more of your filter capacitors are going bad. Or possibly the clips for your tube sockets are loose. I had a thread going with some good advice that I wish Agon would have kept...
Word from the local repair shop is that the tube driver circuit on the right channel shorted out. The short trashed the two ('spensive NOS) EL34's powering the right channel, so there goes some $$ down the drain.

Anyone know how common it is to have a tube driver circuit fail, or what the cause might be?