The one easy thing about trouble-shooting tube gear, is the ease to swap out tubes as a first check for a problem. What you describe is very much what I recently observed with a phono stage (ARC MCP-33) power supply tube. There were loud pops and other times it was an oscillating upper-bass tone. When it happens it can really make you race out of your chair! A quick tube swap and it was gone. A day or so later I put the suspicious tube in and the problem was back which confirmed this was indeed the problem and in the trash that tube went.
Another thing to consider here: since this is a new tonearm, there may now be an intermittent connection. Is the cable attached or the same you used before? Removing these can result in weakening the connection, especially at the din. If the level is high and the connection is intermittent this could cause any link in the chain to cause a "spike" but generally more of a hum.
John
Another thing to consider here: since this is a new tonearm, there may now be an intermittent connection. Is the cable attached or the same you used before? Removing these can result in weakening the connection, especially at the din. If the level is high and the connection is intermittent this could cause any link in the chain to cause a "spike" but generally more of a hum.
John