Chicago Blues Box - blowing tubes

[lowell]

This amp came in making really loud noise when the standby was turned to ON. I put a new pair of 6l6 in there, biased em up and the problem was gone. I then turned off the power but flipped the standby to ON to drain the B+. Again it made that loud popping noise...just once and quick cause the power was off. I turned the amp back on and sure enough the problem was back. I could see one 6l6 flashing so i swapped tube locations and the issue stayed with the tube, not the socket. So...seems this amp is arcing power tubes somehow. I made sure there is negative bias voltage on pin5 and also checked screen and plate voltages...theyre fine. 1k screen resistors are good too. The only thing catching my attention is that the B+ seems very high. With no power tubes installed it is 540v!! Another clue is that with no power tubes, the 2 plate voltages are almost 100v different! So I'm thinking the OT may be compromised. Thoughts? I am also going to measure dc resistance of the secondaries when i get home.

[xtian]

with no power tubes, the 2 plate voltages are almost 100v different!

With no power tubes in, "plate" voltage is at pin 3 of the power tube sockets, right? So, you're measuring voltage flowing through each half (center tap to red, center tap to blue) of the OT primary? And yes, I'd guess if you see wildly differing voltages there, something is bad. Shorted turn?

[lowell]

Correct...heading home to check dc resistance

[lowell]

Ok so oddly enough from CT to each primary of the OT I have 270ohms and OPEN. No power tubes installed. There is also NO reading (OPEN) from primary to primary either. While it seems one primary is completely open I'm still getting plate voltage on both pin3... How could that be? Either way it seems this OT is definitely bad.

[xtian]

Because DC at high voltage can jump small gaps. Sounds toast.

[lowell]

Just powered the amp back on to confirm that both plates have voltage and they do. The plate on the bad primary however fluctuates from 530vdc to 470vdc. I then powered it off and checked DC resistance again and now the BAD primary plate to CT reading is 170Kohms. (yes 170,000). So it's open but must be arced or making a slight connection internally somehow.