The nice amp I got from eBay last week would not come down to the proper bias point. 1.75v was as close as I could get with a set of GE 6CA7's and 5AR4. I had a 5U4G that I substituted to troubleshoot with, and the bias came down into the proper range. The problem wasn't ''fixed'', but I had a clue that things weren't out of control. Learned some things about voltage drop through different rectifier tubes that way. I also ran a 5R4GYB in it and that really has a large voltage drop.
Came to understand the relationship between capacitor input and choke input and why it's necessary when running a rectifier near the upper edge of its voltage range to have low capacitor and or high choke filtering to keep the 'ON' surge to the capacitor from arcing across the plates. The 5AR4 with its super low voltage drop must have plates close together making it vulnerable to arcing when run near its max voltage with high levels of capacitor input.
To fix this one, after checking all the resistors were on the numbers, I first replaced the bias filters one at a time, but still got no change in the adjustment range.
I replaced the selenium rectifier with a good diode and BINGO!!
Fixin' stuff is fun, m'Kay?