You only bias power tubes on an amp not the others so if you have 4 resistors and 4 power tubes there you go.
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Grid voltage for KT88/6550 is typically -55 volts. Less than this plate current can increase and exceed a safe level! I also recommend checking the screen and plate voltages. Current production tubes may not be able to handle high screen and plate voltages - leading to premature failure! Tubes from the "Golden Age" ('40s, '50s, '60s) were built to higher standards compared to today's Russian and Chinese produ
...production! New tubes labeled "Tungsol", " Genelex", "Golden Lion" ... are NOT the same as the originals! Shortcuts have been taken to produce cheap tubes (cathode materials and coatings, less hours spent on vacuum pumping). The old saying "you get what you pay for" holds true for today's new tubes market!
www.ekoukusaudio.com Here's the web address. Try to get a schematic. Also ask for any service bulletins. I suspect that those Russian or Chinese KT88's are being run too hard. They should be willing to put you in contact with a nearby service center that can effect repairs.