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Biasing a valve amp

Hi all

Just had a tube 'blow' on my Ekco EV55SE amp (one of the KT88). No mention I can find anywhere online or in the manual regarding biasing, whether it's auto, fixed etc. 

I've opened her up, and cannot see any test points anywhere on the circuit board? There are 4 variable resistors positioned relatively near, but as my amp has 4 x KT88, 4 x 12AU7 & 1 x 12AX7 surely I'd need 7 variable resistors?

I'm not massively clued up with electronics so any help gratefully received! I can take pics of the board if that would help? 
naturalbritain
jond's avatar
jond
5,807 posts
 
You only bias power tubes on an amp not the others so if you have 4 resistors and 4 power tubes there you go.
roberjerman
4,319 posts
 
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
roberjerman
4,319 posts
 
...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!
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