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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

Showing 4 responses by roberjerman

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!
roberjerman
4,319 posts
 
Find a competent technician that is familiar with tube circuitry and can examine your amp for potential problems. Search for and replace those cheap KT88's with a set of original USA made 6550's (used with 75% emission will do!).
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
 
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.
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