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Tube upgrades for Jolida 502B?

I realize this is something that has come up before, though in searching the archives I didn't come away with a clear path forward. My amp is completely stock and came equipped with Sovtek 6550s. Using it to drive Totem Forest speakers. I contacted Jolida. They recommend KT88s. Their reply is a bit unclear but they might be suggesting Electro Harmonix. They also indicate reliability issues with Chinese KT88s. Response Audio (Jolida dealer and modder) recommends Shuguang KT88s (Chinese). The Tube Store recommended JJ Tesla KT88s on the strength of their great imaging and deep bass (more bass being something I was looking for). I was initially leaning towards SED Winged "C" 6550s. Now I am a bit more confused. I'm curious about KT88 vs 6550 sonics and reliability differences. Thanks in advance to any one willing to take the time to share their experience and preferences.
128x128ghosthouse
tbromgard
814 posts
 
Thanks for the responses. Now can anyone explain how to bias tubes in the 502brc? I dont have the owners manal
wolf_garcia
6,704 posts
 
If it's like the 502p you turn the little bias screws down (a lot) before installing tubes, disconnect any signal to the amp, put the tubes in and turn the amp on and wait 10 minutes or so...then turn the bias up on each tube just until the LED lights...do all of 'em and check 'em in an hour or so and you're done. I bought a Radio Shack meter just to fine tune the bias (and out of curiosity) and found that the LEDs are surprisingly accurate. Note that due to house current fluxuation throughtout the day the LEDs change a little from time to time, but unless your tubes are failing this isn't a worry.
crwindy
123 posts
 
I am considering changing amplifier to cary SLI-80.
My amp is a 502A with some upgrades done at time of purchase.
Could also consider the KT 120.
Speakers are Merlin VSM, and I think they deserve better.
tbromgard
814 posts
 
Wolf Garcia-
I would think the 502b is the same. But do I understand correctly that you turn the screw until the led light comes on? I heard the opposite-that you turn the screw to the left until the led light goes off. Im not sure what to do. I put the tubes in turned it on and seems to operate great. sounds fine. none of the led lights are on...
wolf_garcia
6,704 posts
 
You turn the screw down initially, then turning it up adds bias current until the light just comes on (this merely keeps you from accidently frying new tubes with too much current). Then you're set...backing off from the light being on is fine as there is some wiggle room...50 milliamps or so at least. I've checked this with a meter and adding bias current does correlate with the LEDs coming on so it all works fine on my amp. The LEDs on my amp blink stay on or go off depending on the house current...but the KT120s still stay within a few milliamps of spec...so far anyway.
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