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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
tomtab
97 posts
 
thanks wolf, i was hoping to hear from you about this. and what about biasing. just set it till the led light and thats it. couldent these tubes go for alittle more. i think these amps are set for 50 mg's what would happen if we crank it up alittle past the led setting til say the light goes off. would that be like going to 60 or 70.
jedinite24's avatar
jedinite24
1,503 posts
 
Hi Tomtab

I bought my Jolida 502P from Underwood Wally some time after Garcia got his and Walter didn't comment when I inquired about using the Tung Sol KT-120 tubes in the Jolida. I guess he is playing it safe since Jolida sends the amp with 6550 Tung Sols or KT88s. I've been using the amp for months with the KT-120s and have had no issues.

Also biasing is the same. Just put in the KT-120s and turn the screw until the LEDs light-up. I would still get the meter though to make sure you are at 500 millivolts for the KT-120s. One time I had RCA 6550 tubes in the Jolida took them out and put in the KT-120s. Turned on the 502P and after the amp was fully warmed up I checked the bias with meter and the tubes were at 723 to 865 millivolts. The LEDs were all red but I would never have known without the meter I was so way over the recommended 500 millivolts of the KT-120s.
johnsonwu
469 posts
 
Fear not TomTab. Not only do I have KT120s on my 502B I even have ECC99s replacing the wimpy 12AT7s (of course the driver stage is completely rebiased for ECC99 operating point). I am running a lot more heater current on this thing and the power supply tranny is still holding up AOK after almost a year.
wolf_garcia
6,723 posts
 
According to the 502P manual (or I would never have known) you're supposed to turn the bias screw all the way down when changing power tubes...I wondered what would happen if I didn't, but thanks to Jeninite24 I now know! I also bias the 120s to the 500mv spec and they tend to stay there...the LEDs are pretty accurate and you have 50mv in either direction as adjustment "headroom" anyway...when I check the bias drift from time to time (or really the neighborhood juice drift) its never moved more than .05 to .010 mv with any of the various output tubes I've used, and seems VERY stable with 120s. The LEDs seem to track the available voltage or something (without preamp signal) since they blink, stay on, all go off...whatever, all the time.
tomtab
97 posts
 
thanks guys for the info. 120's will be going in. just one step closer to the promised audio land.
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