Tube upgrades for Jolida 502B?

question about kt120 tubes. i spoke to walter from underwoodwall the other day and asked if i can use 120 tubes in my 502p amp. he said no. i dident ask why but i see other folks here using them. so, whats the story?? is it safe to use kt 120 in this amp. and if yes, do you still bias so the led,s go on??. i understand they use more current or something so wondering if the 502p power supply stiff enough for them. thanks

It has to do with the current supplied from the transformer for the tube "heater" or "filament" elements in the 120s...100mA to 300mA more than KT88s. Walter is simply wrong about this, and based on all the discussion about it in this forum, I'm surprised about that. I bought a 502p a year or so ago and upon getting the approval of the guys at Jolida MD (they said the amp can easily handle the extra current), I took the bold and dangerous step of sticking KT120s in my amp. Yes, I'm clearly a courageous risk taker, but that's how I roll (tube roll anyway). In any case, myself and other courageous risk takers have been using 120s in these amps for a while (in my case almost a year) with zero issues. As part of my ever dilligent KT120 amp explosion death watch campaign, I have yet to hear of ANY amp melting down from using 120s...but I shall continue to my vigilence on behalf of my fellow 120 users, because, clearly, I'm a giver.

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.

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.

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.