I stuck KT120s in my Jolida 502p after being assured (By the Jolida MD people) they would work. They do. Amazingly. The stock 6550s were fine, as are the Sovtek KT88s I tried, but the 120s just sound better in every way I can hear. Then, it was suggested by a dude at the tubestore.com to try their "preferred" 7025s in place of the 12AX7s...another brilliant idea that, combined with the 120s makes the amp just sing...more accurate seemingly, crisp but still tube rich...all good, and the 120s hold the bias settings really well. Now I'm thinking of trying Mullards in place of the EH 12AT7s and if that makes things even better, my head might explode from happiness. |
Risking redundancy I offer the following: My Jolida 502p is currently sporting "matched" KT120s, "Preferred" tubestore 7025s (a Chinese version in the 12AX7 family that really sound great), and tubestore sourced NOS JAN Philips 12AT7s that I just installed...the JAN Philips seem to sound a little more refined than the stock EH 12AT7s, and they have the benefit of having more exposed glowing bits and that's always a wonderful thing regardless of sound quality. "We roll because we can", Wolf Garcia 2013. |
I have to wonder what difference better 12at7s would make as I have been told that particular tube purpose (driver/phase inverter) is somehow less important. I welcome any opinions about that, as long as they aren't meant to intentionally upset me. |
Based on an entertaining conversation with a really cool and tube savvy woman at thetubestore, I ordered a pair of matched section JAN Philips 12at7s...they may or may not be the best option but she was so much fun to talk to (maybe I need to get out more) it's worth the relatively small cost to find out. |
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. |
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. |
I don't think you can put EL34s (or equivalent) in a 502...it's a mostly 6550/KT88 based design. |
Too late...your comment lead to hapless 502 owners all over the world blowing up their amps during xmas...likely igniting trees and festive hifis festooned with decorations (I mean...doesn't everybody festoon their hifis?) leading to holiday audiophile depression. All your fault Stevecham, or shall we call you "the Man who ruined the Holidays for Jolida Owners?" |
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. |
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. |
I do wonder how long the 120s are supposed to last as I've been driving mine hard for about a year and they've stayed absolutely stable, unlike myself. |
The 502p supposedly generates 120 watts in mono, and I suggest a call to Jolida to find out how to make that work. |
There has to be an easier way...I assume a qualified tech could put a switch in there to join the input signals, and "Underwood Wally" has been claiming the 120 watt mono spec for a while so that could make for an interesting call. |
I have no way of measuring the output of my 502p, but I will say the amp with 120s seems to have a bit more headroom, and can play plenty loud in its designated space seemingly effortlessly...I never feel like I "need" more juice. New amps utilizing the extra power available with 120s seem to be coming along all the time...a Rogue Atlas Magnum now does 100 watts pc with 2 120s per side, with what I assume are redesigned trannys and such (at around twice what a Jolida costs). It is nice to have only 4 output tubes to deal with as a money (and heat) saver, and 120s are still a relative bargain. |
