jt1stcav Posted September 8, 2003 Share Posted September 8, 2003 My younger brother is buying a used 6 month old JoLida JD 202A integrated tube amp for his audio system (to replace his two bridged to mono McIntosh MC250 amps), and is wondering about proper bias adjustments. Not having the owners manual to read through yet (the amp should arrive tomorrow) and being a tube novice, we're wondering if the bias is set too high what kind of damage could result to the tubes, the amplifier, or both? Any help would be greatly appreciated! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted September 8, 2003 Share Posted September 8, 2003 Sorry I'm not familar with how Jolida set these amps up so I can't help you with the actual procedure. But I will say having the amp properly biased is very important to the tubes , transformers and the Sound ! I'm sure you will need a descent multimeter ! Craig Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 8, 2003 Author Share Posted September 8, 2003 Thanks Craig. We have our dad's ol' Fluke multimeter and a RadioShack digital multimeter to use. I think we've read that on the bottom of the chassis there are four set screws and what looks like a dip switch to attach the leads of the multimeter to. Tube complement for this amp are: 2 matched pairs (4) of 6CA7/EL34 power output; 2 pcs. 12AX7A pre-amplifier, 2 pcs. 12AT7 power drivers. If the bias is set too high, I assume the tube's lifespan can be greatly shortened. Would the insides of the tubes be burnt looking inside, and if so, are they best not to be used at all until such time as they can be replaced? I think the original owner may have set the bias too high, from what his e-mails indicated to my bro, but not entirely said in so many words. I know if my bro tube rolls, the new matched tubes should be equally rated as close as possible, right? And it's best not to mix and match from different manufacturers (one 12AT7 a Philips and the other a GE for instance, or should the rating for each be more important than who built them?). Sorry for the dumb questions...we're tube newbies not yet in the know! Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted September 8, 2003 Share Posted September 8, 2003 Bias adjustments are a bit of a pain for this Jolida unlike the 302 and 502 which have the trim pots on the top behind the power tubes. The original Jolida 202a has the adjustment on the bottom of the amp. As I remember, the bias settings for the 202 are around 40mv. If you up it a bit, you get a slightly sweeter more lush presentation, but I dont recommend too much over 44/45. Going lower is less bias which can serve to perhaps make the amp slightly more controlled sounding with a loss of some sweetness. IF find the 40 area the sweet spot. You can experiment here a bit but make sure you bias all four EL-34 the same. I do believe that biasing the amp is very easy besides the pain of getting to the adjustment. Jolida usually recs 40mv so start there. Bias the amp after it's warmed up and follow their directions. They are straightforward (follow the cable unhook recs). I would check the bias once a month although many wait longer. It's actually fun to do an gives you a sense of involvement with the amp. The little 202 doesnt have the most solid bass but it's a sweet little amp and a good used deal. Some great mods can be performed as well with better SS rectifiers, coupling caps, and resistors. Also, many replace the volume pot. Going to HEXFREDs in PS is another mod that most do. Still, dont worry about this in teh beginning. Just enjoy the amp. It will be a lot sweeter and more dimensional than that McIntosh. Quote Link to comment Share on other sites More sharing options...
Geo1 Posted September 8, 2003 Share Posted September 8, 2003 The Jolida 102B uses 4 EL34/6BQ5; 2 12AT7; and 1 12AX7. The correct bias setting is 30Mv. I don't know if this is the correct bias setting for the 202A, so take it with a grain of salt. Quote Link to comment Share on other sites More sharing options...
JCturboT Posted September 8, 2003 Share Posted September 8, 2003 Geo, I'm sure the JD102 uses EL-84 while the JD 202/302 uses EL-34s tubes. Jeff Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted September 8, 2003 Share Posted September 8, 2003 The correct bias for the 202a is 40mv just like I said. You can go up and down per your taste however. The 102b uses EL-84 tubes, not EL-34, so is a whole different ballgame. The 102B is a neat little amp also. kh Quote Link to comment Share on other sites More sharing options...
lynnm Posted September 8, 2003 Share Posted September 8, 2003 That amp is similar to my Antique Sound Lab AQ 1003DT. I have mine biased at 450 mv.as measured across a 10 ohm cathode resistor. At that setting my 1003 is running as a pure Class A amp throughout most of its output range. It is driving Khorns and therefore one may reasonably assume that it rarely if ever goes into AB mode. The higher the Bias setting the closer the amp will adhere to the pure Class A mode of operation - ( ie. The amp will essentially run at close to full plate voltage at all times whether the amp is being driven or not ) . This results - in theory - in a more coherent sound with little or no "notch distortion " which may become audible at the point that the amp's output swings out of Class A operation and into Class AB operation. Rather than attempt a re-invention of the wheel - ( not to mention risking a revelation of my own technical limitations ) - I recommend that you look at this article for a better understanding of amplifier classes than I could ever hope to provide: http://www.duncanamps.com/technical/ampclasses.html The downside is that running in a full Class A mode means that your O/P tubes are running "Full Throttle " at all times and will therefore wear out more quickly than if run in A/B mode in which case the O/P tubes only put out full power when there is a demand for full power. (That said all else being equal most EL34's are damned rugged and should give reliable Class A service for years unless they are grossly overstressed by design failures elsewhere in the amp or by failures in associated components..... A good roof will protect against hailstones but will likely leak after being belted by the frozen contents of a 747's toilet tank ) Read the info in the link above in order to comprehend the important technical differences between Classes A,B and AB Some listeners are adamant that they can hear the switchover from low power ( Class A ) operation to High power ( A/B) mode. It is fair to note that most listeners would be hard put to "hear" the difference between Full Blown Class A and a Full blown Class B amp let alone between a Class A and a Class AB amp which by definition is operating in Class A most of the time in any event. There will naturally be a slight extra running cost as the Class A amp will be going Full Tilt Boogie and draw a bit of extra power from your electrical system... No big deal to me however - I suspect it would be about the same difference as using 100 watt bulbs in your table lamps as opposed to using 60 watters - measurable but insignificant in your overall cost of living. If the Bias voltage is set WAY TOO HIGH the O/P tube(s) can fail catastrophically due to excessively high plate current ( and therefore excessive plate heating) causing the internal elements of the tube to expand to the extent that they touch each other and develop shorts which has the potential to destroy the tube and the O/P trannies but in reality any decently designed amp will have protective fusing that will kill the power before a short in an O/P tube can cause significant damage to the power and/or O/P transformers. Such damage (while possible) is unlikely - especially when dealing with a decently designed amp - modern or vintage. Keep us posted re: Your brother's new amp. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted September 8, 2003 Share Posted September 8, 2003 Running any amp much over its designed Bias setting is asking for trouble setting a amp at .45 or 450mV if its designed for .40 or 400mV isn't a big deal but I wouldn't go much further then that. The dangers are as much to the transformers as it is to the tubes. While upping the bias setting will hold the amp in class A a little longer its takes a heck of allot more than a bias adjustment to make it a class A amp. Craig Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted September 8, 2003 Share Posted September 8, 2003 I would find owners of the Jolida 202a in the Asylum. There are a ton of mod posts in there on the 302 and many of the same mods can be done on the 202. I have run the 302 bias at 45 and found it ok and sweet if a bordering on too lush. Still, 40 is the rec from the company and a good mid ground. Just looked it up to make sure - see below: http://www.progressive-engineering.com/klipsch/202_tech.gif Quote Link to comment Share on other sites More sharing options...
Deang Posted September 8, 2003 Share Posted September 8, 2003 I'm going to start running my amp more into Class B. Try to start a new trend. Pretend I'm a Mac guy or something like that. Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 8, 2003 Author Share Posted September 8, 2003 Thanks everyone, for your insight and useful knowledge in this matter. The information you all provided thus far will be a reference point for us to follow. As soon as my brother receives the JoLida, we'll read the manual from cover to cover and make certain to adjust the bias equally for all 4 EL34 power output tubes. Will any multimeter work for this adjustment, or do you need a special meter with settings outside the norm? And at what measurement on the meter's adjustment dial/knob would you set the meter to read the proper reading for the bias adjustment? Not having much (if any) practical experience with the useage of multimeters, we'd probably set the dial to the wrong measurement. Thanks again all...we appreciate all the responses! Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted September 8, 2003 Share Posted September 8, 2003 Speaking of Class B, we are going to have to consider a Signature Adjustment PArty pretty soon...heh. The signatures in this place have slowly gotten longer and longer and longer with spaces between lines and listings of each and every piece of gear. Lordy. We had a big blowout about this a year ago and got them all down. I think the basic rule was if your signature is usually longer than your post, some work needs to be done, knowhatimsayin? There are a few innocent chaps in here with signatures bordering on novellas! A while back, you could take it out of posts. Now, it is in EVERY post. I have an actual TEXT file on how to make it 1 pt text and condensed. Quite a few followed suit but now it's gotten haywire. Some things can best be left to the imagination... Ole Mike Lindsey had the great use of the Audio Asylum link for his sig. no big deal but as long was wer are talkin' Class B.... heh Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted September 8, 2003 Share Posted September 8, 2003 Jeeeez, be careful! Last time I asked a bias question it started WWIII on two forums! Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted September 8, 2003 Share Posted September 8, 2003 "If it measures good and sounds bad - it is bad. If it sounds good and measures bad - you've measured the wrong thing." - Daniel von Recklinghausen I always loved that quote... kh Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 9, 2003 Author Share Posted September 9, 2003 Andy, we sure don't need another WWIII going on...it's bad enough there's one already in progress in Iraq, and also in the Odds & Mods Forum! I've been informed about my sig being grossly oversized from Netscape browser users, but looking "normal" through IE6.0 browsers. I know mine's too long...it took forever for me just to figure out what I have now! Guess I should either shrink my fonts or shorten it altogether...But I like my sig the way it is! It's ME! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted September 9, 2003 Share Posted September 9, 2003 You need a basic Multimeter of reasonable quality digital is easier to read. You set it to read VDC some meter will auto range and give your millivolts which will read 400 mV (most digital meters will say mV or VDC after the number). If its reading VDC then the reading will be .400 . If you don't have a known good meter go spend $30 to $50 at Radio Shack or Sears its worth the expense. Craig Quote Link to comment Share on other sites More sharing options...
Geo1 Posted September 9, 2003 Share Posted September 9, 2003 Errr, ahhh, well, of course the 102B uses EL84's. Glad to see that you guys are proofreading. Keep up the good work. (Where's the red-faced smiley?) Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 9, 2003 Author Share Posted September 9, 2003 Well, it's here! My brother used the RadioShack digital multimeter and set the bias at 41mV for all four EL34s (all tubes are the originals provided by JoLida, and appear to be in good shape). There are slight brownish tinges inside all four EL34s...suppose that's nothing to be concerned about. Eventually, he wants to do some tube rolling; he's considering a quad matched set by Electro-Harmonics, but for now he'll use what's provided by JoLida (are they Chinese or Russian, and does it really matter?). Of course, after a good long audition, he'll provide a little review. Already he's noticed a major improvement over the bridged Macs (you can see the edge of one of the MC250s up at the top left corner)...I can't wait to take the time to listen to this lil' gem myself. Would love the opportunity to try it out through my Cornwalls sometime! Quote Link to comment Share on other sites More sharing options...
jt1stcav Posted September 9, 2003 Author Share Posted September 9, 2003 Another view, with the second unused MC250 to its right. He doesn't know yet what he's going to do with the McIntosh amps, but he assured me he's not selling them! Quote Link to comment Share on other sites More sharing options...
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