Rallye44 Posted April 25, 2009 Share Posted April 25, 2009 As I posted in an earlier thread, one of the two 6L6GA tubes in my Gibson GA50T broke. I replaced them with a pair of JJ (not quite matched) 6L6GC tubes and now the amp is much quieter than it was with the old tubes. Is there a break-in period for new tubes, and after they've been on/used for a while do they work "better" (louder)? Do I just need to get the amp re-biased? I'm a total newb when it comes to tubes, what's going on? TIA! Matt Link to comment Share on other sites More sharing options...
m-theory Posted April 25, 2009 Share Posted April 25, 2009 Output tube should be matched. They don't have to be identical, but they should be very close. It sounds to me as though you need a matched pair of tubes and a proper bias adjustment. Yes, tubes do "break in," but I've personally never heard a noticeable difference from brand new to "broken in." Link to comment Share on other sites More sharing options...
theshrewz Posted April 26, 2009 Share Posted April 26, 2009 I second the re-bias suggestion Link to comment Share on other sites More sharing options...
Rallye44 Posted April 26, 2009 Author Share Posted April 26, 2009 Output tube should be matched. They don't have to be identical' date=' but they should be very close.[/quote'] They didn't have an exactly matched pair. Here's what was on the labels of the two I got (which were the closest two): Ip: 43 Gm: 5,700 Ip: 45 Gm: 5,900 I've been searching the the forum and intrawebs trying to decipher exactly what those numbers represent, and if they're close enough, but I can't seem to find anything useful... Matt Link to comment Share on other sites More sharing options...
appa69 Posted April 26, 2009 Share Posted April 26, 2009 That's probably closer than most stock tubes today. I think you should check into re-biasing also. Link to comment Share on other sites More sharing options...
theshrewz Posted April 27, 2009 Share Posted April 27, 2009 May I suggest that you email "euro tubes". They only sell JJ tubes, and have been very helpful with email tech support. Michael Link to comment Share on other sites More sharing options...
m-theory Posted April 27, 2009 Share Posted April 27, 2009 I've been searching the the forum and intrawebs trying to decipher exactly what those numbers represent, and if they're close enough, but I can't seem to find anything useful... That's because there's no standard for measuring gain. One tech's numbers mean next to, if not entirely nothing to the next tech. The two measurements that you have may be close enough to roll, or they might be a mile off. There's no way of knowing just by looking at those numbers, unless you know the specifics behind the testing that produced them. Link to comment Share on other sites More sharing options...
Eracer_Team Posted April 27, 2009 Share Posted April 27, 2009 M had a Theory of bias.. I would go with that too. Lets say the new tubes aren't drawing as much as the old tubes then the volatges would be lower and so too would be you're output. But if in doubt always have a spare set of tubes Link to comment Share on other sites More sharing options...
m-theory Posted April 27, 2009 Share Posted April 27, 2009 But if in doubt always have a spare set of tubes It's wise to buy in matched quads (assuming the amp has pairs) when you have new tubes installed and bias adjusted. That way, your spares are plug and play. Link to comment Share on other sites More sharing options...
Eracer_Team Posted April 27, 2009 Share Posted April 27, 2009 Quad pair makes sense MT since if he has to replace them in pairs that would keep the Bias settings intact. I think we're a little Bias in here some where. Link to comment Share on other sites More sharing options...
Dave Posted April 28, 2009 Share Posted April 28, 2009 I guess that since the output tubes are in parallel, there could be a problem that is causing lower output, such as a bad tube. You could try swapping the tubes one at a time with an old one. If you get your volume back, you've found the culprit. Bias could drive the tube toward cutoff on one polarity of the sine wave. I wouldn't think that your bias would be off enough to cause extreme volume loss from the old tubes to the new ones, since half loudness roughly equals 90% power loss. A 3db loss would only create a slightly noticeable loss in volume. What do you think, m-theory? If he has a degraded bias resistor, would he experience loud operation from the old tubes and quieter volume with the new ones or distortion on one leg on the signal on both sets of tubes? Link to comment Share on other sites More sharing options...
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