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Post by James on Feb 16, 2006 7:14:33 GMT -7
Fireball,
Did you have the amps bias checked? All I know is I had the same experience..........did not wanna touch the amp because it sounded so bad..........came to find the bias was at around 24,25.....had her tuned up to around 36 and the amp came back from the dead........
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Post by fireball on Feb 16, 2006 9:41:22 GMT -7
No I did not have the bias checked. But isn't that one of the reasons for using Groove Tubes? So that you can just replace the power tubes with the same kind of tube with the same rating (GT KT66 HP #6's in my case) and not have to worry about taking he amp to a tech for a re-bias?
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Post by James on Feb 16, 2006 14:57:05 GMT -7
..........I am not sure about that. Probably a good question for Myles.
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Post by Beckanon on Feb 16, 2006 15:44:28 GMT -7
Re-read the section "Bias Notes" on the Dr.Z website. I believe the only output tube types that are pretty much guaranteed to be "plug-and-play" are cathode biased EL84 amps. You MIGHT get lucky when you buy a new pair of KT66s and they sound just fine -- other pairs, despite being the recommended match #, still might require an amp re-bias. Sounds like you were not as lucky, and the amp needs to be re-biased.
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Post by fireball on Feb 16, 2006 16:54:05 GMT -7
Re-read the section "Bias Notes" on the Dr.Z website. I believe the only output tube types that are pretty much guaranteed to be "plug-and-play" are cathode biased EL84 amps. You MIGHT get lucky when you buy a new pair of KT66s and they sound just fine -- other pairs, despite being the recommended match #, still might require an amp re-bias. Sounds like you were not as lucky, and the amp needs to be re-biased. I guess I'll have to take the amp in and have the bias checked and set. I was always under the impression...and I know I remember reading this somewhere in the Groove Tube literature.... that if you replaced a Groove Tube set that was say #6 rated with the exact same rated tube that a re-bias was not necessary(??). Even with Class A/B amps. So according to Dr. Z's site he recommends the GT KT66 HP's to be biased at 34-38 mA per tube..... is that info correct? I just want to be sure because I'm fairly certain my tech has never come across a Dr. Z amp before.
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Post by taswegian on Feb 16, 2006 17:02:58 GMT -7
The Good Doc seems to like the bias around 28-32 on the route 66 now. It really depends on your personal needs and taste.
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Post by fireball on Feb 16, 2006 17:59:58 GMT -7
The Good Doc seems to like the bias around 28-32 on the route 66 now. It really depends on your personal needs and taste. How does a bias setting of 28-32 mA vs. 34-38 mA as listed on Z's site translate into what I will experience tonally and response-wise when I play the amp? Also, if anything from 28 mA to 38 mA is considered "acceptable" for the bias of a Route 66....I'm wondering how can the bias on my amp possibly be out of spec?? I mean the amp was fine with the previous set of GT KT66 HP's #6.....and now this new set requires a bias?.... yet anything in a 10 mA range (28-38 mA) is considered acceptable? I'm very confused Am I missing something here?
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Post by rickbob on Feb 20, 2006 4:32:09 GMT -7
Fireball, have you seen the thread "BIAS and the Route 66" on this page? It should help anwser your questions. As that thread states, a higher bias number of say 36 will cause the amp to go into breakup sooner (hotter), a lower number will keep it 'colder' and give more clean sounds. Also your rectifier tube could be going (less likely but something to consider), especially if it is newer production.... a good strong old stock rectifier will run forever more or less while the newer production stuff i don't trust very far.....personally I stay away from 'em. edit- don't forget, if you put in a different rectifier, you must have the amp biased for that rectifier!.....
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Post by myles on Feb 22, 2006 11:28:50 GMT -7
Fireball, have you seen the thread "BIAS and the Route 66" on this page? It should help anwser your questions. As that thread states, a higher bias number of say 36 will cause the amp to go into breakup sooner (hotter), a lower number will keep it 'colder' and give more clean sounds. Also your rectifier tube could be going (less likely but something to consider), especially if it is newer production.... a good strong old stock rectifier will run forever more or less while the newer production stuff i don't trust very far.....personally I stay away from 'em. edit- don't forget, if you put in a different rectifier, you must have the amp biased for that rectifier!..... Not exactly .... Biasing on the hotter side of the scale will give more clean headroom, a tighter amp. But, if you go too hot the amp will sound harsh and tube life will suffer. Bias too cold and the amp will sound grainy and not develop full power. The amp will have it's clean tones suffer too if biased too cold. If you stay at 50-60% based on tube dissapation (30 watts for a KT66HP and 25 watts for a Genelex) and based on plate voltage you will be in a great range. The difference between 450-470 plate volts is pretty slight when it comes to bias. So ... if we figure 460 plate volts as a good number we have .... For a 25 watt Genelex or GEC KT66 or Shuguang KT-66 460 V 25 W 40% 50% 60% 70% 80% 22 27 33 38 43 For a KT66HP 460 V 30 W 40% 50% 60% 70% 80% 26 33 39 46 52 As as side note .... using 450 volts at 25 watts and then 480 volts at 25 watts here is the difference that 30 plate volts makes... 450 V 25 W 40% 50% 60% 70% 22 28 33 39 480 V 25 W 40% 50% 60% 70% 21 26 31 36 So here we see that 30 more B+ volts will only mean about a 2mA change in bias which may not even be worth consideration. This also shows that if you are biased at 50-60% with a vacuum tube rectifier and slap in a solid state plug in that you are still going to be pretty safe at your set bias and just have the amp be a bit stronger if you leave things alone with the SS rectifier. And maybe the reason you wanted to try a solid state rectifier in the first place was for a tighter and stronger or stiffer sounding and playing amp so ... keeping the bias set with the Vacuum tube rectifer when the SS one is plugged in could be just what you are looking for.
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Post by myles on Feb 22, 2006 11:39:05 GMT -7
Re-read the section "Bias Notes" on the Dr.Z website. I believe the only output tube types that are pretty much guaranteed to be "plug-and-play" are cathode biased EL84 amps. You MIGHT get lucky when you buy a new pair of KT66s and they sound just fine -- other pairs, despite being the recommended match #, still might require an amp re-bias. Sounds like you were not as lucky, and the amp needs to be re-biased. I guess I'll have to take the amp in and have the bias checked and set. I was always under the impression...and I know I remember reading this somewhere in the Groove Tube literature.... that if you replaced a Groove Tube set that was say #6 rated with the exact same rated tube that a re-bias was not necessary(??). Even with Class A/B amps. So according to Dr. Z's site he recommends the GT KT66 HP's to be biased at 34-38 mA per tube..... is that info correct? I just want to be sure because I'm fairly certain my tech has never come across a Dr. Z amp before. Yes .... same tube at same rating means no rebias needed.
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Post by myles on Feb 22, 2006 11:41:54 GMT -7
The Good Doc seems to like the bias around 28-32 on the route 66 now. It really depends on your personal needs and taste. I always thought of myself as one who biases NOT HOT .... but the Doc runs settings cooler than I do so at times he and I disgree. But .... on the other side of a coin .... Paul Rivera and I also disagree and he biases WAY hotter than I ever would. So .... I sit somewhere between the Doc and Paul Rivera I guess.
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