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Post by Paul (TRANE) on Jun 17, 2008 13:46:12 GMT -7
Myles,
I have a JTM-45 type amp and the current tubes (new production Genalex Gold Lion) seem to have a bad drift issue....if not a short. I will be replacing them and would like to put in some Groove Tubes KT66-C tubes (probably a 6 rating). I usually bias for about 60% of idle using your handy spreadsheet. Just wondering if you find 60% to be a good number for JTM-45 type amps with KT66 tubes? The stock ones were biased at 70% but are dying after only about 6 to 8 weeks of use.
Thanks
Paul G.
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Post by myles on Jun 19, 2008 8:33:17 GMT -7
Myles, I have a JTM-45 type amp and the current tubes (new production Genalex Gold Lion) seem to have a bad drift issue....if not a short. I will be replacing them and would like to put in some Groove Tubes KT66-C tubes (probably a 6 rating). I usually bias for about 60% of idle using your handy spreadsheet. Just wondering if you find 60% to be a good number for JTM-45 type amps with KT66 tubes? The stock ones were biased at 70% but are dying after only about 6 to 8 weeks of use. Thanks Paul G. Paul, 60% is a great number. As far as the drift .... that is caused by handling damage that is commonly incurred in shipping. The 66 is a very fragile tube. Dying in 6-8 weeks is also very odd and not the usual practice. The KT66C work nicely in a JTM45 type of amp and a 6-7 rating is perfect.
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Post by Paul (TRANE) on Jun 19, 2008 14:51:39 GMT -7
Myles,
Thanks. I ordered a set of #6 KT66-C tubes today. I have some extra of the Genalex KT66 tubes but feel kind of sketchy about them. The ones in the amp were burned in and everything and still went south. Basically, the amp would lose volume in a erratic nature (some what similar to how a trem can modulate the volume). I found that if I flipped the amp to standby and then back on then all was well for a while. (I was at a gig without a backup or I would have just switched to a different head). I will be going through the head when I get back home and see if I have any burnt resistors or anything else going on. Hopefully, just a tube issue.
Thanks again for all your help.
Paul G.
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Post by myles on Jun 20, 2008 9:08:09 GMT -7
Myles, Thanks. I ordered a set of #6 KT66-C tubes today. I have some extra of the Genalex KT66 tubes but feel kind of sketchy about them. The ones in the amp were burned in and everything and still went south. Basically, the amp would lose volume in a erratic nature (some what similar to how a trem can modulate the volume). I found that if I flipped the amp to standby and then back on then all was well for a while. (I was at a gig without a backup or I would have just switched to a different head). I will be going through the head when I get back home and see if I have any burnt resistors or anything else going on. Hopefully, just a tube issue. Thanks again for all your help. Paul G. Paul, You are welcome and happy playing!
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Post by Paul (TRANE) on Jun 21, 2008 21:13:33 GMT -7
all right. all is well with the innards of the head no burn marks or anything. Screen grids check out good too. Checked the two tubes with my bias rite. At 465 Volts one was at 36ma and the other at 43ma. They were 38ma and 39ma at first use. I changed them out with a second set of the same type. As a short term solution. The plate voltage was 460 on tube 4 and 480 on tube 5. With the old set they were both at 465. Kind of odd to me but I am not an expert. Biased to 60% and will see how things go from there.
Thanks again,
Paul G.
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