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Post by greenback on Feb 9, 2007 16:02:53 GMT -7
I tried to bias my 1999 used rt66 head, the bias was set at 23 with kt66hp's#4. Anyway, I put in new tubes and could only get the bias to 32ma with bias pot turned all the way. The good Dr. Z said to change the bias feed resistor to a 33k. Has anyone else done this? Does the bias set at 38ma compared to 32ma make a big difference in sound?
Thanks
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Post by Curt on Feb 9, 2007 19:42:16 GMT -7
What rec tube is in it?
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Post by greenback on Feb 10, 2007 5:40:06 GMT -7
The recto tube was a Russian 5ar4 and I couldn't get above 32ma with that one, so I tried a JJ GZ34. Not much of a difference. What kind of bias are you Rt66 players running?
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Post by Curt on Feb 10, 2007 6:54:49 GMT -7
32 to 36 mA for me.
get a nice NOS rec and see what that does, or SS plug.
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Post by jb on Feb 10, 2007 13:53:06 GMT -7
Running mine at 35 currently. I used to run it at 38 and thought it sounded good.
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Post by iggs on Feb 10, 2007 16:56:41 GMT -7
I had a pair of Saratov KT66 which are the "HP" Groove Tube ones and could not set the bias low enough, I sent them back to the store and got ones with lower current draw and the new pair worked great without having to replace the bias pot.
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Post by greenback on Feb 11, 2007 5:52:53 GMT -7
My amp tech said he could add a 33k resistor with a switch to choose between the 47k and 33k. I know 32ma will work but I will always be wondering about the sound at 38ma. The amp already sounds better since I changed the bias from 25ma to 32ma. At some point I will try a NOS rec. The tubes are saratov hp's rated at 34.9 and 35
Thanks
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Post by jb on Feb 11, 2007 7:37:08 GMT -7
The amp to me sounded brighter and punchier at 38. I amso running #4's so it seemed to help the sound. My next set will be #6's which should help.
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Post by Telemanic on Feb 15, 2007 17:32:22 GMT -7
Ah yes, one of the down sides of current production tubes. Due to economics of lower production numbers, than say in the old days, it seems that more tubes slip by the QC dept.'s, that are a little out of spec. or that will drift. Ive seen it with nos too, but less often. Look how many of certain brands of pre tubes that may have a 30-50% reject rate by a reputable tube dealer! I have had sets too that i could not bias in, they were just too far on the high side of what would be a normal variance. - I currently run a set of nos GEC's at about 39ma, and they sound great!! For me 23, seems a bit low, while im gettin on to the high side. Other guys are right tho, there is other contributers to the sound, .... the devil is in managing the variables, and makin the stew too your own recipe!
PS, the 39ma is with a set of nos GEC's, $$$, .. but very nice!! And contrary to popular and well received cautions, they will when wired ultra linear, as with the 66, handle the voltage nicely.
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