yarmo
Junior Member
Posts: 69
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Post by yarmo on Aug 24, 2014 10:32:21 GMT -7
Hi guys,
I replaced all the tubes today and with the bias pot completely turned clockwise (the starting point), the reading on the DMM is already at 39.5 mA and nowhere else to go but upward?!! I've waited an hour and the reading is now at 39.3 mA. I usually set the bias at 36 mA.
Tubes are JJ EF8O6 gold pin - Gold Lion 12ax7 -- Gold Lion KT66 -- Mullard GZ34 - all brand new.
Is there a tube that might be faulty or perhaps the 12ax7 is rated for higher gain (btw I know very little in tube interaction electronics-wise!)?
If I should post this elsewhere, I'd be happy to oblige.
Thank you very much!!
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Post by strat59 on Aug 25, 2014 3:51:56 GMT -7
Hi Yarmo, You could also try: > ask the experts > amp questions/troubleshooting > ask Myles
Myles knows everything about tubes/tube amps and works with Z-amps all the time. He graciously shares his time & knowledge answering questions. At one point he wasn't getting messages - hope that has been resolved. Randy
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Post by headshrinker (Marc) on Aug 25, 2014 8:38:19 GMT -7
The preamp tubes shouldn't affect what you are getting from the power tubes. I would guess you have some power tubes that are way out of specs. It's also possible that your rectifier is putting out a ton of juice. Don't think that's likely though. I would put your old power tubes and rectifier back in and see what readings you get. If they are normal, probably not an amp problem, but the new tubes. If they are out with no room for adjustment I would start to suspect an amp problem. Next put in the new rectifier with the old power tubes, see what you get. If normal with some room for adjustment, it's not the rectifier and amp itself is good. If good with old tubes but not now it's likely the rectifier. Next try the new power tubes with one of the rectifiers that checks out good. If you still get a high reading it's the power tubes out of spec. Talk to whoever you got them from at that point. From some dealers new tubes may be matched but matched out of spec.
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yarmo
Junior Member
Posts: 69
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Post by yarmo on Aug 25, 2014 9:29:47 GMT -7
Thanks guys!
I did put the old rectifier tube with the new power tubes and the bias went down to 36 mA. Still, I had nowhere to go but upward. I left it there because I had rehearsal last night and an upcoming gig this thursday. I played for three hours and all seems well for now.
I'll put the old power tubes in next week to try to eliminate the amp as being the source of the problem. I'll keep you informed.
Thanks again.
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Post by thunderstruck(formerly doc001) on Aug 25, 2014 11:11:44 GMT -7
yarmo, it sounds like a resistor might need to be replaced in the bias section. Not a big deal. Any knowledgeable tec should be able to do it easily. You should email the Doc!
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Post by Jaguarguy (Mike) on Aug 25, 2014 14:32:42 GMT -7
Here is Doc's response to Yarmo's other thread on his problem: Doc: "Try to remember to request MEDIUM range tubes when ever your replacing output tubes from your supplier. The KT 66's you received are probably high range or higher transconductance output leaving little or no swing on the bias pot. You can increase the bias resisters value by say 10 or 20k and increase the bias range for those tubes. The bias resistor is the resistor connected to the bias pot. Or you can leave it alone if you if you like the sound, KT 66's can be safely biased to a max of 44Ma / tube at idle. So 39 or 36 will be OK, I like 32 to 34 mA my self. DR.Z" Read more: ztalk.proboards.com/thread/63948/bias-problem-route-66#ixzz3BRSNY1fx
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Post by thunderstruck(formerly doc001) on Aug 26, 2014 7:55:34 GMT -7
I used to bias mine a bit hot. I did notice when I switched from shuguangs to the gold lions that medium/average tubes were in need of a different resistor in the bias section. Otherwise the GL's would not bias up same as the shuguangs.
Doc nailed this one!
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yarmo
Junior Member
Posts: 69
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Post by yarmo on Aug 27, 2014 11:07:50 GMT -7
The odd thing is that I did request medium tubes and have been using GL KT66s for since they came out.
There are numbers on the box but damn if I know what they mean :
Set that I had no problems :
lp : 46 (both) Gm : 4,500 (both)
Set with the problem :
lp : 45.1 and 45.3 transconductance : 3850 and 3900
Thanks again for the help guys!
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Post by thunderstruck(formerly doc001) on Aug 27, 2014 13:20:01 GMT -7
Maybe Gold lions have changed. Or you got some bad tubes. New tubes are notorious for being poor quality.
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