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Post by jeffrsn on Mar 3, 2009 5:26:19 GMT -7
I just ordered a set of NOS Sylvania tubes (a 6v6, 12ax7 & 5y3gt) for my Airline Valco amp and I don't know jack about biasing (My Z amps are powered by EL84's so I know they aren't at issue).
Can you guys enlighten me please?
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Post by benttop (Steve) on Mar 3, 2009 6:45:21 GMT -7
There are a couple of methods, but if you've never been inside your amp and don't know much about that, I recommend you stay out of there and have a tech do it for you. There are lethal voltages inside there, even when the amp is off. That is not an exaggeration - you can be killed if you touch the wrong thing. If that doesn't bother you, the best thing to do is get one of those bias tools you can get. Weber makes one, and Groove Tubes use to but I don't know if that's still available now that they are owned by Fender. You stick the bias tool in the tube socket and plug the tube into the bias tool, then use the trim pot inside the amp to adjust the bias while watching your multimeter. You'll need to know how much cathode current you want for the amp and power tubes you're biasing. Set it there and you're done. Don't forget that part about death and danger and all that - it's kind of an important safety tip.
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Post by jeffrsn on Mar 3, 2009 18:07:41 GMT -7
thanks Steve.
I've swapped tubes on my Ghia but rather than flirting with death I reckon I'll take the Valco to my equipment tech
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Post by benttop (Steve) on Mar 3, 2009 19:27:06 GMT -7
Prolly a smart idea - getting kildt kinda ruins a guy's day... ;D ;D
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Post by dixiechicken on Mar 8, 2009 7:18:03 GMT -7
DC here! When you bias like "benttop" describe - take your time and dont rush things. Let the amp warm up first for 10 minutes or so. (even more - depends on the power tubes - my Route66 have the KT66 tubes big bottles - they take almost 30 minutes to reach proper working temp)While the amp is not properly warm - the current tends to drift over time. Adjust to lets say 32 milliAmps - wait 5 minutes and current may have drifted to 36 milliAmps. On my Route66 this drift is quite pronounced. YMMV Just take your time and be careful, dont rush things and you should be fine. If possible keep one hand in your pocket at all times. Good luck! Cheers: Dixiechicken!
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Post by jeffrsn on Mar 8, 2009 14:46:31 GMT -7
OK, so, I took the amp to my tech who told me it's a fixed bias amp, so I changed the 6V6 & 12AX7 tubes (one at a time as per Myles' suggestion) Thanks for the information guys. That's what I love about this forum.
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Post by benttop (Steve) on Mar 8, 2009 19:33:00 GMT -7
OK, so, I took the amp to my tech who told me it's a fixed bias amp, so I changed the 6V6 & 12AX7 tubes (one at a time as per Myles' suggestion) Thanks for the information guys. That's what I love about this forum. The term "fixed bias" generally means "fix the bias when you change the tubes" so I hope you had him adjust it. On the other hand, Cathode Biased usually means you don't have to make any bias adjustments when you change the tubes. In both cases though, you want matched pairs. If the amp only has one power tube, I can almost guarantee that it's cathode biased, not fixed bias. But I've been wrong before - there's always something new to learn. So, one or two 6V6's?
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Post by jeffrsn on Mar 9, 2009 17:17:23 GMT -7
One 6V6. I changed the 12AX7 first, then the 6V6; nuttin blew up and I didn't electrocute myself. I'll continue to keep an eye on it. thanks again Steve
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