Mbira
Well-known member
I have been doing lots of guitar amp work these days. I want to see what the general consensus is for biasing power tubes. What I was taught was to feed a 1K signal into the 1/4 inch input, and then hook up the scope to the speaker jack (while a speaker is connected) The black goes to chassis ground, and the red goes to the tip of the output jack. I am then looking at the slope of both tubes and seeing if I get an early cutoff (in the middle of the wave). If so, I adjust the bias until there is the most even looking slope.
I have a couple questions about this technique. First, is it a good one? I have seen other techniques with soldering on resistors, etc, but I want something that is quick and cost effective for the customer. The problem I have seen with this method is that sometimes it seems like the trace is real crisp and the cutoff is very pronounced. Other times, it is hard to see the cutoff, but adjusting the bias just makes the whole slope change angles a little (did that make sense?). Also, sometimes, changing the bias doesn't seem to get the the cutoff smooth all the way-in this case, does that mean I need to change the bias resistors? Other times, one side of the wave is clipping while the other isn't-even when the bias is equal on both tubes.
I think that is all the questions for now...
Thanks.
Joel
I have a couple questions about this technique. First, is it a good one? I have seen other techniques with soldering on resistors, etc, but I want something that is quick and cost effective for the customer. The problem I have seen with this method is that sometimes it seems like the trace is real crisp and the cutoff is very pronounced. Other times, it is hard to see the cutoff, but adjusting the bias just makes the whole slope change angles a little (did that make sense?). Also, sometimes, changing the bias doesn't seem to get the the cutoff smooth all the way-in this case, does that mean I need to change the bias resistors? Other times, one side of the wave is clipping while the other isn't-even when the bias is equal on both tubes.
I think that is all the questions for now...
Thanks.
Joel