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In Reply to: Re: Citation II AC balance posted by Jim McShane on April 6, 2007 at 19:01:07:
Jim, I use a method similar to your scope method. The difference is that I invert one signal and adjust for as close to zero as I can get on the added signal.
Follow Ups:
Just a thought - since one tube already is inverted (due to the action of the phase inverter stage) simply summing will work even better. Get the no-signal traces to overlap as accurately as you can, set both channels to the same scale (I use 1-2 volts and AC couple the probes).Then when you sum/add the two forms your goal is to get as close to a straight line as possible.
If you invert one scope channel you are "undoing" the phase inversion already present. Don't invert one channel, you'll see the improvement in the procedure right away.
TOP SECRET HINT! - Shhh, don't tell anybody I told you...
After you get the straightest line possible on your scope rotate the balance pot clockwise (as viewed from the top) about 2 widths of the slot in the pot shaft. This will give a tad more THD, but a much nicer distortion spectrum - a "waterfall" pattern, if you will, with each higher order lower in amplitude than the one before.
"After you get the straightest line possible on your scope rotate the balance pot clockwise (as viewed from the top) about 2 widths of the slot in the pot shaft. This will give a tad more THD, but a much nicer distortion spectrum - a "waterfall" pattern, if you will, with each higher order lower in amplitude than the one before."This suggests an alternative method of just tuning the thing by ear in the first place. No need for any test instruments at all!
I have had amps with AC balance controls. What I learned from them is that AC balance is not as critical as I might have guessed. Twisting the knob definitely had an audible effect, but it seemed to me more one of flavor than quality. What I mean by that is that if it's a good sounding amp to begin with then a *little* imbalance didn't ruin that; there is no narrow sweet spot where everything comes together. Instead, my experience was that there is a fairly broad range where the difference in sound was small.
When I tried tuning by ear and then checking with a scope, I found that generally I did not select perfect balance. I never made any careful notes (never mind quantitative measurements) so I can't say for sure, but I doubt I zeroed in on any repeatable level of 2nd HD or anything like that.
Of course, I'm not suggesting that gross imbalance is OK, just that perfect balance might be one of those things (like tremendous channel separation) that is not as important to good sound as we might guess. As you suggest, a little imbalance might actually sound better, perhaps depending on the rest of the system. That's up to the individual, I suppose...
As I said in my first reply on this, AC balance is a non-critical adjustment. We agree Dave.But you can hear the effect of achieving the "waterfall" pattern. The problem is if you go too far you quickly ratchet up the THD.
I set the amps I restore up on the spectrum analyzer, but I have found that the "two slot" trick gets pretty close to that ideal spot, assuming the amp and tubes are in good condition.
My main point was to emphasize that the stock meter is not the optimum instrument to set the AC balance. It was good for Stu to put it in the amp, but it works much better for bias than balance.
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