greiswig wrote:I had the opportunity to test my hypothesis a bit: in our large room at church, I opened up the amp quite a bit and played first with the head on the cabinet, then with the head about 10' away on the floor. For both situations, I played with the phase switch in both positions, for a total of 4 conditions.
Results: with the amp on the cabinet, the distinction between phases was pretty clear. One phase in particular was easier to generate feedback from and punchier overall. With the amp removed from the cabinet, the distinction between the two phases was much less clear, and really seemed to relate to the sensation I had from the guitar itself (i.e. what I was hearing/feeling from the guitar directly).
So I think this supports my idea that there is a feedback loop between the cabinet and the tubes/circuit that could be either positive feedback or negative depending on the polarity of the cabinet. But I'd like to have some other people try the experiment themselves.
Which phase was it that you got the better feedback with?
With the speaker pushing when you attacked the strings?
Structo wrote:
Which phase was it that you got the better feedback with?
With the speaker pushing when you attacked the strings?
Tom, I don't mean to be difficult in saying this, really. But the answer is "I don't know." If you trust the tranny manufacturer, the way it felt best to me was "in phase."
I don't know how one would measure which direction the cone was moving when the strings picked short of slow-motion replay, and even there I wonder whether the cone would move the same direction every time you picked the same string because of factors I've already mentioned in this thread. That would be an interesting experiment itself.
Also, note that some of the difference was not just on initial attack, but during sustained feedback. This is when the string oscillation is rotating around it's lengthwise axis.
Let's contact the Mythbusters! They have more resources and time than any of us.
Another thread prompted me to resurrect this one. Anyone else tried this experiment?
greiswig wrote:I had the opportunity to test my hypothesis a bit: in our large room at church, I opened up the amp quite a bit and played first with the head on the cabinet, then with the head about 10' away on the floor. For both situations, I played with the phase switch in both positions, for a total of 4 conditions.
Results: with the amp on the cabinet, the distinction between phases was pretty clear. One phase in particular was easier to generate feedback from and punchier overall. With the amp removed from the cabinet, the distinction between the two phases was much less clear, and really seemed to relate to the sensation I had from the guitar itself (i.e. what I was hearing/feeling from the guitar directly).
So I think this supports my idea that there is a feedback loop between the cabinet and the tubes/circuit that could be either positive feedback or negative depending on the polarity of the cabinet. But I'd like to have some other people try the experiment themselves.
I don't remember which book it was, but Gerald Weber tackled this subject in one of his books, using the 410 Bassman as the example.
While I was at Peavey, they made a huge deal out of phase coherence in guitar amps. One of the guys in the transducer group told me once that the parts of a speaker suspending the cone are somewhat like a accordian bellows, and that the direction of the folds allows for slightly more forward cone travel, and acts somewhat like a brake in reverse travel. I never really paid much attention to it, but it does make a bit of sense when I think about it now.
The "normal" and "reverb/vibrato" channels on all those old fender amps we all love are out of phase, and nobody seems to care. Just do what sounds best to your ear. The only time I've ever worried about it, or heard anything of note was in my younger years with multiple amp setups. (too much to carry these days....age has made me lazy) You can get some pretty odd things going on if two amps are out of phase in the same rig.
I used to have my head on top of my speaker cab but I rearranged things so now the VOX 2x12 is standing vertical and the head is on another stack of gear.
I never tried this cab with the amp on top but I hear what you are saying.
Makes sense that the vibrations that are mechanically transferred between the cab and amp would contribute to feedback.
I did some work on my amp a week or so ago and I need to get back inside and adjust the PI trimmer as I seem to have lost my harmonics a bit.
That also seems to be key in having an amp feedback sensitive, having that PI balanced to where the notes bloom into harmonics.
