talbany wrote:Upon request I dragged the amp and cab downstairs..The amp is sitting in a separate room separated by a partition so virtually no acoustic coupling going on.. The cord is 12 ft clearly stretched out...Master vol on 2 1/2 no lator straight in.. PAB on Delta King Hollow body same licks...Clearly the amp is more picky about which notes it chooses to feedback on and the harmonics generated seems more random..You be the Judge!!
I've attached my spectrum analyses of the whole test and of one shorter frame
I can see that the pitch of the note is absolutely constant (apart small sections with Tony's vibrato) hence the preamp resonating "on his own" hypothesis is not valid: the fretted note is essential part of the resonance
what's going on with time is that harmonics relative amplitudes change and this is perceived as the pitch going higher
after some time harmonics around 700Hz become prevalent
there's also another "resonance" around 2000Hz
given the frequency response of the skyliner tone stack the above is absolutely unespected
cheers
teo
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Teo, could you explain what these traces are showing? Why the three sections? I can see it is frequency vs. time, but I'm not familiar with spectrum analyzers and how they work.
I'm inclined to agree with Greg on the NOS parts, based on my recent experience. When HAD used them they weren't all that old, mostly "recent surplus" in many cases, but I think he used what he did because he knew what the differences were.
Remember, HAD used a spectrum analyzer from pretty early days so he knew exactly what was going on in his builds. I think that knowledge contributed to his early success.
HAD's basic circuits get you most of the way there, BUT in my builds I am finding that, for example, in my #13/#40 ODS, changing out preamp plate and cathode resistors from modern Dale RN65/Riken carbon films respectively to '70s Mepco-Electra/late '60s Electra metal films respectively does get you a noticeable tone sweetener. I should add this particular amp has old iron in it too and was already sounding pretty authentic.
Last edited by David Root on Mon Mar 12, 2012 3:02 pm, edited 2 times in total.
Structo wrote:Do you guys think having ceramic sockets can play a role in this feedback?
I remember reading that the ceramic sockets tend to be microphonic because of the hardness and being directly coupled to the chassis.
It's one of the reasons that phenolic sockets are recommended for guitar amps.
Tom's got a point here - possibly the effect is a mechanical coupling and not electrical - the OT, tube sockets and of course that wonderful chassis that was used for this particular specimen...
FWIW..My Music Man amp has Fender CTS pots/ Phier resistors. KOA Speer's and NOS Mallory caps and Sprague Atoms and is laid out just like my 102 variant.. I did use the pre amp out on my 102 variant into the MM output section and note the feedback was definitely there.. So I have determined it is in my preamp..So it is not in the output section... So we can axe the tubes/OT/and output section filtering and PI as being the reason why the 102 variant is so sensitive..The lead dress is pretty much the same in my Music man amp..Although I cannot get the amp to feedback with the same intensity as my 102 variant!!..Why?
BTW.. My Music Man amp does carry with it some nice feedback!!!.. Just not with the same intensity as My 102...No clue about the ceramic sockets!!
Teo
Thanks for the spectrum test VERY COOL!!..So since the analyzer confirms the use of the Skyliner stack then I can rule out any micro-phonic part..
Tony
" The psychics on my bench is the same as Dumble'"
The socket mechanical FB theory was proposed earlier in the thread, by Gil I think. Isolating the amp from the cab acoustically pretty much kills that hypothesis wrt the speaker output. I don't know how much vibration an OT produces when energized with signal, but that seems like the only other possible source of mechanical FB.
martin manning wrote:The socket mechanical FB theory was proposed earlier in the thread, by Gil I think. Isolating the amp from the cab acoustically pretty much kills that hypothesis.
That was not me, Martin. I think that amp has some mojo going on which is either "electronic" feedback somewhere, or something else that causes it to generate harmonics. Think like a diode effect, subtle signal clamping somehwere along the way. Can't prove any of it, though.
martin manning wrote:The socket mechanical FB theory was proposed earlier in the thread, by Gil I think. Isolating the amp from the cab acoustically pretty much kills that hypothesis.
That was not me, Martin. I think that amp has some mojo going on which is either "electronic" feedback somewhere, or something else that causes it to generate harmonics. Think like a diode effect, subtle signal clamping somehwere along the way. Can't prove any of it, though.
I posted where I am playing the open strings..After the initial attack and sustain the notes eventually fade but the dominant harmonics remain.. At least this is what I hear (most of the time)..IYO would this be a clue of either the so called diode effect, or perhaps a kind of electronic feedback?..Thinking out loud
Thanks!!
Tony
" The psychics on my bench is the same as Dumble'"
on the X axis there is time, the (whole) image correspond to the entire Tony's test, the other smaller image is a focus on the first three notes
on the Y axis there is frequency displayed in log scale (to relate them to notes on the displayed piano keyboard)
amplitude of the signal at each frequency is displayed with shades of color of increasing intensity, you can clearly see the fundamental frequency of the note (usually the lower trace) and all of his harmonics (traces above the fundamental)
on the first note for instance you can see that there is an harmonic around 600Hz onsetting after 1/4 of the length (when the feedback gets in) and becoming dominant in the last 1/4
analysis has been carried out by a fourier transform of size 1024 with 50% overlap, followed by frequency reassignment analysis
I currently cannot think something that doesn't involve the string and the pickup into this behavior...
When Tony stops the string the sound ends immediately, I think something resonating entirely inside the amp will cause some sort of tail to the sound
It's a sort of Fernandes sustainer working with just one pickup instead of two
Pickup inductance & capacitance + cable capacitance + v1a input capacitance +... constitute an almost resonant circuit, and pickup via magnetic field can engage the string into play too
Not all tube input stages do that, at least at such a level, but on the contrary we all know that transistor input stages never do that...
I've been trying to get my non-hrm to produce similar feedback, especially on the clean channel.
While I cannot reach the same intensity, I'm finding that adjusting the HEATER wires near V1, as well as the treble and middle wires has noticeably increased harmonic feedback. I can now get my amp to produce runaway clean channel non-PAB feedback in multiple places on the neck (baker).
Could some one else try this and report back? Or am i just crazy?
I've been trying to get my non-hrm to produce similar feedback, especially on the clean channel.
While I cannot reach the same intensity, I'm finding that adjusting the HEATER wires near V1, as well as the treble and middle wires has noticeably increased harmonic feedback. I can now get my amp to produce runaway clean channel non-PAB feedback in multiple places on the neck (baker).
Could some one else try this and report back? Or am i just crazy?
Interesting, I don't think heater wires has been suggested as a possible source of the feedback. Can you tell us more specifically how you moved the heater, mid, and treb wires? Perhaps we can verify it.
I've lost track of what has been tried and experimented on.
Did we go over the phase inverter and whether or not having a closely matched or balanced 12ax7 is the key?
I just got a GE grey plate that is supposedly 1700/ 1700.
I'm going to try that in my PI and then I have a few other GE grey plates for V1 and V2.
This is a long thread, so it is easy to miss what has been tried. Tony verified that the source is not the PI or power section and that it is also not the tubes.
