"Good" Feedback

[TopOfTheMountain]

What, if anything one thing, contributes to the "good" feedback one gets when holding a note? I've played low, medium, and high front-end gain amps and it varies. I've had it and not had it at all volumes also. I'm thinking it's more in the power amp section.
The Peavey Classic series with it's higher gain doesn't seem to produce it nearly as well as my Fender Deluxe Reverb (slightly modded). My '77 Marshall is kind of in the middle. I've played many, many amps and am just wondering if there is any theory behind this.

[Wilder Amplification]

What, if anything one thing, contributes to the "good" feedback one gets when holding a note? I've played low, medium, and high front-end gain amps and it varies. I've had it and not had it at all volumes also. I'm thinking it's more in the power amp section.
The Peavey Classic series with it's higher gain doesn't seem to produce it nearly as well as my Fender Deluxe Reverb (slightly modded). My '77 Marshall is kind of in the middle. I've played many, many amps and am just wondering if there is any theory behind this.

Volume and room acoustics.

[Steve Conner]

Also the quality of your guitar and pickups (some guitars are microphonic and just whistle and screech) and the phasing of your pickups and amp. (If you can't get that good feedback, try swapping the speaker terminals over.)

And EQ, an amp with a scooped midrange won't feed back so readily.

Not to mention where you're standing relative to the blast from the speakers.

[Gibsonman63]

I have an 85 Marshall combo that does it beautifully. The guys are correct, don't scoop the midrange. Also, I notice that the bridge pickup alone works best. Proximity to your amplifier is another factor. The TS-9 works well to bring it out as long as you don't go insane on the settings. I have also noticed that once I get it going there are a couple of fun tricks you can do. Try lightly bumping the whammy bar and you can get higher and higher overtones. The other fun trick is to use your phase 90 and keep increasing the speed after you obtain infinite sustain. Once you get it going you get cool warbling sounds like an alien invasion and the volume control on your guitar controls the frequency.


I like the big knob so much on the Phase 90 that I put them on my other pedals so I can adjust with my foot while I am playing.

[daz]

It's mainly midrange as others have said. The particular mid frequency and amount are critical too, and if it's produced in the OD stages it's even better. Volume and room acoustics help, but this is what really make it happen. One way you can see this is when tweaking the pre stages of an amp you're designing. As i've spend a stupid amount of time trying every circuit variation known to man with mine i have found configurations where it just doesn't happen easily at all, all the way to the opposite end of the spectrum where even at TV volume levels it will just start feeding back and go into high harmonic feedback at the drop of a hat. And one thing i've noticed it great acoustic feedback and great tone in general are not mutually exclusive, in fact one is the product of the other. At least thats been my experience, tho it's actually in regards to marshall style tone because as others have said, scooped tone won't do it easily, and it cannot be said that scooped tone isn't great. I don't like it but some do, so there is an exception to that rule.

[TopOfTheMountain]

Well, to be pricise I will pick one room where we have played. I used a Les Paul and a Peavey Classic 20. I basically run the EQ with everything up. I have tweaked my amps to do this without too much bass or treble. I can lean the guitar into the amp and it is very hard to get it. Halfway through the night I decided to change amps just for the heck of it. I hooked up my Deluxe Reverb (with lower gain) and got it all over the stage (almost). Last week I played my Marshall and it would do it every now and then.

I do believe my Fender has more power amp distortion. That is what got me thinking is there a certain stage in the amp that lends itself to feedback.

The Deluxe is running a SS rectifier so it's not the sag that does it.

[daz]

Volume will make it happen in any amp, but my point was it's the mids in the tone that make it happen easiest. Put a EQ in the front end and do a reverse V to accentuate the mids PRE distortion then watch what happens. Your classic 20's tone stack is i believe POST preamp where a lot of distortion is happening even if it's a cranked amp situation, and like i said it's the mids that are in the signal AS it's being overdriven that make feedback effortless. The further back in the signal flow it is the less it will affect the ease of feedback. Volume will get it, but it will happen much easier regardless of volume with what i'm saying.

I had a C20 and it's not a particularly middy amp. Plus the tone stack give a midrange thats not the type of mid you really need for this. Set that EQ with the mid boost centered around 700Hz and see what happens.

[TopOfTheMountain]

Hmmmm. Now that's interesting. You are right. The tone stack in the Classic 20 is late in the preamp. As far as that goes, so is the Marshall's. The Classic has been modded for less of a "scoop". Maybe I'll experiment with moving the tone stack.

[daz]

Yes, so are a marshalls. But a marshall tend to generate mids in the pre stages so you don't need the tone stack to do much. What you should try IMO is removing C5 and in it's place put a .0047uf and 470k resistor in parallel. Unfortunately thats a bitch to do in those amps, or at least the C30. But i suspect the C20 is hard to work on too. But if you do that what it will do is allow mids to pass freely while attenuating other frequencies. Just like those marshall 470uf/470k filters, but the .0047 will pas more mids then highs like the marshall filter. I did this to my amp and it really give you a nice mid boost of the right kind to get that feedback. but you can experiment with where in the pre it works best and what cap size works best. But again, not so easy on that amp. On a point tot point like mine you can try several values and places to put it in 5 minutes. The pv takes 20 minutes just to disassemble and re-assemble it. But in any case theres a suggestion for you.

[Wilder Amplification]

One thing that always worked for me with Les Pauls and EMG pickups (two things going against regenerative feedback) was to stand close to the cab but perpendicular to it, not facing right up on it. Can't explain why that worked....I just know that it did and results were pretty consistent no matter where I played at.

[TopOfTheMountain]

Jon, even better than that...I've taken the bottom of the guitar and touched it to the amp or cabinet. Instant feedback! Of course it looks stupid.


Daz, I've already changed a lot in the preamp of that amp. I believe I settled on just a minimal approach and went with .022uf caps and 1Meg resistors between each stage. When you say that the .0047uf will pass more mids than highs, that has me wondering. Wouldn't the smaller cap just have a higher cutoff and pass everything above a certain frequency?

[Wilder Amplification]

Daz, I've already changed a lot in the preamp of that amp. I believe I settled on just a minimal approach and went with .022uf caps and 1Meg resistors between each stage. When you say that the .0047uf will pass more mids than highs, that has me wondering. Wouldn't the smaller cap just have a higher cutoff and pass everything above a certain frequency?

That would be correct. Going off of the 1/2piRC formula, a 22nF/1M combo would have its -6dB down point at 7Hz while a 4n7F/1M would shift it up to 33Hz at the -6dB down point. Doesn't seem to make much of a difference going off of the mathematics of it all, but sonically you might be surprised.

[daz]

Doesn't seem to make much of a difference going off of the mathematics of it all, but sonically you might be surprised.

Exactly. I don't know the math generally, and it doesn't always make sense anyways. I stopped using the duncan TSC because what it showed me rarely sounded like i expected. I won't comment on the math behind it, i just know it worked for me to get the right kind of mids flowing that gave me much better feedback/sustain. 0047 may not be the best value either....i haven't explored that nearly enough, plus every amp is different in where the peaks and valleys are.

[TopOfTheMountain]

Wow. Very interesting. I used the .22uf just because that's what "everybody" uses. I didn't know it went down that far. It's a wonder it's not all flubby cut off at that low of frequency. Is there a chart or something to let me learn more about the cap/resistor relationships? I've always thought that .0047uf would be way too thin. I guess not.

[daz]

You think so, eh? Look at the classic 30. It has a 470pf coupling cap ! But the thing is, nothing is too small when it's used with a resistor to form a filter like i described. Marshall uses 470pf caps all the time like that and as long as it's parallel to a resistor you're passing ALL frequencies but they are attenuated a bit due to the resistor. But the frequencies the cap passes get thru w/o being attenuated so they *in effect* are boosted the same as if you took a EQ and cut all the frequencies you want less of and left the sliders that you want more of at 0 Db. By the way, when you say "it's a wonder it's not flabby" i'm not sure what you mean, but a .0047 wouldn't cause flab unless you DUMPED the frequencies it passes such as if you connected it to the signal and ground. But in this case you are accentuating it and in effect as i explained CUTTING flab.