Desireable interactive guitar player feedback generation

[MelodianChew]

Hello, TuTians,

I wasn't exactly sure which category I should post this question, but anyway, this is a question about a phenomenon that I have rarely seen addressed in guitar amplifier forums. And that is:

Which components or stages of a guitar amplifier should I direct my attention to, for modification or parameter tweaking, that would have the most success in enhancing the phenomenon of having that entirely desirable sort of spontaneous, sustaining feedback, that can happen while you are playing, that seems to be a natural by product of SOME certain rare amplifiers? What is it that I can change or modify about a circuit in a tube amplifier to build this kind of interactivity with the guitar into an amplifier? Is such a thing even known?

I KNOW that this kind of playability in an amplifier is possible, within reach, and NOT built with some special kind of "un-obtanium," because I had the opportunity to play through such an amplifier, ONE time. This was when I was in a studio during some recording sessions, in which I was tasked with dubbing in a little solo part into a section of a song. I only had my guitar with me that day so, the engineer just grabbed one of his friend's heads that happened to be at the studio that day. When I plugged into that amp, the tone completely floored me. I wasn't even ready for it! 

No. seriously. When I plugged my guitar into that amplifier, hit a chord to check for tune and then played a couple of notes, just merely bending into vibrato, the thing CAME ALIVE!  How easily it would just sing... and sustain perfectly into these notes that were some portion of the harmonic series, that seemed to work with every note I played. I was so astonished at the incredibility of the tone from that amplifier, that I actually DID NOT get to play on the solo in that song that day. 

I know this sounds silly, but every time I tried to do a take, I got completely distracted by this amazing electric tone that I had never heard coming from my guitar before. So much so, that those emotions totally devolved my playing into nothing but fits of laughter and Tourette's Syndrome like cursing, just stuck in postures of complete exasperation, high-fiving nothing but air; like some over-excited Patriots fan watching some epic touchdown. Embarrassing. I simply couldn't keep it together. And because time is money in a studio, after about 3 or 4 tries, I got quickly switched out to give someone else a try on the solo. (Which he nailed, by the way.) 

I'm sorry for such a long dramatic anecdote of my first time with a really great amp, but up until that day, I had NO IDEA that kind of tone was possible in a guitar amplifier. Of course, you guys will relate, because that kind of feeling is, after all, why we are all here. So, I guess what I am asking is, what do I need to do to get, build, modify or acquire an amplifier like that one? And I'm talking about getting that singing spontaneous feedback, that would just miraculously appear at the end of a run.

The gear I used couldn't have been more straight forward. That is why I KNOW that tone was ALL the amp. I had my own guitar with me that day, which was an all stock 1996 Japanese Fujigen made Fender, '68 reissue Strat in naturally finished swamp ash, and a maple neck. This guy's mythical amp that I plugged into was a non-master volume, non-plexi (i think), '68-72 or so, 100w Marshall Super Lead head, plugged straight into an older, slanted Marshall 4x12 cabinet. The engineer told me that the amp WAS modified in some way, but he didn't know how. And curiously, I WAS NOT playing at any deafening kind of level or even really loud at all.  

He had grabbed that amp for me to use because it was very similar to my OWN, which was a stock '69 100w Marshall Super Bass that I plugged straight into a vintage 30 loaded slanted Marshall 4x12 cab. But it didn't sound like that!

So, can that kind of feedback come from any type of amp? If so, what kinds of things should I focus on in an amp build, or WHICH KIND of amp build should I target in building to get that kind of feedback? Does it have to be a Marshall style circuit? I do love that Marshall kind of tone, by the way. Could I get something like that from a modified or tweaked 18w clone or a JCM800 micro kind of kit, or something else? 
(Am I playing 30 years and still an amp noob?   )

Thanks for ALL of your kind help, 

Scott

[K O'Connor]

Hi Guys

Sustain begins with the instrument being set up properly. One thing to avoid, as TUT5 points out, is to NOT crank the pickups up as close as they can be to the strings, especially with humbuckers. The magnetic tug will damp the vibration of the strings particularly during the release part of the note envelope. back the pickups off just a little and the natural sustain of the instrument will be unencumbered.

Most players associate sustain with their lead sound, relying on saturated distortion and/or compression from the preamp or amp. That is certainly one form of sustain. But as we saw in TUT, clean sustain is the core of achieving long ringing notes. A simple matter of adjusting each stage for maximum symmetric signal swing. Some amps will achieve this without it ever being thought of by the builder. Examples of this are the Traynor Bassmaster (YBA-1A not the YBA-1) and Custom Special, or the high-voltage Marshall plexis, later called Super lead and Super Bass. There are many others.

Many hobbyists and builders have followed the TUT advice for gain stage adjustment and have created outstanding amplifiers with singing tone. Almost any tube amp can be similarly tweaked; it may be more of a challenge with tiny amps like Champs et al.

Another way to achieve this kind of feel is to have excess gain. Guitar pickups have a range of output signal and if you look at that and compare it to the speaker output of a given amplifier, you can easily calculate how much gain the amp must have to be pushed to its max. Most amps have a lot more gain than this. For example, Fender amps were designed to match Fender single-coil guitars and therefore have quite a bit of gain. The voicing gives a warm clean sound but not one that sustains well with most other kinds of pickup, at least not in the "clean sustain" way that TUT describes.

You can build an amp with a minimum tube count and have beautiful clean tones, but they may still seem a bit "dry". Adding some more gain can imbue an effortlessness that makes it feel like the guitar is playing itself without leaving the beauty from the initial circuit.