bloom into feedback....

[redshark]

What is it inside an amplifier that makes it bloom into harmonic feedback? Trainwreck express or Dumble OD special and selected clones has this quality..what is it exactly that cause this? how can you help an amp to do this?

I recently modded an old traynor yba1a and just revoiced the channels, made it adjustable bias but this amp has that quality, more than any other comercial amp I've heard...I noticed resistors instead of choke, solid state rectifier similar to the express....but the preamp has nothing in common with it.

Help me understand this please...

[tubeswell]

I've got a 5G9 tremolux clone that blooms into feedback when you stick a pedal in front that is set up 'right' in terms of overdrive. AFAICT, its a combination of gain, sag and freq networks output stage and OT. The feedback is just the freq harmonics making the geetar strings start vibrating again, usually happens with lots of gain. The freq depends on how you have set up the signal chain to pick up on the 'right' frequencies (mid to high freqs). The sag and bloom is a power supply thing - not necessarily related to resistor filtering (the 5G9 has a CLC filter), but more a peak voltage demand thing that causes temporary current starvation - the 5G9 has a 5U4G rectifier, so this may assist the sagginess in this regard. Also related to transformer saturation AFAICT. The trannies needs to be sized to break 'smoothly' into saturation conditions when it happens. There's quite a lot to it, and I admit that for me its more a result of a flukey outcome than deliberate contrivance.

[marcoloco961]

http://www.freewebs.com/valvewizard/index.html

Start here. It will explain how harmonics are created, which type are created and where.

[PCollen]

I've got a 5G9 tremolux clone that blooms into feedback when you stick a pedal in front that is set up 'right' in terms of overdrive. AFAICT, its a combination of gain, sag and freq networks output stage and OT. The feedback is just the freq harmonics making the geetar strings start vibrating again, usually happens with lots of gain. (snip)...

Also involved is the players physical position relative to the speakers. If you look for it, as is the case of a player like Santana or Beck, you will see them play within, or move into, a 'zone' where they can achieve and control the feedback.

[redshark]

Thanks guys!! Please keep it coming..I would like to understand more this thing....

[Alexo]

I'll buck the trend and postulate that the guitar is a big factor in this equation.

Electronically, you're doing the same thing as holding a mic in front of a PA speaker. The frequency that is the most resonant in this feedback loop is the one that will "bloom," as the inverse is often done when the soundman cuts that pesky 1K or whatever that keeps feeding back.

So I guess I shouldn't just say it's the guitar, but it's a combination of the amp and the guitar coming to an agreement on the frequency of the note you're fretting, or often a higher order harmonic. In any case, the resonance of the guitar (not just magnetic resonance/frequency response of the pickups, but the physical resonance of the body and neck) form a significant part of the equation. ...ever notice how much "bloom" an acoustic guitar can get?

[Analog Assassin]

When I was younger, I had modded an old Fender Pro Reverb to "Boogie" specs via a Torres kit.
Although I basically destroyed the value of that amp, it had the most incredible sustain and effortless feedback.
All I had to do was rub the string against the fret, and this smooth feedback (not high end squeal) would begin oozing out.
I think the biggest part of this was not the extra gain, although that helped. It was the midrange boost. I don't remember the value, but it was a switching pot that replaced the original treble pot. And it switched a cap in, I believe it put it parallel with the original treble cap.

[iknowjohnny]

IMO it's certain mid frequencies amplified in the gain stages, especially in amps with preamp OD. The mids are what bring that in any case.

[skeezbo]

redshark
Everyone has experienced this mechanical effect: if you get closer to your amp, or play louder (even clean) or have a more resonant instrument you can get feedback that will sustain a note. Partials that are consonant harmony-wise and that float within a few octaves above the fundamental note being played seem especially desirable. The difficult question you ask is this: How do you design an amp that can produce this effect, to be used as a musical tool, at reasonable volume levels?
These "bloom" tones are a part of the harmonic structure of the individual note when played clean, too, of course, but here is something to design for: harmonic distortion seems to emphasize these bloom partials in comparison to the volume of the fundamental, allowing the singing to happen at a lower apparent volume. Design a preamp that has beautiful in-tune harmonic overdrive and you will encourage bloom. Or play a smaller amp cranked and let your power tubes create overdrive as well.
I think iknowjohnny has it right: engineered harmonic distortion sounds most natural to me when it is the mid frequencies that are broken into harmonics and the lows stay tight and the highs stay sweet. Tone stacks, however, generally produce a dip in the mid frequencies, so classic amp designs with the tone stack that comes late in the preamp (Fender tweed Bassman, classic Marshall and Vox designs) have more mid content in the preamp overdrive stages and seem to have more "bloom" than a Fender blackface or similar design with the tone stack and its inherent mid dip after the first gain stage. More recent designs use multiple stages after the tone stack to create "bloom", some with mid boost, tone stack defeat or larger-cap bright switch circuitry to increase the mid range content early in the signal path.

Skeez

[tubeswell]

I've got a 5G9 tremolux clone that blooms into feedback when you stick a pedal in front that is set up 'right' in terms of overdrive. AFAICT, its a combination of gain, sag and freq networks output stage and OT. The feedback is just the freq harmonics making the geetar strings start vibrating again, usually happens with lots of gain. (snip)...

Also involved is the players physical position relative to the speakers. If you look for it, as is the case of a player like Santana or Beck, you will see them play within, or move into, a 'zone' where they can achieve and control the feedback.

Yes I assumed everyone knew that, but I took from the OP's question that the topic was; what was it in an amp that; firstly, predisposed it to (sag and) bloom, and secondly; predisposed an amp to encourage feedback? Hence my response; i.e., power supply conditions (for the sag and bloom) and gain and freq response (for the feedback) AFAICT (which, as everybody knows, is Irish for 'a faict' )

[Structo]

In the overdrive special amps, it seems to be a combination of a few different things that have to be right to get the bloom.
Even the guitar can play an important role along with the type of pickups used.
First is tube selection. As we all know, every tube is different.

Second, the ODS amp should have a balance trimmer between the PI plate resistors. This varies between 5K and 10K.
The value of the plate resistors may contribute but I have been able to achieve bloom with both the 100K/110K and the 110K/120K pair.

The critical piece in these amps is the trimmer adjustment. It can take a while to learn how to do it and and to find the sweet spot.
As you make very small adjustments, lightly play the strings with the amp turned up pretty loud.
At some point you will start to notice more overtones and harmonics wanting to take off.
This is generally the place to leave it.
It usually ends up being around 5-6v DC higher on the input side of the inverter after adjusting.

[iknowjohnny]

It usually ends up being around 5-6v DC higher on the input side of the inverter after adjusting.

This is interesting. So would you say that if your PI has a much larger difference between the plates like say 20v, that closing that gap is likely to help with bloom?

[redshark]

Thanks for the opinions guys.
I know all tube amps have this effect to an extent but I've noticed some amps have that quality to take a note and take off more than others.

[mcrracer]

Interesting subject. Can a SE amp or should a SE amp be capable of this Blooming effect? What if it had an Express type preamp? Or is the Blooming strictly a power supply phenomenon?

[jaysg]

I've found that a very lively amp/guitar setup as described, won't do it as easily in a band setting. Anyone else experience this?