🚀 go-pugleaf

ptaylor wrote in message <3639AF1C.7E72A029@uswest.net>...
>Hi..
>This may seem kinda weird,but I was wndering if it was possible to make
>tubes "Ring",kinda like how transistors can/will oscillate..
>like if you were to overdrive a tube in a certain way..
>Like if you had a tube preamp,would it be possible to feed a signal
>through it,and at the other end have a signal with "ringing" at certain
>frequencies..Something like a resonance,perhaps..??
>
>I dunno.. I just thought I would ask..


Assuming you're talking about power amplifiers, oscillation is usually
caused by the output stage of the amplifier not coping well with the load it
is driving. To cut a long story short, the output stage decides that the
voltage at the output is too low, so it increases it. The load behaves
weirdly, causing the voltage to overshoot, so the output stage reduces it.
Again, the load causes the voltage to undershoot, so the output stage
increases the voltage, and so on over and over again.

In principle it can happen either to a valve amp or to a transistor amp -
I've personally seen it happen in both at various times. Modern amplifiers
are carefully designed so that their outputs won't oscillate even when
driving very badly behaved loads. An ideal load would be a simple resistor,
but real loudspeakers are a strange combination of electromechanical
components (DC motors, effectively) and passive components like coils and
capacitors, so the job of the amplifier designer is made much more
difficult.

Sarah

In article <363c2909.0@212.32.20.195>, "Sarah Thompson"
<sarah@telergy.nospam.com> wrote:

> PS: I should point out that this has *nothing* to do with the feedback
> oscillation that you can get from an overdriven guitar amp. This is caused
> by the sound from the speaker(s) making the strings of the guitar vibrate -
> with enough gain this overcomes the string's natural tendency to dampen over
> time.

   I dunno about that.  I would say it has a *lot* to do with this (acoustic
feedback).

   In both cases, you have a system, one electrical, one mechanical, with
a complex exponential natural response.  And in both cases that system is
coupled to an external medium having a reactive (i.e, capable of storing
and re-releasing energy) nature.  Finally, in both cases there is a path
by which the output of the system is fed back to its input at some point,
the result being that an ordinarily well-behaved (damped) response becomes
unstable to the point that it self-generates oscillatory energy.  The
mechanism is the same, the theory is the same, the analysis is the same,
and the only difference is the topology and the physical nature of the
individual elements of the system.

   I would add, too, to your original post, that the ringing or oscillation
is rarely in hi-fi amps due solely to interaction between the output stage
alone and the load; but it depends instead on the characteristics of the
several stages comprising a global feedback loop.

-Henry

I was just wondering if this type of thing might be possible in a
pre-amp..I have an older recording(I have no clue what it is though)
that has some very interesting sounds,Like harmonics within the
preamp,mostly noticed with guitars..I don't know if this is the cause of
the sound,or maybe just feedback of some kind..(doesn't sound like it
though.) it sounds quite strange,it sound kinda like the sound that a
burnt out light bulb would make if you were to shake it,except at much
higher frequencies..I was thinking maybe this was oscillation in the
tube pre-amp,or maybe distortion,or maybe just from being overdriven in
a strange way..?? I dunno.. I was hoping someone may have heard this
type of thing,or maybe have an explanation..



ptaylor wrote:
>
> Hi..
> This may seem kinda weird,but I was wndering if it was possible to make
> tubes "Ring",kinda like how transistors can/will oscillate..
> like if you were to overdrive a tube in a certain way..
> Like if you had a tube preamp,would it be possible to feed a signal
> through it,and at the other end have a signal with "ringing" at certain
> frequencies..Something like a resonance,perhaps..??
>
> I dunno.. I just thought I would ask..
>
>                      Pat Taylor..,

>Assuming you're talking about power amplifiers, oscillation is usually
>caused by the output stage of the amplifier not coping well with the load
it
>is driving. To cut a long story short, the output stage decides that the
>voltage at the output is too low, so it increases it. The load behaves
>weirdly, causing the voltage to overshoot, so the output stage reduces it.
>Again, the load causes the voltage to undershoot, so the output stage
>increases the voltage, and so on over and over again.
>
>In principle it can happen either to a valve amp or to a transistor amp -
>I've personally seen it happen in both at various times. Modern amplifiers
>are carefully designed so that their outputs won't oscillate even when
>driving very badly behaved loads. An ideal load would be a simple resistor,
>but real loudspeakers are a strange combination of electromechanical
>components (DC motors, effectively) and passive components like coils and
>capacitors, so the job of the amplifier designer is made much more
>difficult.


PS: I should point out that this has *nothing* to do with the feedback
oscillation that you can get from an overdriven guitar amp. This is caused
by the sound from the speaker(s) making the strings of the guitar vibrate -
with enough gain this overcomes the string's natural tendency to dampen over
time.

Sarah

henry_pasternack@my-dejanews.com wrote:

>Each of these remedies dampens the sound by breaking the feedback loop
                        ^^^^^^^
That's "damps"!  An excellent article, otherwise.
-- 
========================================================================
          Michael Kesti            |  "And like, one and one don't make
                                   |   two, one and one make one."
          mkesti@gv.net            |          - The Who, Bargain

In both cases, your answer is more technically complete than mine. However,
the original poster wouldn't stand a snowflake in hell's chance of
understanding your reply. I felt it better to avoid waving my dribbling ego
around and instead tried to answer the question in a way that the
uninitiated in the ways of the dark gods of analogue design might gain some
useful benefit from.

Sarah

Henry Pasternack wrote in message ...
>In article <363c2909.0@212.32.20.195>, "Sarah Thompson"
><sarah@telergy.nospam.com> wrote:
>
>> PS: I should point out that this has *nothing* to do with the feedback
>> oscillation that you can get from an overdriven guitar amp. This is
caused
>> by the sound from the speaker(s) making the strings of the guitar
vibrate -
>> with enough gain this overcomes the string's natural tendency to dampen
over
>> time.
>
>   I dunno about that.  I would say it has a *lot* to do with this
(acoustic
>feedback).
>
>   In both cases, you have a system, one electrical, one mechanical, with
>a complex exponential natural response.  And in both cases that system is
>coupled to an external medium having a reactive (i.e, capable of storing
>and re-releasing energy) nature.  Finally, in both cases there is a path
>by which the output of the system is fed back to its input at some point,
>the result being that an ordinarily well-behaved (damped) response becomes
>unstable to the point that it self-generates oscillatory energy.  The
>mechanism is the same, the theory is the same, the analysis is the same,
>and the only difference is the topology and the physical nature of the
>individual elements of the system.
>
>   I would add, too, to your original post, that the ringing or oscillation
>is rarely in hi-fi amps due solely to interaction between the output stage
>alone and the load; but it depends instead on the characteristics of the
>several stages comprising a global feedback loop.
>
>-Henry

In article <363d7b61.0@212.32.20.195>,
  "Sarah Thompson" <sarah@telergy.nospam.com> wrote:
> In both cases, your answer is more technically complete than mine. However,
> the original poster wouldn't stand a snowflake in hell's chance of
> understanding your reply. I felt it better to avoid waving my dribbling ego
> around and instead tried to answer the question in a way that the
> uninitiated in the ways of the dark gods of analogue design might gain some
> useful benefit from.

   Yes, but I wasn't replying to the original poster -- I was replying to
you.  I'm sorry you have responded so... egotistically.

   Anyway, I disagree strongly with your statement that acoustic feedback
and electrical feedback have nothing to do with one another.  Nor do I
think it's at all impossible to explain this subject in ways that will
be accessible to beginners.  I find it interesting to draw engineering
parallels between related but distinct phenomena.  I think you did a
disservice to the original poster and other readers by drawing a sharp
and needless distinction between acoustical and electrical feedback.

   Had I been in your position, I would have said the following.  We
understand intuitively and from experience that the strumming of an
electric guitar string produces an output from the loudspeaker, which,
if strong enough, may create a sympathetic vibration in the guitar body
that sets the strings vibrating anew.  This feedback (as it is known)
may build to the point that it becomes self-sustaining.  The vibration
will continue until a hand is placed on the strings, the amplifier
volume is turned down, or the guitar is moved away from the speaker.
Each of these remedies dampens the sound by breaking the feedback loop
from strings to pickup to amplifier to speaker to sound and back again.
The likelihood that feedback will occur depends on the properties of
all of these links in the chain.

   Similarly, in a feedback amplifier, a portion of the signal at the
output is fed back to the input.  It is arranged in such a way that the
feedback signal cancels, rather than reinforces, the input signal.  The
result is that the output signal is diminished, but also a portion of
the internally-generated noise and distortion of the amplifier as well.
The ability to cancel noise distortion in this way is one reason why
negative feedback is so useful and popular.

   There is a down side to negative feedback.   If the amplifier drives
a non-ideal load, one that tends to store and re-release energy after
a short delay, there is a tendency for beneficial negative feedback to
get turned around and become harmful positive feedback.  In *exactlly*
the *same* manner that the vibrations of a guitar string are sustained
by acoustical feedback, the oscillatory signals in the amplifer may
become self-sustaining.  This is a common and very undesirable condition.

   The likelihood that an amplifier will oscillate due to perturbation of
its feedback loop depends in part on the interaction between the output
stage and the load, but also upon the properties of all of the stages
contained within the feedback loop.  Controls against unwanted oscillation
may be placed at any or all stages in the amplifier.  A great deal of
effort goes into the design of a good amplifier to insure that it is
well-behaved when driving difficult loads.

   I don't believe in dumbing down my answers, and I think you could
have done a lot better job with your original posting.  But, hey...
what can I say?

-Henry
-----

Whatcha gonna do when they come for you?  Bad boys, bad boys...


-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/       Search, Read, Discuss, or Start Your Own

In <363CD0C1.6C431BC0@uswest.net>, on 11/01/98
   at 01:21 PM, ptaylor <ptaylor@uswest.net> said:

>I was just wondering if this type of thing might be possible in a
>pre-amp..I have an older recording(I have no clue what it is though) that
>has some very interesting sounds,Like harmonics within the preamp,mostly
>noticed with guitars..I don't know if this is the cause of the sound,or
>maybe just feedback of some kind..(doesn't sound like it though.) it
>sounds quite strange,it sound kinda like the sound that a burnt out light
>bulb would make if you were to shake it,except at much higher
>frequencies..I was thinking maybe this was oscillation in the tube
>pre-amp,or maybe distortion,or maybe just from being overdriven in a
>strange way..?? I dunno.. I was hoping someone may have heard this type
>of thing,or maybe have an explanation..

Vacuum tube preamplifiers can be "microphonic". The "elements" (they are
just wire grids or perforated sheet metal ) inside a tube will move around
a bit if the tube is jostled (like your lamp filament). As the elements
move, the tube characteristics change. You can experience the sonic
consequence by gently tapping on one of the smaller tubes. During loud
musical passages the sound field in the room may excite the tubes giving
rise to a type of acoustic feedback. Some tube designs use shock mounts on
the tube sockets for this reason.

This is one possible explanation, however, guitar players use all sorts of
distortion inducing boxes to get their "sound". Guitar amplifiers are
generally very poor designs by hifi standards, but one must respect the
idea that the amplifier is part of the "sound". There are many well
meaning technicians out there who thought they were doing the guitar owner
a favor by "fixing" the amplifier to bring it up to hifi standards only to
reap tons of abuse from the owner because his "sound" was ruined.

-----------------------------------------------------------
nitebird@voicenet.com (Barry Mann)
-----------------------------------------------------------

henry_pasternack@my-dejanews.com wrote:

>    Anyway, I disagree strongly with your statement that acoustic
> feedback
> and electrical feedback have nothing to do with one another.  Nor do I
>
> think it's at all impossible to explain this subject in ways that will
>
> be accessible to beginners.  I find it interesting to draw engineering
>
> parallels between related but distinct phenomena.  I think you did a
> disservice to the original poster and other readers by drawing a sharp
>
> and needless distinction between acoustical and electrical feedback.
>
>    Had I been in your position, I would have said the following.  We
> understand intuitively and from experience that the strumming of an
> electric guitar string produces an output from the loudspeaker, which,
>
> if strong enough, may create a sympathetic vibration in the guitar
> body
> that sets the strings vibrating anew.  This feedback (as it is known)
> may build to the point that it becomes self-sustaining.  The vibration
>
> will continue until a hand is placed on the strings, the amplifier
> volume is turned down, or the guitar is moved away from the speaker.
> Each of these remedies dampens the sound by breaking the feedback loop
>
> from strings to pickup to amplifier to speaker to sound and back
> again.
> The likelihood that feedback will occur depends on the properties of
> all of these links in the chain.
>

I know we are straying from the original question but lets get some
facts straight about acoustical feedback:  Acoustical feedback occurs
when the in-phase gain at a particular frequency is exceeds unity.  The
acoustical feedback is then invoked if a sound stimulus (in your case
strumming) is present.  The feedback loop however is not dependant on
the strings then resonating, but a ratio of distance(s) to the feeding
frequencies' wavelength.

By way of proof I suggest that you do the following:
1)    Stand with a guitar near an amplifier and speaker and wear some
ear plugs (safety first!)
2)    Turn up the gain (past unity) whilst holding or stopping the
strings
3)    You can invoke feedback from an extraneuos source eg whislting or
turning the radio on (or strumming the guitar)
4)    move around and the feedback frequency will change.

I find it useful to use a hard compressor/limiter in the chain to
prevent anything getting broke.

Your hand placed over pick-ups will act as a sheild and will disrupt the
feedback path.  Your hand placed to the side will act as a reflector and
may induce feedback at a different frequency.


Paul
--
/*************************
Paul Conroy
Microphone Research Centre
TIP - CSIRO
PO BOX 218,
Lindfield NSW 2070,
Australia
pconroy@tip.csiro.au
*************************/

In article <363E6078.DB9D30DE@tip.csiro.au>,
  Paul Conroy <Paul.Conroy@tip.csiro.au> wrote:

> I know we are straying from the original question but lets get some
> facts straight about acoustical feedback:  Acoustical feedback occurs
> when the in-phase gain at a particular frequency is exceeds unity.  The
> acoustical feedback is then invoked if a sound stimulus (in your case
> strumming) is present.  The feedback loop however is not dependant on
> the strings then resonating, but a ratio of distance(s) to the feeding
> frequencies' wavelength...
>
> Your hand placed over pick-ups will act as a sheild and will disrupt the
> feedback path.  Your hand placed to the side will act as a reflector and
> may induce feedback at a different frequency.

   The guitar acts a microphone having some transfer function.  The
guitar body, strings, and pickup are a coupled mechanical system,
each of which contributes to some degree to the microphonic behavior
of the whole.  It is as much the properties of the guitar as it is
the acoustic path from speaker to pickup that determines the likelihood
and frequency of feedback.  You're saying the strings are relatively
insignificant in the overall equation, which is fine with me.  The
underlying principles remain the same.

-Henry

-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/       Search, Read, Discuss, or Start Your Own

Okay.. I do understand how various types of feedback work,So my
"snowballs chance in hell" was more like "a snowballs chance on a hot
day", I have a simple home-brew guitar that I slapped together one day
while i was bored..(It sounds okay,all things considered..) I know how
acoustical feedback works,And I have a fair,yet rough idea of how
negative feedback is used in amplifiers..

I just had to spit that out...





Michael R. Kesti wrote:
> 
> henry_pasternack@my-dejanews.com wrote:
> 
> >Each of these remedies dampens the sound by breaking the feedback loop
>                         ^^^^^^^
> That's "damps"!  An excellent article, otherwise.
> --
> ========================================================================
>           Michael Kesti            |  "And like, one and one don't make
>                                    |   two, one and one make one."
>           mkesti@gv.net            |          - The Who, Bargain