Newsgroup: it.arti.musica.strumenti.chitarra
Titolo: Suonare mismatch, qual'è la verità ve ra?

Periodicamente qalcuno chiede se è pericoloso suonare "mismatch", ovvero 
disadattato, con impedenza del carico diversa da quella in uscita 
dell'ampli.
Ecco pezzi tratti da un FAQ di alt.guitar.
Vi sono considerazioni motivate da ragionamenti tecnici.
Ognuno tragga le proprie conclusioni e agisca come ritiene opportuno.
Non ho voglia di tradurlo:
"Can I plug a X-ohm cabinet into a Y-ohm head?
Rich's answer:
``NO!!!!!!!!!!!!!!!''
Short answer:
In general, you should match impedances. If you must mismatch, mismatch 
in the right direction, but stay as close as possible. So what's the 
right direction?
With tube gear it's better to have the speaker impedance lower. 
Connecting a 4 ohm load to an 8 ohm amp may be OK, but connecting a 16 
ohm load to an 8 ohm amp is probably not. Connecting a 16 ohm load to a 
4 ohm or 2 ohm amp is begging to destroy the amp. Running too high of a 
load on tube gear can fry anything (and occasionally everything) in the 
outputs, including tubes, transformers, resistors and tube sockets.
With solid state gear it's better to run a higher impedance. Connecting 
an 8 ohm load to a 4 ohm amp should be fine. Running too low of a load 
with solid state will fry your output transistors, and possibly more.
Lord Valve."
Mh.
Ci accontentiamo noi di un dogma non argomentato?
No di certo.
Read on:
"# What happens when I mismatch output impedances in my tube amp?
* Lower impedance will _generally_ stress the power tubes gradually more 
and more as the volume goes up.
* Higher impedance will _generally_ stress the power tubes less as the 
volume goes up until the powertubes cut off at high volume levels. This 
will create voltage spikes on the plates of the power tubes called flyback.
If the flyback spikes are kept within sensible limits (say 1000V-1200V 
for EL84/6V6 etc., and 1500V for 6L6 type amps), then it's unlikely that 
any harm will be done. It's when they go way higher than this that the 
tubes, transformer or sockets can flash over causing expensive damage to 
the amplifier.
In summary, lower impedances _may_ stress your power tubes and shorten 
their life. Higher impedances are OK to a point, then it becomes Russian 
Roullete.
Where running high impedances are concerned, much depends on:
* Quality of output tubes * Material used for the sockets * Leakage 
inductance of the output transformer * Quality of insulation in the 
output transformer * Amount of negative feedback in the power stage * 
The volume you play at * The level of mismatch
For running low impedances, the amount of extra stress will be 
influenced by:
* The class of the amplifier (A/AB) * The volume you play at * The level 
of mismatch
Given all of the variables above, it's difficult to hand out blanket 
statements like X is good but Y is bad. The only totally safe advice, is 
to ensure that what's stamped on the cab matches what's stamped on the amp.
If you can't do this for practical or emergency reasons, follow LV's 
advice and go low instead of high - there's a lot less to go wrong that 
way...
-Duncan Munro "
Sempre più interessante.
Ma siamo paghi di cotanta spiegazione?
Ancora no:
"Why is impedance mismatching so dangerous?
The crux of the problem lies in the inductive nature of the output 
tranny. Inductive loads are pretty special things, since they STORE 
energy in a magnetic field. A property of this effect, as has been 
pointed out, is that the voltage can soar to levels above the supply 
voltage in the amplifier-- sometimes WAY above. You can't do that with 
any other kind of load other than inductive.
Now the transformer doesn't have an impedance of its own; it only 
reflects an impedance from one winding to another in proportion to the 
turns (or voltage-- they are the same) ratio squared.
So imagine that you've got an open secondary. This impedance is for all 
intents and purposes infinite. Thus, regardless of the turns ratio, the 
primary impedance is infinite as well. (leakage inductance and parasitic 
capacitance-- two unavoidable nasties of real-world trannies-- will 
limit this to some finite number less than infinity, but suffice it to 
say its really high.) This means that the primary will act like a 
constant current source, attempting to keep changes in currents through 
its windings to a minimum. This will be an important point later.
Operating into such a humungous load impedance will cause the plate to 
swing HUGE voltages, according to V=IR. Especially with 
tetrodes/pentodes, which are much better at cranking out current, the 
delta Ip will stay the same regardless of the load. Consider what 
happens when the R goes sky high.
Now, if the load were NOT inductive, the maximum possible voltage 
generated would be equal to the rail voltage. No problem. This is how it 
is in SS amps. But with tube amps, that's not the case.
The primary danger here is in the development of these extraordinarily 
high voltages, which can punch through winding insulation, arc over tube 
sockets, even arc inside the tubes themselves. Once an arc has struck 
you can be pretty sure it will happen again. And again.
This is not good. Probably the worst scenario is that the OPT primary 
arcs to the core, which is grounded, and that will cause mega current to 
flow. The OPT is toast, and the power supply will be too unless 
something stops that current in a big hurry.
So that's what can happen with too high a load. Admittedly, this is an 
extreme case scenario here, where you've got an OPEN secondary, and thus 
a very very high primary impedance to work into.
Notice above how I pointed out that tetrodes/pentodes will have a worse 
time of this than triodes. This is because of their much higher dynamic 
plate impedance, which can also be described as their being an 
approximation of current sources. The pentode will just keep cranking 
out plate current-- regardless of what potential the plate is. The 
electrostatic shielding effect of the screen grid continues to "pull" 
electrons from the cathode with the same force. Thus the plate current 
is largely independent of the plate voltage, a mark of high plate 
impedance. That current is what "builds up" (so to speak) when working 
into a high load impedance and generates the excessive voltages. It's 
almost as if you've got a constant current load (the unloaded OPT) on a 
constant current generator (the plate of the pentode)-- obviously if 
these two devices are "concerned" with currents, not giving a flip about 
the voltages involved, you can get some pretty crazy effects.
With a triode, the much lower plate impedance limits the extent to which 
the plate voltage will swing about uncontrolled. As the plate swings 
high, for example, the attraction of electrons from cathode to plate 
will increase due to the higher voltage. More electrons will be pulled 
to the plate, regardless of what the control grid is doing. More 
negatively charged electrons means less positive voltage, so the voltage 
is "automatically" decreased. This is a direct measure of plate impedance.
In fact, running a triode into a very high impedance is done all the 
time with interstage transformers, which generally are very lightly 
loaded. The inherent degeneration in the plate circuit keeps the peak 
voltages from becoming a problem. Actually, triodes "love" current loads 
of very high impedance-- the tube is operated in its most linear fashion 
and is free to do what it does best-- generate an output VOLTAGE.
You can think of the dynamic plate impedance of the tube as forming a 
voltage divider, with the inductive tranny between plate and B+, and the 
tube itself between plate and ground. Obviously, with the low plate 
impedance of a triode, the voltage cannot swing madly about. Now 
consider the very high plate impedance of a pentode, and how much higher 
those plate voltages can swing.
OK, that's the situation of too HIGH a load impedance. So what about too 
LOW of an impedance? Let's consider a dead shorted secondary on the OPT.
Now the primary presents a very low load to the tube, a low impedance, a 
vertical load-line. We will notice that the tables have exactly turned.
Since the triode's plate is like a voltage source, it will attempt to 
pass incredible amounts of current in a heroic attempt to make the plate 
voltage swing. Operating into a dead short, it cannot do this, so 
something eventually will give. The cathode will attempt to emit way 
more electrons than it can, and it will have a short, hot life.
The pentode, however, is more of a current source, so it will continue 
to pass the total plate current in accordance with the screen voltage 
and the control grid voltage. These have not changed with the alteration 
of the load, so the pentode will continue to merrily pump its current 
swings into a dead shorted load.
Take a look at some plate curves, if you need to. Find some for pentodes 
and for triodes. Better yet, find some for the same power pentode 
connected as a triode (g2 connected to anode).
First the pentode case: look at the way the curves lie on the page. 
Imagine a horizontal load line (infinite load, open secondary) drawn on 
the graph. Notice how the pentode doesn't look like it would work this 
way-- an infinitesimal control grid voltage change would produce a 
gargantuan change in plate voltage. The tube is NOT happy. Now imagine a 
vertical load line (zero load, shorted secondary). The pentode's peak 
current for a given control grid voltage doesn't change much at all-- 
the vg1=0 plate curve is nearly horizontal for most power pentodes, 
cutting right across all of the various plate voltage points. It doesn't 
matter what Vp is at all-- no matter where you draw that vertical line, 
the peak plate current is pretty much the same. The tube is happy.
Now the triode case: imagine the horizontal load line now. Notice how 
the plate voltage is almost PERFECTLY proportional with respect to 
control grid voltage. No matter which tube you try, or what current you 
draw that horizontal line at, it will be VERY linear. The tube is happy. 
Then consider the shorted output tranny case, with a vertical load line. 
Notice how the vg1=0 curve will produce a humungous plate current since 
the plate curves are so much "steeper" than the pentode's case. The tube 
is NOT happy.
What the heck does all this mean? Well, hopefully you aren't running ANY 
tube amp into a shorted or open load... Since no pentode is a perfect 
current source, and no triode is a perfect voltage source, the actual 
characteristics are somewhere between the two idealized cases. As LV 
pointed out, you're much better off running a pentode amp into a lower 
load impedance than it expects. For those of you with triode output 
tubes, or a triode switch, you're better off running into a HIGHER load 
impedance. If you don't see why by now, reread this essay. It's also a 
good idea to take a high value power resistor, say 470R, and hard wire 
it right from the OPT secondary's 16R tap to ground. This will dissipate 
a very small amount of power under normal conditions, but will limit the 
extent to which the primary impedance will tend towards infinity in the 
case of a disconnected load.
For what it's worth, I've been deliberately "mismatching" load 
impedances by one tap for years. In other words, either a 4R or a 16R 
load on an 8R tap, and so on. This small mismatch will limit output 
power and will change the clipping points of the output tubes, but will 
not damage anything in a properly designed amplifier. Keep in mind that 
a higher load impedance in a pentode amp will put additional stress on 
the screens, so you may want to have at least 1k stoppers installed. A 
lower load impedance will cause more plate current to flow, and if 
you're running the tubes at the edge of acceptable quiescent plate 
dissipation that may push them over the edge into the red zone. If 
you've got an old vintage amp you'd hate to see get damaged, by all 
means, play it safe and don't mismatch at all. But if you're wondering 
about how it sounds, and the amp's got good trannies in it, then 
mismatch away. Just keep it within ONE TAP please, for safety's sake.
-Ken Gilbert, by way of PMG "
Siccome nessuno di voi ha le palle per leggere tutto vi riassumo.
"Don't mismatch.
If you really want to:
Transistor - go high
Triode - go high
Pentode - go low"
A questo punto molti di voi osserveranno.
"ma il 5050 è a pentodi e sul manuale dicono "go high", perché?"
E che cazzo ne so, scrivete a Randall e chiedete spiegazioni.
-- 
*** per rispondere via e-mail rimuovere la "X" dall'indirizzo ***
       \--------------------------------------------------------\
        \     Diego Krota   
http://www.geocities.com/dkrota
      \
\-------\------------------------------------------------\-------\
  \   Never do anything you wouldn't be caught dead doing  \
   \--------------------------------------------------------\

E che cazzo ne so, scrivete a Randall e chiedete spiegazioni.
Credo di aver capito perchč. Negli articoli da te riportati fra le altre
cose si legge che il mismatch verso l'alto č dannoso (con gli ampli a
valvole) perchč sul plate delle valvole finali e sul primario del
trasformatore d'uscita si generano alte tensioni, che crescono al
crescere del carico collegato sul secondario e del volume d'uscita.
Queste tensioni possono creare archi sulle valvole o (peggio) tra
avvolgimento e nucleo del trasformatore, producendo disastri degni di un
monsone caraibico.
Evidentemente Randall ha calcolato tutto e verificato che collegando un
carico da 16 ohm sull'uscita da 8 ohm l'aumento di dette tensioni non
arriva mai ad un  livello tale da creare di per sč una situazione
potenzialmente dannosa. Se questa mia congettura fosse vera
confermerebbe ancora una volta una cosa che sospetto da tempo, e cioč
che in Mesa/Boogie hanno le palle e fanno bene il loro lavoro... e se lo
fanno pure pagare :-(
Ciao
SH
--
Copulo ergo sum

"I've been deliberately "mismatching" load
impedances by one tap for years. In other words, either a 4R or a 16R
load on an 8R tap, and so on .
This small mismatch will limit output
power and will change the clipping points of the output tubes, but will
not damage anything in a properly designed amplifier"
Cazzaroni...io intendevo questo.... Il tipo dice che in teoria da uscita
otto a cassa 4 o 16 non danneggia......
MA limita la potenza di uscita e cambia il punto di clipping delle valvole
finali!!(Anche se non ho capito in che senso...clippa prima o dopo??)
Quindi tutti i possessori di casse 2x12 con coni da 16 ohm che le usano in
stereo.....Hanno un comportamento diverso in termini di clipping...(chissa
se è udibile)
IO ME LA FACCIO CON I CONI DA 8 OHM...hehe cosi tolgo l'acqua dallo
scoglio....taglio le corna al toro...o era la testa???!?!Per me è la
soluzione ideale perchè poi in base agli ampli posso collegarli in mono ed
avere 4 o 16! e sul mesa non ho probblems!!
(cazzo perchè invece quelle di serie le fanno con coni da 16??? che possono
andare a 8 o a 32 in mono?)
--
Baciamo le mani....
Don Vito

andare a 8 o a 32 in mono?)
La 2x12 Mesa *di serie* può andare solo a 8 ohm in quanto i due ingressi
sono collegati in parallelo.
--
Copulo ergo sum

 otto a cassa 4 o 16 non danneggia......
Cazzarone lo dici a tuo cugino.
Non c'è scritto che non danneggia.
Gli ampli a valvole sopportano in una certa misura maltrattamenti.
Suonare a palla è un maltrattamento se l'ampli non è progettato bene.
Accenderlo senza carico è un maltrattamento.
Togliere due valvole da un doppio push pull è un maltrattamento.
Usare un powerbrake è un maltrattamento.
Suonare col bias mal regolato è un maltrattamento.
Spostarlo a valvole calde è un maltrattmento.
Suonare prima che siano calde è un maltrattamento.
Eccetera, eccetera, eccetera.
Questo non vuol dire che facendo una delle cose elencate l'ampli esploda 
istantaneamente.
E' come dire "oh, ho iniziato a fumare oggi, un pacchetto al giorno. Tra 
quanto tempo mi si bucheranno i polmoni?"
Nessuno può darti una risosta tranne che "ti stai maltrattando".
Suonare mismatch è un maltrattamento.
E' un rischio che, in certe condizioni, è sufficientemente basso da 
poter essere corso.


Cazzarone lo dici a tuo cugino [cut]
Credo che fosse un'esclamazione, non un sostantivo..
(come dire "ma porca pupattola" solo che lui ha esordito con
"cazzaroni") :-)
SH
--
Io so' innocente signor presidente
Io nun ho fatto niente
Mě madre č vedova e mě padre pure
                              (Bombolo)

Diego Krota wrote:
|| Joe wrote:
||
||| Cazzaroni...io intendevo questo.... Il tipo dice che in teoria da
||| uscita otto a cassa 4 o 16 non danneggia......
||
|| Cazzarone lo dici a tuo cugino.
|| Suonare mismatch è un maltrattamento.
|| E' un rischio che, in certe condizioni, è sufficientemente basso da
|| poter essere corso.
CAZZARONE...ma perchè hai saltato il pezzo sul clipping....e delle casse
originali??.....M'importa na sega il maltrattamento LOL
In caso l'ampli essendo un mesa...(e quindi dotato anche di intelligenza
artificiale) telefonerà al telefono azzurro!!
E poi io lo devo usare match..è solo na curiosità!;-)


 artificiale)
Te difetti anche di quella naturale.


artificiale) telefonerà al telefono azzurro!!
Minchia, ma allora sei fissato, eh?
Lead



Lead, è il triste destino di chi compra Mesa... Una volta che li provi
diventi fanatico :-)
SH
--
Viviamo in un paese libero,
lasciatemi dare libera espressione
alla mia stupidità.


Il tuon non cià l'intelligenza artificiale...perchè è grosso preistorico e
poco evoluto!!Il mio si....è stato già maltrattato e quindi è sensibile!
Si incazza e chiama il telefono azzurro via midi.....!!ROTFL


 diventi fanatico :-)
sarà perchè sono er mejo :PP
--
Diablo

 Si ma cazzo fallo ora il pignolo....RISPONDI!
Non so cosa cambi in termini di suono suonando mismatch e non so nulla 
di casse Mesa.


Diego Krota wrote:
|| Joe wrote:
||
||| Si ma cazzo fallo ora il pignolo....RISPONDI!
||
|| Non so cosa cambi in termini di suono suonando mismatch e non so
|| nulla di casse Mesa.
Ecco sei un Ignorante PIGNOLO!!:-)


diventi fanatico :-)
ehehe... eccerto, porca zozza. Paghi 2 melloni un aggeggio usato...
minchia, se te lo tieni e lo idealizzi :|
Lead