Sujet de la discussionPosté le 29/07/2006 à 11:12:42REGLAGE DU BIAS : Proposition de Tutoriel
No, but I can try to explain it. To calculate approximate bias, you'll need a Weber Bias Rite BR-A2. Since you're running a CL/50, I assume you're running a matched pair of EL34s? The BR-A2 is for pairs and will allow you to measure plate voltage and cathode current (more on that below)
The first step is to find the standard rating for a given tube type's maximum plate dissipation. I like the good old RCA Tube Manual, in pdf format here. There are times this site doesn't have what you're looking for, Automatica is a good place to look.
So here we see the maximum plate dissipation for an EL34 is 25w.
Carefully remove the chassis from your head. Make sure it is unplugged and off! Once out, locate the bias trim pot on the 'board (for reference), and remove the power tubes. EDIT - Keep the amp's speaker output plugged into your cabinet/speakers. You need them to serve as a load to protect the power section of the amp while it is on.
Then, you plug the Bias Rite sockets into your tube sockets and then your tubes goes into the top of the Bias Rite probe sockets. Then you turn on the amp, wait 60 seconds, and switch off the standby switch.
Switch the Bias Rite to the Vp setting (plate voltage). You will see a reading in the likelihood of somewhere between 300-500v. Whatever you read here, this is your plate voltage. Write it down. For sheer example, let's say yours reads 400v.
Next, you take the maximum plate dissipation (25w for an EL34) divided by plate voltage. Here, we'd get:
25w divided by 400v = 0.0625amps, or 62.5ma.
Next you take the amperage you just calculated, and multiply it by 65% (+/- 5%) so you're getting the tubes in the appropriate crossover range that Steve Fryette recommends.
62.5ma multiplied by .65 = 40.6ma
Slowly adjust the internal bias trimpot until your Bias Rite (now set to B+ for cathode current) says 40.6ma. Be careful not to touch anything (especially caps) on the circuit board - there are lethal electrical currents present.
Turn the amp off, let the tubes cool down, remove them, remove the Bias Rite, place the chassis back in the head, put the tubes in, and rock out!
Long story short, the calculation method is:
[(Maximum plate dissipation / Plate voltage) x 1000] x .65 = ___ma cathode current. Bias to this value.
NOTE - this is at your own risk. If you're not comfortable around electrical currents, measurements, etc., I'd leave this to a tech. If you take your time, are careful and mindful...I really think you'd be able to do it. Just wrap you rmind around it before you start. Don't try to figure it out as you work on it .
Voilà ce que j'ai déniché sur le forum vhtusers.com...je pense que ça résume assez bien le règlage du bias.
Maintenant,si une personne qualifiée pouvait confirmer la manip,y ajouter des choses ou même mieux,la traduire afin d'en faire un tutoriel (avec mon aide si besoin),ça serait
Que vaut cette affirmation, relevée dans le même article cité plus-haut (http://optimisetonampli.chez-alice.fr/lampe.htm#top) :
"Le remplacement par des tubes appairés et de même référence, si le réglage du bias précédent à été fait correctement (ou déjà fixé d'origine dans l'appareil), permet de s'affranchir d'un nouveau réglage. Ainsi chaque tube possède les caractéristiques électriques proches des tubes précédents, mais un contrôle des tensions de polarisation est préférable ainsi que si possible un réglage de balance (équilibre des courants du transformateur de sortie via les tensions négatives de grille de commande G1), afin de minimiser le bruit."
Si je comprends bien, changer ses lampes par un des tubes appairés (évidemment de la même référence) permettrait de ne pas avoir à régler le BIAS. Que doit-on en penser ?
