Bon, bah, j'ai une réponse plus ou moins valide...
J'avais juste pas les bons mots clé dans gogol.
En gros, C'est cathode bias, mais avec un courant d'anode plus faible ("...Addendum not being a tech or such, I wonder as well if the plate voltage is so low on these amps to deliver only 20 watts - someone measured theirs at about 230V - would it matter which tubes?..." - Citation extraite du Marshall Forum)
Et juste après, il y a la citation suivante:
Santiall: "I did design the 20W amp that was used in the mini-Jubilee, the original Origins and some other amps that did not see the light. That design DOES work with any octal tube, even 6V6. I don't know what circuit is in the Studio series but I'd be very surprised if it isn't just the same as the mini-Jubilee with the preamp modified accordingly. In fact those amps were already planned when I was still in Marshall and that's what was being done, just a preamp change (which is actually the same if you think about the 100W amps, same power amp, different pre).
Regardless of circuit changes, the amps will work with the other tubes. Actually the EL34 is harder on the transformers than tetrodes like the 6L6, 5881 and so on. The KT88 and 6550 draw a bit more of heater current but that was taken into account when specifying the power transformer.
In Marshall's defense it is normal that if the amps come with EL34 they told you to only use those but that doesn't mean it won't work or will be dangerous to use other tubes. I'm sure if you ask if you can use a 12AT7 in the preamp they'll tell you it is designed for 12AX7 too."
Et un peu plus loin:
Santiall: "well, the main reason why I said that is because it was part of the design parameters and it has been tested.
hmm, let's see if I can explain this. The working principle of cathode biasing is that if the current increases, the bias gets more negative (grid to cathode voltage) until it finds an equilibrium point. In short, the current limits itself and it just happens that the octal pentodes and tetrodes used in guitar amps all end working around the same zone. Should the current keep increasing, then the bias will try to stop it increasing, it is negative feedback. Obviously the circuit was optimised for a particular tube but it still works with others delivering more or less distortion/ power but it doesn't blow up.
Also, these amps run at such low anode voltage that the margin is quite big, hence the flexibility. The only tube that runs near the maximum parameters in this circuit would be the 6V6. Had these amp had higher anode voltage then yes, I'd not have advised to change tubes. In a 50 or 100W amp, the tubes would be powered at 450V-550V or even more, in these amps around 'only' 280V-300V
I had the same circuit running EL34s for certain series and 6L6/5881 for some other series that never saw the light perfectly without issues.
IF it had fixed bias, then I'd have said you can use any tube but you need to re-bias accordingly like in the 100W amps. A JVM, 2555x, YJM, and pretty much any amp with enough bias range will also take "any tube" (apart from the 6V6 as the maximum voltage will be exceeded).
Hope it is clearer, also, with "any tube" I mean any octal tube with pinout similar to the EL34 and a minimum anode voltage of at least 280V also know as 6L6, 6V6, 5881, KT66, KT77, KT88, KT90, EL34, 6CA7, etc. etc. just in case someone plugs in there an octal preamp pentode or something and it doesn't work but I guess this was pretty much clear...
Pour info, le Santiall en question, c'est Santiago Alvarez, le monsieur à qui l'on doit l'AFD100, le YJM100, les Astoria, Class 5 (pas certain de celui là), SL 5, le mini silver jubilee 2525h dont le poweramp a servi pour la série studio, apparement.
Je pense que l'on peut lui faire confiance, là dessus, mais, vu qu'il ne travaille plus chez Marshall, on pourrait aussi se dire que:
1 - Il n'est plus au courant de l'évolution de projets tels que la studio séries, depuis son départ.
2 - Ne connaissant pas les raisons de son départ (départ volontaire, licenciement???) Il pourrait être suffisamment frustré de ça pour inciter les gens à faire n'importe quoi avec leur ampli histoire d'égratigner la réputation de Marshall.
A prendre avec des pincettes, donc.
Ceci dit, des utilisateurs semblent avoir monté autre chose dans leur Studio séries, que des EL34, et ça semble le faire... Donc...
PS: Marshall Specialist? Jamais entendu parler... C'est un modèle récent?
@+
Fab.
Les Rockeurs ont du coeur! Certes, mais ils ont surtout des grosses c**illes!!!
Ma Face de Bouc:
Moi
