Nach meinem derzeitigen Verständnis impliziert großer Raa eine Auslegung auf höhere Betriebsspannungen und geringere Ströme (oder ne dickere Röhre...), vollkommen analog zur Situation bei SE, wo eine EL84 an 250V Ua gerne 5.2k sieht, an 300 V jedoch lieber 7k. Was in bezug auf die Ausgangsfrage wohl eher nicht auf Katodenbias hindeuten würde - da verliert man ja faktisch die Potentialdifferenz zwischen Katode und Gitter an der Betriebsspannung.
Unabhängig gibt es bei mir ja das fehlgeschlagene Bausatzprojekt mit 2xEL84 und wahlweise UL. Der Trafo hat ebenfalls Raa=8kOhm, Ua ist jeweils 250 V.
Was mich zu der weiterführenden Frage führt, ob man bei einem UL-AÜ nicht auch die UL-Anzapfungen für die Anodenspannung nutzen kann und die Anzapfungen für die Anoden isolieren. Z.B. wenn die Ausgangsspannung des vorhandenen Netzteils nicht reicht bzw. die verwendeten Endröhren bei der verfügbaren Anodenspannung eine geringere Lastimpedanz sehen sollten, oder auch, wenn man aus irgendeinem Grund eine "andere" Impedanz des Lautsprechers benötigt. Oder hab ich da einen Knoten in den Hirnwindungen?
Yep. You´re on the money as far as Ra-a vs load vs voltage. Correct.
Being one of the few variables when it comes to electronics i for one believe it to be imperative to take to heart that this is one of the few instances where it´ll all work.
Frequency response asf will of course be affected,but the mainstay of it is still the mere fact....if it sounds good,it IS good.
UL in turn.
Since its dawn down in Australia back in the mid 30´s it is one seriously misconcepted setup. The thing with UL,from "our" point of view,is that it is less forgiving from a sonic aspect.
Hence many a designer and builder have taken to that and simply state that UL isn´t usable for guitar (or bass for that matter) work. This is of course false,and what needs to be taken in IMO is the mere fact that UL is more of a "crap in=crap out" kind of deal. In short it is simply less forgiving.
There´s been several takes on the UL principle,which again most are not aware of. There is no need what so ever for the UL winding to be directly connected to the anodes.
Just take a look at how the UL setup works in an old Suprem amp. First the anodes gets what they need..then a dropresistor and THEN heads for the UL winding before hitting the screens of the powertubes.
IMO what´s to be taken to heart here is that there´s more than one way to skin a cat. Ie;what i´m saying is that it isn´t the technical approach that tells us right or wrong,it´s the net result that does.
Ie;how the whole thing SOUNDS.
Enter,using zener diodes for the screens. g2 is an often misunderstood part of what´s going on too. Not to get into the deep end of the pool let it suffice to state that screens were never designed to handle any appreciable amount of load. This can be read in clear text from data sheets if and when need be.
What a zener,of ample wattage,will bring is a fluent line between the interaction of the anode and the screen and hence prevent the screen from ever even reaching the state where it can go positive vs the anode.
In contrast to when using resistors,UL,chokes or whatever this becomes a sort of dynamically floating around constant. This also means that the actual dynamics of the setup will change as the interval between the anode and screen will not change,as will it with the other setups.
This..will to a large effect "improve" how the amp works near full throttle in as much that the sag from the PT will now primarily affect supply voltage only. NOT the inherent voltage difference normally associated with anode vs screen under full load.
In short as the voltage drops due to load the inherent difference between the screen and anode will still stay constant and hence the anodes will never be alone at being starved. Ergo,the anodes will at all cost be the point doing what they were designed to. Make power.
From a practical point of view this becomes rather interesting for low power amps,like an 18 watter,as that provides a platform for the amp to go full power sans any real drawbacks,or whatever you want to call it.
At that the transients will change too. The response of the amp will become way snappier,as far as the powerstage,and this too will have IMO a positive effect on how you actually come to use a low wattage amp.
