Wow this topic is back from the crypt 
Thank you very much for the answer and clarification.Not all of any given tube designation are the same, or for that matter labelled and sold under the intended designation, so Cliff will most likely need to be idealising the specification to some extent. Tubes of the same model number from different manufacturers can have significantly different internal structure. While many of the "corksniffer" tales tend to exaggerate how transformative of tone whichever rare and overpriced Mullard, Telefunken, RCA or whatever tube set is, they do sound different from one another to varying extents.
Throw in on top of that: some of them might be "gassy", have screen resistors that have drifted in value, have varying degrees of microphonics, tube sockets that are not connecting perfectly, and a whole host of other unintended variables, and the omelette starts to become really quite complex. (Cliff, if you are reading, not sure anyone needs tubes half or completely failing to be modelled).
So neatly avoiding answering your question, I think in a lot of ways I prefer not to know. The last 20 years has been a minefield of mismatched tube brands and factories from all around the world, or see what you can find NOS that wasn't a reject, and if you are lucky you'll get a set that sounds good and is reliable (why is it that you could buy a 60s or 70s Marshall or Fender, still with original tubes, and go out and do 100 gigs straight without even thinking about tubes; late 80s onwards, you sometimes couldn't even set the bias without losing a tube?). I'm pretty happy just to know that modelled 5881 is a good 5881 (maybe RCA, GE or Philips), and that I can play with bias point and transformer matching without worrying about forking out for a new set because I have been running the plates too hot, or I got a matched set where one decided to prematurely commit suicide.
I'll bet I still have a brand new quad of matched EL34, a pair of 6V6, and probably a quad of 5881 or 6L6 GC in my spare tubes box, all of known reliability, or good quality NOS. I used to lose sleep over "what if" with power tubes, but for the last 5 years I've been using Axe FXes, and I am completely over it. This isn't yet perfect I am sure, but for me, it's close enough and getting closer all the time.
Liam
Oh yes, definitely, but not quite from the crypt. Thread was started in the whole ancient times of... wait a minute... last month.Wow this topic is back from the crypt
Good thread, and not one that i have noticed. I have changed a couple of words in my previous post to clarify and bring more clearly into line with how I understand the Axe FX models to function.
I agree, but that would involve coding so that all of the power tube model's operating parameters are updated on making the change. I am not certain that's realistically achievable. Can you imagine, "I swapped tube type in my real Marshall JTM45 BBRI from 6L6 to KT66, yet when I did the same in the Axe FX the breakup point did not change in exactly the same way. Why is this?" And the answer is that you HAVE to make some changes in the amp, adjusting the bias, possibly changing screen grid resistors (been a while, can't remember), and no two amp techs will do this in exactly the same way. Worst of all, if the amp tech was able to get all the operating points ideal for the KT66, the tone change would be exactly the same as we hear when we change tubes in the Axe FX currently.
. Let’s play guitar lol. I m not against a partial answer by the way
Different valve types have different internal impedences due to the physical differences in cathode, grid, and anode spacing, construction, and materials. This makes them work best at a particular load impedance and voltages in the transformer primary. Frequently, different manufacturers' valves will have different materials and construction even for the same type.Don’t know if you understand my question as English and me …
Did you have 10 tubes in a table, put one, then another, compare the eq values, so tube B add this in the eq … maybe the answer is secret as I am asking “how do you model tubes?” . But it’s interesting to know the process . Is this a result of a comparaison between tubes in the same circuit or an individual value of something ? A comparaison start to a a reference tube so … hm . It’s maybe too complex to me
To add to Joe's excellent answer, yes, the simple answer is fairly simple, but the full answer is secret, and I am sure Cliff would not want to tell anyone exactly how he does it. In its simplest terms a tube is simply modelled as a valve. (In fact in UK english we call them valves for that very reason). They are a valve with an output of current flow, and that flow is modulated by applying an input voltage (i.e. the signal from the preamp) that controls the flow. The current goes from there to an output transformer, where it is turned into a higher current at lower voltage that drives the voice coil of the speaker. Tube, transformer and voice coil interactions are what my PhD supervisor used to refer to as "non trivial", which was a back-handed way of saying that it's complicated, and there may actually not be a well-defined mathematical solution in the end. If there is no good mathematical model, there will be no good DSP emulation.Don’t know if you understand my question as English and me …
Did you have 10 tubes in a table, put one, then another, compare the eq values, so tube B add this in the eq … maybe the answer is secret as I am asking “how do you model tubes?” . But it’s interesting to know the process . Is this a result of a comparaison between tubes in the same circuit or an individual value of something ? A comparaison start to a a reference tube so … hm . It’s maybe too complex to me
