jfet4tube and mu-amp

[demym]

Hi,

big noob here, but i'm having fun emulating some amps with the jfet4tube method (i am actually building a Carvin Legacy emulation).

I've noticed that it seems to be another method for doing this emulation, called mu-amp.  Ignorantly speaking, the jfet4tube seems to me (at least, looking at schematics) as a more fascinating way of emulating, cause you end up with the same exact circuit of the original amp. What are the differences with the mu-amp method, and which one is better to use ?

Thanks in advance

Demy

[Ripthorn]

A book could be written (and probably has) on the mu-amp.  As for which is better, that is purely a question of preference.  Some will say one, some will say another.  The only way to tell what is best for you is for you to build something both ways and decide.  As for info on the mu-amp, you could do a search here or on the net and then check out books like the Art of Electronics.  Sorry to not give you a nice compact answer, but your questions really require quite the long answer that I don't think I am completely capable of.  RG or Mark might be able to give you a better answer.

[R.G.]

As you've no doubt read, neither JFETs nor MOSFETs are exactly a replacement for the vacuum triode, which is what most guitar signal circuits use. Both are more similar to signal pentodes, and each has quirks that are different.

It's enticing to be using "the same circuit", as that makes things easier. However, the circuit is not operating at the same voltages, and JFETs don't distort in the same way as 12AX7 tubes. But it can be fun to play with as long as you don't convince yourself that a 2N5485 is a triode. 

Mu-amps are not like any single tube. The mu-amp circuit was known from early tube days as a way to get more gain out of two tubes used together than used in cascade. It's an old, old circuit that National Semiconductor applied to JFETs.

There isn't any "mu-amp method" that I know of, other than just sticking in a mu-amp for every triode section. JFET mu-amps have a much higher gain than a 12AX7 stage, and distort in a way that pleases a lot of people, but is not the same as a triode.

You see a pattern emerging here, I'm sure. There isn't a "better" way, other than what your own personal ears like to hear. This is the reason some say one, some say the other. It's like the song "... you say to-may-to, I say to-mah-to..."   

Of course the name of that song is "Let's Call The Whole Thing Off". 

[petemoore]

  Takes another Jfet to build one...only slightly more 'tangled' than a 1-Jfet gain stage.
  I like the advantages of the self-bias nature of the circuit, and the ''Jfet Mu Tone''. Various tones actually...what comesoutta depends greatly on whatgoesinta...but the Mu-Sound adds and takes away in ways I've grown fond of.

[demym]

Thanks for your replies guys. Now it's more clear to me.

I know that a jfet will not sound as a tube, but.... after building on perfboard the first stage of the legacy (which, from schematics, is supposed to be also the first stage of the clean channel).... what can i say..... the sound coming from my amp remembers me a lot of that mr.vai clean sound !... maybe i'm too much suggestionable :-)

i've breadboarded all 4 stages of the amp (the crunch channel)... having breadboarded also the marshall jcm900 jfet4t project, i hear some peculiar differences between the two sounds;

good for me that i have a big fantasy !! :-)


I'm now waiting for components to perfboard the remaining 3 stages (and then the tonestack, also).

I would love to build an all valve preamp (as the AX84 project, etc.) but i'm very scared about high tension, too early for a noob of the 9V...

Anyway, this is all too exciting !

Thanks once again people, have a nice time !