Umble Circuit - Seems to have serious design flaws

[wavley]

Yay!

I love me a good Troll, sign me up for this... I'm ready for some Scarlett!

[george]

Still waiting.

[vigilante397]

Still waiting.

Popcorn refills, get your popcorn refills! Popcorn refills right here!

[duck_arse]

we've all got our seats and popcorn, but what time does the matinee start?

[PBE6]

Maybe he's filing his patent first. I hope we haven't missed the previews... *munch* *munch*

[garcho]

^ yeah, the lawyers must have his JFET overdrive guitar pedal all tied up in the IP dept.

[Frank_NH]

"Those that can, do;  Those that can't..."   

[PBE6]

"Those that can, do;  Those that can't..."   

..., teach. Those who can't teach, teach gym.

I'll get my coat.

[wavley]

Excuse me,

But I think that it would be really awesome if mojokorn built me an Umble, but with a burst button switch.

Thanks,

Jack

I'm waiting to see if he misses the opportunity to build a custom pedal for our most famous forumite.

[GibsonGM]

^ yeah, the lawyers must have his JFET overdrive guitar pedal all tied up in the IP dept.

Patience, you know...it's something never-before-seen in the musical effects world - it will take time to sort out just how it all works! 
I imagine they're having it sent to Roswell for reverse-engineering...

[disorder]

Dolby and Apple

I work in the extended circle that is bay area audio companies and this says a lot right here, if your attitude on this messageboard hasn't said enough already.

[deadastronaut]

it would probably sound better in dobly...

[stallik]

 
Only when set to 11

[amz-fx]

For those who like jfets...  the Roland Cube 60 from around 1984:



Best regards, Jack

[Frank_NH]

Actually Jack - my main gigging amp right now is a "vintage" 90's era Roland Blues Cube BC-60.  Awesome amp!  Maybe it is the JFETs...

(Sorry for the thread derail - now back to the latest episode of "Umble Watch")

[duck_arse]

it would probably sound better in dobly...

.... and then he'll probably want "....... sausages".

didn't dobly have some serious flaws as well?

[thehallofshields]

I'm curious to see how JFETS can more accurately be biased.

I hope this guys unorthodox posting model pays off.

[garcho]

^ more "accurate" bias?

[Bill Mountain]

For those who like jfets...  the Roland Cube 60 from around 1984:

Best regards, Jack

Looks cool.  How does it sound?

[Frank_NH]

^ more "accurate" bias?

I have a spreadsheet which calculates the DC bias and does a general circuit analysis for a self-biased JFET gain stage.  There are four different bias points that are calculated, asumming known JFET characteristics (idss, Vp) and supply voltage:  (1) midpoint bias, where the drain current = idss/2 and the drain voltage = Vp/2, (2) fixed source resistance and drain voltage, (3) fixed source and drain resistances, and (4) Fezter valve JFET bias.

In case (1), source and drain resistances are calculated to satisfy midpoint bias.  In case (2), the drain resistance is calculated.  In case (3) the drain voltage is determined for the given source and drain resistances.  And for case (4) the ROG Fetzer equations are used to determine source and drain resistances.  Note that in all cases, the full analysis calculates all voltages, currents, gains (DC and AC), transconductance (gm), etc.

So, the point is that there is no "accurate" bias - just different bias points which result in different voltages, currents, and gains.  I should note that for a given JFET, the Fetzer bias has a more modest gain than choosing other source resistors. Also, for many amp sim circuits (like the Umble) where JFETs replace tubes, the designers generally retain the original tube circuit's source resistance.  I suppose there is no rule that you have to do that if want to replicate the original circuits tonal "character" as long as the tone stacks and other tone filtering elements are similar.  IMHO that's where the magic mojo is to be found...