New circuit: the QUITAPENAS Jfet amp emulation

[Xavier]

NOT VERIFIED YET

What is this?

It's a JFET amp emulation of a known "tweed" combo amp which name I can't disclose here. The original amp has a very pronounced "sag" effect when pushed, and delivers a very liquid and creamy , singing OD tone. Reminds a lot to Larry Carlton / Robben Ford.

This is a direct translation from the original schematic, please keep in mind that C6 and C10 appear as "N/U" (non used) at the original schematic. Current values are only suggestions.

As soon as it's verified I'll post it in my gallery.

I don't know how close this will sound to te original, but if it is, makes a worthy build.

Have a nice weekend !!!

[zjokka]

wow, looks interesting!

did you develop it yourself? not so clear what's not verified: circuit or the layout?
very curious about which amp it is, don't want to ask, because there seem to be clues all around. But sure you can mention the name?

my latin really fails me.. Quid ampenas? the "What Amp?"

j

[Xavier]

wow, looks interesting!

did you develop it yourself? not so clear what's not verified: circuit or the layout?
very curious about which amp it is, don't want to ask, because there seem to be clues all around. But sure you can mention the name?

my latin really fails me.. Quid ampenas? the "What Amp?"

j

I have just translated it as is from the original schematic. Quitapenas is a spanish word that means basically "pain reliever". That word applies also to an alcoholic shot which mixes cognac and absinthe . It's for real men only.

It's not my development, just my transfer from the original. It's a little bit like the Highway 89 situation. All I can say is the amp is US made in California, has chicken cream knobs and it's covered in tweed, with a 12" speaker and EL84 power tubes . A very good sounding amp.

The layout is not verified, the schem obviously is. I don't know how to use ExpressSCH, hence my direct posting of the layout only. It should be correct though, but it will only be verified when someone actually builds it.

However, it's NOT a Dumble emulation.....and sure NOT a Classic 30 emulation.

[bassmeister]

I believe R2 = 1,5 Ohms is a typo. 1k5 perhaps?

[Xavier]

Yes it's a typo

Damn.....that's why it's unverified yet

[toneman]

 
This is a JFET "emulation" of a Tube amp 
Specifically a "Tweed"   
That would be a Fender Tweed 
What other 'tweeds" are there? 
So, is the 3 FET "emulation" what U have "cloned" 
Just redraw the schemo, and post 
I like to see the schemo with the perf   
thanx
T

[Xavier]

This is a JFET "emulation" of a Tube amp 
Specifically a "Tweed"   
That would be a Fender Tweed 
What other 'tweeds" are there? 
So, is the 3 FET "emulation" what U have "cloned" 
Just redraw the schemo, and post 
I like to see the schemo with the perf   
thanx
T

No it's not a Fender, but it's covered in tweed also. It's a two channel reverb combo with 1x12", made by a well know manufacturer. It's just that is not one of the "big names" such as Fender, Marshall or Peavey. However (another hint ) it's an amp you will never see in a retail store, except for some showrooms in the US

I'll post the scan of the schematic next week for you non believers .

But believe me. This amp DOES exist and it's not my invention.

[Dirk_Hendrik]

I might be wrong but wouldn't an input and an output cap be in place? A DC coupled device in front might distort the FET bias level of the input FET, causing undesired results.


Other than that this might be a good one for me to finally get my self in te field of FET emulation. Never done that. I should give it a try.

[mattumbi]

Carvin...?

[zjokka]

Other than that this might be a good one for me to finally get my self in te field of FET emulation. Never done that. I should give it a try.

in my short diy stompbox history I very swiftly went from collecting GE trannys to ordering some interesting FETs with any order I place. Did the Flipster (knocked me off me feet) and now drilling the Umble and Professor Tweed pcbs. Should finish another Fat Boostered (also with FETS)...

zj

[Xavier]

I might be wrong but wouldn't an input and an output cap be in place? A DC coupled device in front might distort the FET bias level of the input FET, causing undesired results.


Other than that this might be a good one for me to finally get my self in te field of FET emulation. Never done that. I should give it a try.

You may be right. however other emus like the Flipster or the T-chief don't have them neither. Anyway it's a matter of adding just 2 caps. Suggested values?

[Jay Doyle]

As far as I can tell, I'm much better at looking at a schem, but if that tone stack is anywhere near standard, you won't need an output cap, the tone stack blocks DC on its own.

For the input, with a 2.2Meg input impedence, I'd go with a .01uF cap, but you could go lower...

One thing, I wouldn't expect the same 'sag' effect you speak of because of the nature of common emmiter JFETs and 9V circuits. I think you could get there, but the biasing arrangement will be a hinderance. One of the reasons I don't like the trim-on-the-drain bias scheme is that you don't get to chose the drain/emitter ratio or have as much control over what level of current you run the JFETs. Having a higher current level for the FETs might allow you to get a little closer to a 'sag' effect as the JFETs will pull more current when they run hard.

BUT, I think that the 'sag' effect comes more from a Class AB output stage than a class A. This is because in a class AB stage the current draw is much higher at maximum output so that draws down the voltage supply. All of the JFETs in this circuit are class A. In class A, current draw at full power is the same as at idle.

http://www.aikenamps.com/Sag.html

None of this means that your circuit won't sound good or great! Just that I don't think it will get that 'sag' effect you mention.

[toneman]

BTW,
what's that "P67" pot all about 
is this a "clue"   heh heh 

[orgaqualia]

kendrick?

[Noplasticrobots]

I agree, has to be Carvin.

[Xavier]

BTW,
what's that "P67" pot all about 
is this a "clue"   heh heh 

Nah, it's just that when I tried to write the right word (Gain) the software gave me an error message, so I left it as is..

Jay, thanks for the advice. I guess the input cap is a series cap?. As for the sag effect, that's good food for thought. So you mean by rising the drain current higher than the 4.5V point could kind of provide the effect? BTW what an interesting link....

I'll build it next week and see how it sounds.


EDIT: OK I have found the schematic on the internet so it's not a secret anymore:

It's the second channel of a Carvin Vintage 33 amp (you guys are smart ). The current model is called Nomad 50 but it doesn't sound half as good as this one , which was discontinued like 6-7 years ago.

What I think it's interesting, if you look at the following schematic you'll find an array of 4 back to back silicon diodes to ground after the 2nd FET stage, which DOESN'T appear on the schematic I have used , which is a newer one .......

http://www.carvinmuseum.com/pdf/amps/VINTAGE33.pdf

[Xavier]

348 views.....someone must be thinking on building it right?

please leave your comment here when you do !!

[SISKO]

MikeLandau?

[wui223]

I am curious about the trace for R5 and R6 in the layout, it seems wrong for me.One more is the connection of Mid pot,as stated in schematic the lug 3 is leave open and the lug 1 connected to lug 1 of Bass pot. But in the layout, it is different. One more question, how to tune to the VR1, VR2 and VR3? I guess there are the trimmers for biasing. Why the diodes in schematic are missing from the layout?

[Xavier]

I am curious about the trace for R5 and R6 in the layout, it seems wrong for me.One more is the connection of Mid pot,as stated in schematic the lug 3 is leave open and the lug 1 connected to lug 1 of Bass pot. But in the layout, it is different. One more question, how to tune to the VR1, VR2 and VR3? I guess there are the trimmers for biasing. Why the diodes in schematic are missing from the layout?

Yes you are right. If memory doesn't fail, R5 and the cap beside it should be in series. I will correct this next week, thank you.

As always, the trimpots should be adjusted to half the supply DC, that means 4.5V if you are using a 9 volt battery, but I always fine tune by ear.


Yes Sisko, it's me.