Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Schematics and Layouts => Topic started by: stratitis on April 30, 2020, 05:49:11 PM

Title: Does anybody know this double JFET amplifier stage?
Post by: stratitis on April 30, 2020, 05:49:11 PM
What is the point or trick of those serialized/cascaded J-FETs (Q1, Q2)?
 
(https://s12.directupload.net/images/200428/a9gszoxq.jpg)

Can anybody name or explain this technology, please?
Title: Re: Does anybody know this double JFET amplifier stage?
Post by: phatt on April 30, 2020, 09:54:27 PM
Google "Cascode Fet" or "Fet uAmp"
Basically higher gain from a stage, any advantage is open for debate. :-X

The Roland cube amps I've played/heard are not awe inspiring to my ears.
Due in part to TOO MUCH Bandwidth and small box.
Any extra mojo in this particular model is likely due to the fact there is no DSP crap to stuff it up.

Great guitar tone is dependent on what you expect it to be which depends a lot on what you have already heard/played.

Dig deep into why for long enough and you will find that tone shaping of the WHOLE circuit from PU to speaker is key to the outcome.
Tone knobs on the front panel are only a fine control but what you need to understand is the Whole System Tone. If you look close you will find the most sort after amps were actually very simple designs and had major limitations, one being limited bandwidth, and shocking distortion figures.
Even Hank Marvin's famous "Clean Sound" had a lot of triode rattle. Without that limitation it would sound dead.
When the Vox "ValveTronic amps" came out I thought it might be worth a visit to store,, oh what a disappointing event that was. I ended up plugging into a Fender Deville which I don't really like but it was at least more real.
Phil.
Title: Re: Does anybody know this double JFET amplifier stage?
Post by: Loudthud on April 30, 2020, 11:03:59 PM
Not exactly a cascode circuit because the gates are connected together.

A quick experiment on the curve tracer shows that it knocks Idss down to about 60% of the single JFET value. Gain is also lower. The curves look slightly more linear.
Title: Re: Does anybody know this double JFET amplifier stage?
Post by: phatt on May 01, 2020, 06:07:50 AM
opps, Yeah I kinda thought that it was a little different but the Guy is asking and with a google search he might find something. Thanks for the correction.
The Q still remains is there a term for this topography?
Phil.
Title: Re: Does anybody know this double JFET amplifier stage?
Post by: stratitis on May 01, 2020, 07:42:58 AM
Not exactly a cascode circuit because the gates are connected together.
Completely different, I would say:

(http://www.circuitstoday.com/wp-content/uploads/2012/03/cascode-amplifier-circuit.png)

Yes, I wanted to know the name of the Roland Blues Cube gain stage circuit, if there is one, to google for it.

A quick experiment on the curve tracer shows that it knocks Idss down to about 60% of the single JFET value. Gain is also lower. The curves look slightly more linear.

I've simulated the circuit in LTspice.
I used generic JFET model from LTspice. Input source: 100mVp, Ri=10kohm
 
Results:
(https://s12.directupload.net/images/200501/6sgrx5z6.jpg)

Gain for dual FET stage:
270/160=1.7

Gain for single FET stage:
600/160=2.75

Dual FET output signal is slightly more distorted than single FET.

What's the benefit of dual FET?
Title: Re: Does anybody know this double JFET amplifier stage?
Post by: joecool85 on May 01, 2020, 09:29:53 AM
When the Vox "ValveTronic amps" came out I thought it might be worth a visit to store,, oh what a disappointing event that was. I ended up plugging into a Fender Deville which I don't really like but it was at least more real.
Phil.

I agree.  I owned a 50w valvetronix amp and while it was "ok" at certain sounds, it was never great.  Honestly, for much of what people play, a simple Fender Frontman 25r was doing better.  But the Vox did look amazing!