Tweaker 15 input stages..

[leadfootdriver]

I can make some sense of this.. The 1st is a JFET and then it goes to a MOSFET before the 1st Tube state..
Can anyone explain why he chose SS parts before a tube stage?

https://el34world.com/charts/Schemat...tweaker_15.pdf

[Helmholtz]

Input stage is a cascode consisting of a Jfet and a Mosfet. Has high input impedance and gain. Requires less space and is cheaper than a tube.

https://en.wikipedia.org/wiki/Cascode

[uneumann]

I realize it's been done before... but it does work very well. It also has higher headroom than a tube.
An additional current mirror makes PS noise and ripple a non-issue, so it's very quiet.

https://sites.google.com/site/string.../solo-100-v4-3

[leadfootdriver]

Thanks!
Why would it be desirable to have high input impedance vs low like you would get with an active pickup??

[Chuck H]

Thanks!
Why would it be desirable to have high input impedance vs low like you would get with an active pickup??

Because not all pickups are active, basically. a lower input impedance could cause problems if a plain ol' vintage guitar were in use. There would be significant loading that would lower the knee of any capacitance in the circuit and ruin the "tone". A high input impedance is a standard for a reason. Ideally you want to have low output impedances and high input impedances. To put it bluntly. An input that only managed the lowest possible output impedance devices (like active pickups with a 25k volume pot) would be pigeon holed from a marketing point of view.

[loudthud]

Gibson made some low impedance guitars and amps in the late 60's to early 70's. The amps have disappeared, the guitars you occasionally see. I have a Les Paul Recording Bass that can be used into a normal amp with a microphone transformer inline. Without the transformer, it sounds very weak. These guitars were passive, no battery in the guitar, just low impedance to lower the noise.

[Helmholtz]

... just low impedance to lower the noise.

Not sure if passive low impedance PUs generally provide better S/N (given adequate shielding). They do generate less noise but also less signal, so require more gain.

They can provide extended HF response, though, if that's desirable.

[leadfootdriver]

What if I hit the front end with a clean boost? Would the Cascode distort?

[uneumann]

What if I hit the front end with a clean boost? Would the Cascode distort?

Yes - if you overdrive any circuit it will distort. The distortion behavior is different than that a tube (no surprise). The circuit is very clean at low signal levels and gradually increases it's distortion as signal levels increase. However, once it clips, it clips hard and distortion increases abruptly, something I would avoid. Fortunately, it's easy to avoid given high headroom. IMO, this input stage works well when distortion is created (intentionally or not) in later stages of the amp.

[leadfootdriver]

I guess I'm trying to get an idea of how much headroom is in the Cascode.. It seems there is only 3 gain stages total then, so the Cascode must in it's own way a clean boost to the latter stages to get distortion..

[J M Fahey]

It´s not "cheaper" at all, and if it were so, everybody would use it.

Musicans are not aware that a 12AX7 in China goes for about $2 or less if bought by the container, I can buy excellent Russian 6N2P, much better than a 12AX7 being "the same thing but built for Military/Industrial use" for $2 in small quantities any day of the week, so less than $1 per gain stage.

The SS version costs a little more per stage.

The point is it is *better* , much better, only, as said above, does not clip the same way, although it can be tweaked to be very very close.

Better in what way?

* NO Hum (no filaments)

* much less noise

* NO MICROPHONICS !!!!!

* all the gain you want, easily 200X , might get 500X if needed but that would destroy headroom.
As shown it´s about 100X which would be a practical limit; comparethatbto 12AX7 50X normal, sweating blood to get 60X

It will behave much more like a pentode than like a triode.

All those "improvements" make SS great for input stages (as used here) , cathode followers (although clipping is somewhat different, since they can source all the current the load wants, even Amperes) and Power Tube drivers (long tail PI).

Since internal Cathode impedance is very low (a few Ohm) and Plate internal impedance VERY high, a true current source, even more than a Pentode, in this example gain is pracically 100k/1k (plate load/cathode resistor).

Notice they show an "unused" cathode resistor + capacitor pair (NP means not placed) R20+C23; if somebody has weak pickups he may add there, say, 1k + 1uF or so.

Very good amplifier as is but if somebody wanted to add an über gain mode, he could add this "unused" boost with a footswitchable relay.

On the other hand, they are "wasting" a good triode in the effects loops end, V1A, as a cathode follower which adds nothing to sound, but, of course, since they already had it .....

Notice they did not use that "extra" triode at the input stage.

Some Dumblemodels used a plain FET input.
In theory to accept passive Piezo Guitars, but used as a clean boost to overdrive later stages for "the JFET input actually sounds pretty good with electric and the overdrive channel when dialed in just right. A little sparkle and drive. kinda cool. ".



Mesa Boogie also used an "SS Tube", the FETRON, originally designed for very long life Telephone Line repeater.

[leadfootdriver]

Awesome, thanks for that.
Would the Cascode be similar in headroom to an EF86?? I've never played one.

[eschertron]

I found this on the blue gtr website several years ago (RIP Steve) and bread boarded it for testing (swapping out resistors, adding bypass caps, etc). I like it but have not built it into any amps...yet. Took good notes, though

https://blueguitar.net/new/articles/other/hi_v_ss.pdf

Since a typical first stage triode will have a gain of around 50x, a 100mvRMS input gets 5vRMS, a 1volt input gets 50v, etc. Hardly enough to make use of the tube's natural compression to make an audible difference. That's my argument for why NOT to use a tube in the first stage. Add to that the fact that a SS input can have its gain set to more than 50x, the next (tube) stage can be overdriven without adding a booster pedal to the stompbox string. Maybe with a switchable boost on the input?

IMHO unless you're looking for an insane amount of gain, headroom is not an issue with this. This circuit can output a much higher PP voltage than a triode in the same place.

[Helmholtz]

Prof. Zollner analyzed that there is some interaction between the PU signal and the beginning grid conduction (which often starts at some negative Vgk).
Based on distortion measurements, he attributes sound differences between input tubes to the large observed variation in grid current between different samples.

[leadfootdriver]

I found this on the blue gtr website several years ago (RIP Steve) and bread boarded it for testing (swapping out resistors, adding bypass caps, etc). I like it but have not built it into any amps...yet. Took good notes, though

https://blueguitar.net/new/articles/other/hi_v_ss.pdf

Since a typical first stage triode will have a gain of around 50x, a 100mvRMS input gets 5vRMS, a 1volt input gets 50v, etc. Hardly enough to make use of the tube's natural compression to make an audible difference. That's my argument for why NOT to use a tube in the first stage. Add to that the fact that a SS input can have its gain set to more than 50x, the next (tube) stage can be overdriven without adding a booster pedal to the stompbox string. Maybe with a switchable boost on the input?

IMHO unless you're looking for an insane amount of gain, headroom is not an issue with this. This circuit can output a much higher PP voltage than a triode in the same place.

Wow, it's like nearly identical.. The reason I mentioned EF86 is because the early Vox's used the EF86. SO since the Tweaker has a Vox setting, I wonder if part of the decision to use a SS Cascode is for the headroom like the EF86..