New at runoffgroove.com: Professor Tweed

[B Tremblay]

The Professor Tweed is a FET emulation of the Fender Princeton amp.  Check it out!

Article, schematic, and perfboard layout: http://runoffgroove.com/professor.html
Sound clips:  http://runoffgroove.com/salvo.html#professor

[puretube]

wow! a new record in the C/W *  score !

[* : Circuits per Week]

[Mark Hammer]

Okay, now yer pushin my buttons, Tremblay!!!

As the proud owner of a 5F2-A since 1976, my sense of identity is challenged by the notion that 3 FETs could replace this marvel of glass and iron (and tweed).  Now I'm gonna have to put them to the test.

One thing I am glad to see, though is the implementation of the one-knob tone control which many of those whose viewing of schematics is limited to effects may not be familiar with.

Imagine the following.  Imagine the wiper (2) has been rotated fully so that it rests at point 3.  What you have there is a volume control with a treble bypass cap (470pf) and a treble bleed cap (4n7) with a 1meg resistor in series (severely restricting treble bleed).  As you start to rotate the knob, the resistance in series with the 470pf treble bypass cap increases at the same time as the resistance in series with the treble bleed cap decreases.  So the one knob adjusts treble boost and treble cut at the same time.  Pretty neat, huh?  The thing to remember is that, like all treble bypass caps on volume controls, its effect interacts with the volume control setting.  The lower the volume setting, the more impact the treble bypass has.  As you turn the volume up, you need to turn down the tone more and more to notice any audible change from having the tone full up.  On the one hand, the interdependent nature makes more work for the player/user.  On the other hand, it means a lot more interactivity.  Since this designs also includes a master volume (which my little tweed baby doesn't), the flexibility aspect wins hands down over the inconvenience

I might point out that my 5F2-A, as well as being fabulous for blues guitar,  is one of the finest harmonica amplifiers I have ever heard.  One harp player I jammed with years ago had a complicated setup with the requisite bullet mic and a Twin.  I suggested he plug into mine and it was like we had been instantly beamed to Pepper's Lounge.  In his words "You're not leaving here with that amp tonight".  So, if it emulates as well as you hope this may well be an interesting project for those harp players who want a little box that will make them sound like "instant Chicago".  Who knows, you may not even need your bullet mic anymore.

[Gary]

I wouldn't lay down that bullet mic, yet!  I'll be looking for a report of the pros/cons.

I've seen one of these amps and the guy that owned it refused to let me touch it!  Oddly enough, he was a harp player.  He owns a few Valcos, too.

[Marcos - Munky]

Cool, another circuit!!!

[koolimy]

Wow... You guys are posting up new stuff like crazy now...
I remember that the Matchbox came out not too long ago...

[Gary]

Last week was the Matchbox.  Since the Matchbox seems to be a slow burner (pun), we put the Professor out for those that don't have access to J201 fets.

[Mark Hammer]

I might point out that on a real 5F2-A. lifting the negative feedback resistor (22k) puts one squarely into the icepick through the forehead tone-zone.  Does the negative feedback have the same roll here, that of keeping the harmonic distortion in check?  If so then some sort of variable, or at least switchable (e.g., 22k / 100k / 500k)  resistance there would be a worthwhile mod.  

You will note as well, that the locus of traditional "presence" controls was in a parallel RC network alongside the 1k5 resistor on Q2.  The 22k feedback resistor and 1k5 resistor (well, its equivalent, actually) served as a voltage divider for the feedback signal.  A variable resistor in series with a cap would set a differential amount of feedback (i.e., attenuation of the feedback signal from the output transformer) for the higher-frequency content so that there would be more and/or less feedback for the high-end.  With less negative feedback, all the added grit to the signal would be cancelled and the amp would sound cleaner.  One of the reasons why early amps without the negative feedback, or with the ability to adjust the amount of negative feedback are desirable and sound so great.

[puretube]

"instant Chicago"

would be a very nice name for a new pedal...
(with an equivalent sound, i.e.)

[Mark Hammer]

... as opposed to an early Liverpool emulator called the "Instant Hamburg"?  :lol:

[puretube]

you mean: "The Beatler" ?

(rather a name for a starkey rhythm ace...)

[Gary]

Mark,

Lifting the 22k does drive the distortion way up.  We omitted the "presence" setup since it seemed a bit redundant.  The value of the coupling cap (10n) on the feedback loop makes a huge difference and you can indeed tune the feedback via this cap.  It seems to do exactly as you mention.  I highly recommend playing with this cap's value, as the effects are very noticeable.  10n got the nod from both of us, so that's what we've stated as the "stock" setup.  As you work up through the values from, say 10n to 1 micro, you'll hear the diffrences.  As always, breadboard this circuit before you build.  If you work up something different with the source side of the second jfet that sounds more like the real thing, let us know.  I'm dying to know how close this thing is to the real deal.  Having only heard one of these amps once, we kinda shot for a conglomeration of tweed tone.  Please tell us how far off we are.

BTW, I guess "instant Chicago" is much better than "instant Air Supply!"

[Gringo]

Wow, do you people at ROG actually sleep?

Another one that goes to the (almost endless) build list

[Lonestarjohnny]

Mr B.
You guys are cookin the Midnight oil at Runoff, I just put the finish on my Sili-Face, tried it out with a Gibson GA-5, It work's like Gangbuster's for sure, It's a nice addition to my board, now I know what my next build's gonna be, where's my J201's,
JD

[Bucksears]

You's guys are fryin' my brains ova heah!!!! I'm OD'ing on OD's!!!!
Man, my 'to do' list is growing by leaps and bounds.
The Matchbox and the Professor would pretty much round out my versatility of OD's and distortions.
Top notch work guys!!

[Ed G.]

Hah! I thought woodshedding was in my future. WRONG! I'm breaking out the breadboard as I type (well, not exactly this moment, but...)

Seriously, those sound clips sound fantastic.

One question: Does the tone control give a wide range from bright to dark? (I suspect it does) because playing a strat through a super reverb, I find a lot of pedals too bright/harsh. I always have to add extra tone filtering and I don't usually like the end results.

[RedHouse]

Wow that actually did sound Fender'ish, I'm impressed.

Ok then, it's time to dust off the good'ol 2204 schematic and see if that can be done.

Any thoughts on how to implement a CF with the FET's?

[Ed G.]

The BSIAB and Doug's "Sweet Thing" series of pedals use a fet as a source follower. Try that.

[puretube]

imho a presence control plays its best role, when deriving the F.B. signal from the actual power output at the speaker...

[bwanasonic]

BTW, I guess "instant Chicago" is much better than "instant Air Supply!"

Is there a pedal that does "intant Poughkeepise" ? That,  and "instant Idaho" , are the tones I've been searching for.

Kerry M