Thoughts on a 1-tube pre/clean boost in a pedal?

[DenaliJoe]

First: I'm posting this in Amps, not Effects, since it is really more of a pre-amp than a "stompbox effect"- but if this is in error- my apologies!

Next- I have a friend who plays a simple electric rig- guitar straight into into a Carr Rambler- fingers only, no pick- lots of jazz and blues and improv in various gigging situations. For what he does- it totally works- and he likes the openness and touch sensitivity of playing straight into the amp. Playing pretty much fingers only myself, I get it.

But sometimes he needs a little "more", a push or a boost for soloing, etc.

There are plenty of clean boost stompboxes out there, and he's tried several. I've even wired him variations on various themes. But he always comes back to feeling like he is playing through an effect- and well, he is right.

I realized the one thing we hadn't tried is some kind of a clean tube booster.

I hunted around for schematics and ideas, and didn't find what I'm looking for. Everything is either a "low-voltage, starved plate, angry swarm of bees sound" pedal, or an op-amp driving a tube into an even angrier swarm of bees, or a 2-tube design with a full EQ section like the EHX or Hoffman-style tube pedals.

I don't want a distortion box, just another tube stage of boost. I don't want a full eq section- the guitar has a tone knob and the amp has EQ. I just want to get "more" and retain the open clarity of playing direct into an amp.

I've made 1-tube 12au7 high voltage buffers in pedal form before, so am familiar with the Hoffman-style power section...and thought switching that 12au7 buffer idea to a 12ax7 might do it? And since I cant get to my soldering iron until the weekend, I scribbled the below on paper and thought someone might have experience- or see it for the fun experiment I think it will be.

It's nothing special- just a typical 12ax7 gain stage into a CF. I was thinking go a little higher on V1a Rk for headroom. Caps are "big" , to pass everything through, but maybe need to be adjusted down. I don't want a recovery-gain-needing EQ, so instead of a Bax or TMB or even a tweed-style knob I thought just a switchable bypass cap to shape lows and a cut control to tweak highs- but I'm looking at that now and thinking I don't like how that will change output impedance...

It's an idea and a sketch and my little hopes someone has done something similar?

Thanks!

[sluckey]

That volume control is really gonna mess up the operation of the cathode follower.

[PRR]

That's a heck of a lot of gain.

The hi-cut doesn't.

IMHO the cathode follower "works" but will overload easy. Maybe not before the amp input?

[pdf64]

It looks to be a halfway house between an ac CF and a dc CF. As it is, with the CF grid referenced to 0V, it won't function, as the CF triode will be biased into cut off.
http://www.valvewizard.co.uk/dccf.html
http://www.valvewizard.co.uk/accf.html

Excellent clean boosts can be built using silicon technology.

[terminalgs]

I've built a few of these types of pedals.  The 12AX7 will make a lot more gain than you really need.  You'll probably overdrive the Carr input stage with any device that produces more that +6dB or so. If you use a 12AX7 pinout, you can swap in a 12AU7 or 12AY7, but even a low Mu tube like a 12AU7 will produce so much gain that you'll overdriver V1 of the amp into distortion. regarding your design:

• I would put the volume control after the coupling cap and before the output jack.   YOu have a resistor with an asterisk there.  I'd make that a  50K pot  with a resistor on top of it ( to lower gain)  center lug/wiper output to output jack.    To do this you'll need a bigger coupling cap .  A .1 would be ok, or even a +/- 1uf.
• find out the input impedance of the Carr amp.  if its 1M, then skip the CF use a 500K or 250K pot with a ~500K resistor between it and the V1 coupling cap.
• skip all the cathode bypass cap stuff on V1.  If you friend wants Jazz tone then he wants all the low end he can get.  No need to cut any of it with bypass caps smaller that 25uf.
• for a tone knob,  I'd try something similar to what you have only connect it the plate of V1.  maybe start with a cap in the 100-500pf range.

Triode Transformers/Classictone sells a PA522 originally spec'ed for a Dynaco preamp.  it s a 330-0-330V+12.6V which is lots of B+.  The PA511 is a 120V main version, the PA522 is a 120/240 version.  I've wired the PA522 for 240 but giving it only 120 yields 165-0-165+6.3V which is works great for preamps as well (and lets you use 6SL7's or other 6.3V tubes..)Regardless of which tube you use, I would try several old variants.  I wouldn't use a new JJ or EH.  You can get a handful of 12AU7s and swap them out until you find a really nice sounding one.  I think the best tube for the job is a 6SN7 or a 6SL7,  your selection is limited to used, but they sound so good.  Look at the Ampeg B15, they used the 6SL7 until 1970 or so.  Gemini's Rockets, even the SVT and VT40s used 12AX7s in V1 but they continued to spec 6SL7's for V1 in the Jazz fav B15.


(BTW I edited this post but somehow made it a reply and quoted myself..,I deleted that second post....)