Tube Stompbox Project -- What good preamp circuits for blues?

[Transmogrifox]

My bro picked up an Epiphone Casino and I really enjoyed playing it through my Peavey Classic Duel 212 with a little JFET OD I have on my breadboard.

He talked about how a friend of his brought over a tube stompbox one day and he said it sounded sweet, even just going direct into his PA system.  He failed to mention which tube box his friend had.

Anyway, I have so far really liked the sounds I get from the Peavey Classic series amps so I'm leaning toward that basic circuit topology for a start. 

Does anybody have recommendations of tube pre's you really like for a bit of gritty blues, but with enough gain (like the Peaveys) to push it into something more like a Kenny Wayne Shepard type of tone?

As an aside, since switching power supplies is what I do for a living, I thought I'd make it work off 12V in a little light-weight box.  This circuit hardly takes any space using SMT components and looks promising with what I saw on the o-scope driving a resistive dummy load.  Not a high-gain regulator, so it will put any 60 Hz/ 120 Hz buzz from the 12V rail onto the 320V B+, so it will probably end up needing some high voltage bulk caps and resistor going toward the plates or it won't be quite so clean as one would hope.



Thanks for any input on good tube pres + mods.

[GibsonGM]

At 12V, you'll be wanting to look at low voltage tubes.  Any chance you'd be interested in doing some multiplication there, getting into at least the 100V+ range?   The well known old faithfuls (12AX7) work best at voltages up in that realm, and beyond.  If you're comfortable using higher voltages and know how to do so safely, that's what I would recommend. 

Other than that...there are only a few common (classic) topologies.  What you do between stages and whether you bias warm/cold will of course determine a lot about the character of your preamp.  Mid hump, or mid scoop?  That kind of stuff.     

To experiment, I'd set up 2 or 3 basic triode gain stages roughly center-biased, maybe 100k on the plates and like 1.8k (over 100V, mind you) on the cathodes.   Feel free to change values of course.  Then play with the gain you're feeding each stage after the input, as well as how you're coupling.  Drive the tone stack with a cathode follower after the 3rd gain stage.  Feeding this to a tube amp will drive it nuts!   Watch your output levels, ha ha....seriously, don't feed 80V to your amp! lol

Check out the "GTFO" on here, that's neat one.  Harder than you want, but there is good stuff on the schematics I've seen...

>> After playing with trafos for a long time and getting really annoyed by balancing heater vs. PS, I think it might be easiest to drop $50 on a decent transformer for a good project.  Making regulators and messing around gets old FAST, and keeps you from real tube work!

[Transmogrifox]

To make it clear, this will be operating the HV to the tubes at 320 VDC with a switching power supply (see schematic above).  12V input does not mean 12V B+.  I want to make a real tube pre in a stompbox then step the output down to line levels, not a starved-plate OD pedal.

I am looking for recommendations for some amps that have been known to be good ol' standby amps for the blues sound so I can learn some things from looking at the preamp schematics.  As stated above I am familiar with the Peavey classic series amps and schematics because I own one and have spent quite a bit of time looking over what makes these circuits tick.

I'm an electrical engineer so I know how to safely muck about with high voltages, and tear schematics apart to glean the salient features.  I don't think I need much help with realizing a circuit that will do what I want it to do, I'm just not sure what it really needs to do yet.   I'm just looking for some reference material to help identify what stackup of stages is a good choice for non-flabby sounds.  For example, what makes the difference between a crunchy OD used for hard rock and the meatier, yet lest saturated Chicago blues sound?  It seems there's a fine line between keeping enough low end to be meaty, yet not so much that it becomes muddy or fuzzy.  At the other end, I don't want it to break up into a piercing splatter like some Fender amps I've played.

I appreciate your input (GibsonGM) about experimenting with the 2 or 3 basic triode stages.  This is the basic topology common to all amps, but as you said -- what you do in between the stages makes all the difference.  The basic idea is really simple:  Filter-amplify-clip-filter-amplify-clip-EQ then output level conversion.

I do have a SPICE simulation of just that.  As you know the magic happens  where frequency emphasis/de-emphasis happens between stages, and this is where I would like some input about tube preamps that stand out as good overdrivers with moderate crunch.

The two sounds I want to do well are these:
1)  Clean right on the edge of breaking up, where 100% on the volume knob makes it break up on a hard pluck.  The "milking" it in and out of distortion.  My Peavey Classic doesn't do this very well as it is -- needs some mods to get there.
2)  Ability to push it over the top into a much more sustained, well-overdriven sound (Kenny Wayne Shepard, for example).  The Peavey classic does this well on its dirty channel with the right tone settings and gain.  Of course, it's such high gain it does metal pretty well too.

These two approaches are somewhat at odds with each other, so I fully expect to have relays installed to do Clean, Dirty, Overdrive channel switching, where clean will tap from the first stage like a Matchless HotBox, Dirty from Stage 2, and Overdrive from Stage 3.  My simulations show me these each give about the right amount of distortion for the desired goal.  

I am hoping to pull off the tone stack with a simple hard-wired  post-eq network and then a single BMP style tone knob.  It will be something of a 3-trick pony where it does one thing really well, and does the other two reasonably well.

I have looked at GTFO, and I agree it doesn't look like it's aimed at the kind of sound I'm going for (with this one, although I really want to build it sometime).  The Real McTube seems more like a starting point, but I don't see that it really aims at my target either.

Another approach I could take is just re-purpose the Ibanez Tubescreamer frequency characteristics back into a real tube format.  I know the mods I like with this circuit, and maybe it would sound good using real tube stages for clipping.

I know that was wordy, but maybe makes it more clear what kind of sound I'm going for.

[GibsonGM]

DOH! Early morning fuzz-head, I didn't look closely at your schematic to find the B+.  My bad.

Just an opinion, but the Fender Bassman or any of its variants would be a good place to start.  With 2 or 3 resistor changes, the GTFO would be fine for blues.  The tone shaping is what's given it the massive scoop.  As you mentioned, the TS mid hump?  Easy to incorporate just about anywhere you want....tons of gain on tap there to eat up in insertion loss, if needed.

The differences between the 'classic blues amps' are almost nil (again, my opinion...change a  couple of caps and R's, and now it's a Marshall).  That's why I suggested just winging it, starting with 100k plate resistors and a warm bias.   22n coupling caps, maybe a bypass cap of 470p around the voltage divider to stage 2, etc.

I just put together the first 2 stages of the GTFO the other day, for grins, on 180V B+.  100k Rp on V1, 330 on V2.  Couple of tweaks on bypass cap values (560p), and a 47u output cap.  Run this into my Hot Rod Deluxe (flabby Fender blues amp, 40W).  You can't tell the difference between the sound of this and a Marshall, I bet.

Turn the gain down between V1 and V2, and up the output, cut some bass....there's your SRV creamy solo in "Little Wing".   The diff, to me, between hard-driven and blues tone is simply how much you attenuate between stages.  If you're running into a tube amp, 2 stages should be MORE than enough!! 

Just some thoughts, by all means keep looking at designs and for opinions!  Those little differences DO mean something, I don't want to sound too offhand about it...

[Transmogrifox]

Thanks again.  If anything you give me some confidence the tweaking will be simple R's & C's.

I'll look at the bassman schematic.  I remember our high school had one of these and the sound was beautiful on electric guitar when cranked.  I was sad to hear it got abused, beat up, and then thrown away.  If I had been aware of it's fate I would have gladly taken the remains and applied some TLC to keep it going.  I don't think anybody there really understood what a treasure an old Fender Bassman is.