Thanks for all the info and hints. I'm about to order the parts that I don't have in my stash and as soon as they arrive, I'll start breadboarding. Provided nothing unexpected gets in the way, that is.
As far as the higher voltage goes, I happen to have an LT1054-based voltage doubler put together, so I'll give it a try at roughly 18V too.
Have a good one and stay tuned!
M

I'd try 10pF to 22pF, base to collector, similar to the trick employed to smooth down silicon transistors in fuzzes, but that's a matter of taste
I had a bit of downtime this arvo and threw the Vulcan on LtSpice: the frequency response is VERY similar to the Randall RG100Es, so you'll probably hot rodded marshall vibes. That doesnt mean that it will sound like a Randall, though: clipping thresholds and gain increments are very different.
I was also surprised to see that the circuit, as is, is already shaving off some dBs above 6kHz.
I'd be tempted to try it at higher voltages, with low hFE transistors, just for the sake of it.
Oh, and the diodes do mimic grid clipping: they chop almost half of the waveform before the gain stage. Very clever design with just a few parts

Thanks for the reply, Umlaut.
Looking at the schematic, I also expect it to sound like a Marshall on steroids. Like a DSL or a Bogner Ecstasy perhaps?
Speaking of snubber caps, do you mean in parallel with collector resistors?

Hi there!
I have this one on my to-breadboard along the line.
Seems to be marshall-voiced, judging by the filtering, and pretty high gain.
Might be wrong, but perhaps the diodes mimic grid saturation as in a valve amp.

Indeed breadboard first, especially if using other trannies, and check the voltages. Personally I'd try snubber caps on the last two gain stages.

Good luck!

I have a problem with a multi watt dual rectifier head.
It keeps blowing fuses and a rectifier tube.
Using the light bulb mains lead method, even with both rectifier tubes out of the unit and on standby it seems to be drawing quite a bit of current. No change with power tubes also removed.
Is this likely to be the large filter caps going or any other suggestions?
Thanks in advance

Hi, everyone, first build-related post of mine   
Soon, I'm going to build Joe Davisson's 4-stage Vulcan preamp using the schematic posted on his website and I have a few questions before I embark on this project.
1. Has anyone tried it?
2. If so, how does it sound? Judging by the number of gain stages, it's a hard rock / heavy metal machine.
3. I'm going to use BC337 transistors because I have a lot of them (not much of anything else tbh). Apart from pinout and biasing, is there anything I should specifically pay attention to?
I'm going to breadboard it first and, of course, I'll keep the forum updated on the progress. That, however, may take a while because adult life is the way it is
Stay tuned.

Thx G1!

Attached some screenshots just after the push-pull mimicker:

Nice work and great name for your project.   

Attached schematics:

Hi all,

First post here, long time lurker though - I figured I'd start with a little something that I have been working on in the past weeks.

I wanted a fairly neutral preamplifier circuit to use with dirtboxes and various AIABs, but with the ability to have some flavour and zing of its own. After several iterations and massive overhaulings to an otherwise unremarkable starting point, I arrived to this.
For the sake of simplicity, it can be split in two parts:

First schem shows the input stage, tonestack and limiter/soft clipper.
- Input stage is somewhat ripped from the Ampeg B2. I did try jfets, transconductance stages and whatnot, but in the end I wanted transparency, so buffer + variable gain stage it is. There is a "bright" switch too, because we still want to have a nice and chimey clean sound on its own.
- TB tonestack is a Voigt, just for the sake of it, and because I wanted a) passive, and b) non-interactive. I tweaked the values to get a flat response at the middle of the pot rotation. Fender have a pretty nice passive and non-interactive tonestack in some of their models (Frontman, etc) but I was looking for a frequency response more in line to what I am used to (FMV). Completely subjective choice, though. The drawback of a Voigt tonestack is its high sensitivity to load and very high gain loss, so it is followed by a makeup gain stage.
- Mids control is your bog-standard mids filter: just a scaled-down FMV with treble and bass removed. The switch changes the frequency response of the mid-dip from JC120 to JCM800-ish, which is convenient to have depending on what dirt flavours one is using upstream of the preamp.
- From there we go into a GK G.I.V.E-styled soft clipper. Depletion mosfets gave me a slightly mellower sound than the prescribed J113s from mr. Gallien, so I went with that. The advantage is also their regularity in specs (no massive differences from one device to another), so it allows for the trimpot on the drain to be removed altogether. The resistor in the feedback loop of the transimpedance stage determines the amplitude of the wave, and I kept it fairly low because of what comes after. This stage is the first to clip, and does so in a very gradual and symmetrical way, not too far removed from a LTPI.

The second schematic shows the push-pull power stage mimicking and presence/resonance circuits.
- I had this crazy idea last year when tinkering with the Peavey T-Dynamics circuit. I wanted to have a reasonably simple approximation of some squish and dynamic, without having to resort to feedback from poweramp or other complicated contraptions. The Traynor Dyna series have a pretty cool way of emulating that (sans the crossover distortion), but I hated the idea of using only one half of a LM13700. The Ampeg SVT-8Pro has another very creative way of doing that, which I breadboarded and tried, but no luck: sounded like crap with a clean preamp, and only good if some distortion was added upstream. In the end, I took two of Ampeg's flexwave stages in parallel and made them "push" in opposite directions, while clipping half of the wave: the result is that of a semi-slow compressor that gradually introduces crossover distortion. It is followed by back-to-back 1V Zeners, on account of, y'know, power amp clipping and such, and also to make 100% sure that the following stages do not hit the rails.
- Then comes volume control, followed by a Presence and Resonance circuits VERY similar to those of Rocktron power amps from the late 80s-early 90s. The main difference is that I added a pot for each, to be able to tweak them individually, as opposed to a simple mix pot from the original circuit.

Other considerations:
- I am using LF353 opamps because right now I have more of those than TL072s, and because they sound fairly nice here. Feel free to try other opamps if you are breadboarding or building it, but should make no difference.
- For the last clipping stage, other clipping configurations can be used, provided it is not above 4V ptp.
- Other Jfets or DMOS should work well on the soft clipping stage. Experiment, etc.
- The tone stack is tweaked to the frequencies of standard FMV tonestacks, minus the interaction, but can yield good results with the original Voigt values and, say, the contour from Marshall's Valvestate, or the Focus from several crate amps.
- Resonance is tuned at 120Hz (closed back 412 cab), but other freqs can be used by changing C26 and C27.

Disclaimer:
- NO CLAIM is being made on the superiority of this circuit above any other circuit, quite the opposite. I simply followed some random ideas I had and put all this together; it works and it sounds how I wanted it to sound, and does what I wanted it to do, and that's all I need.
- Yes, I am aware that simpler means can be used to the same goal, but again, my point was to get an idea to reach a concretion (and have fun and learn as I do so). I am only a hobbyist, not an engineer (I wish)

I will work on some modular veroboard layouts in the coming weeks, but as of now, circuit is still on the breadboard, so please do let me know if you have comments, suggestions, observations or criticisms, or if something is terribly wrong somewhere in the circuit.

Cheers and enjoy!

/Alex