Joe Davisson's Vulcan preamp sound sample

[fentom_lord]

Hello, everyone. I just wanted to share a recording of Joe Davisson's 4-stage Vulcan-based guitar preamp. I used BC337 transistors because I had a lot of those. You can listen to it here:
https://m.soundcloud.com/user-996907905/joe_davisson_vulcan_preamp_bc3

Guitar: LTD MH 50 with pickups from Dean Vendetta 2
Amp: single-ended EL91 Fender Champ-a-like
Speakers: some 1980s 12" marked Celetion (mind the lack of S)
Settings change as I go. I do not touch the guitar's volume knob, I just play with the knobs on the preamp.
The values I used for the tone stack, presence and level are different than those on Mr. Davisson's schematic because I only had pots for tube amps. All related caps have been scaled accordingly. Moreover, I had to use 22k collector resistors to get down to roughly 4.5 VDC. Everything else is stock.
Enjoy.
Oh, and please forgive my sloppy playing 😅 I don't play as much as I used to...
M.
EDIT: I forgot to mention that it's still on my breadboard, hence the noise. I'm going to try and tweak some values to see what happens.

[fentom_lord]

I added another gain stage and tweaked it again. Here's how it sounds now

https://m.soundcloud.com/user-996907905/joe_davisson_preamp_modded_bc3

Again, I'd like to apologize for the sloppy playing. Oh, and I realized that in the previous recording I had the bright switch on.
Enjoy.

[Mark Hammer]

I've made one of Joe's Vulcans and a Blackfire as well.  Hard to go wrong with any of his ideas.  Very thoughtful.  NOT for easy listening jazz, though.   

[fentom_lord]

That's true. His ideas have enormous potential, even if you simply copy and paste one gain stage or delete one or two of those. In the last recording, the preamp has one extra stage and some minor RC tweaks. I can draw a schematic if anyone's interested.

[soggybag]

I've built a few of these everyone I gave them to likes the sound. Besides the sound, what's great about this is it uses bog standard and easy to source parts that are not so specific.

What's going on with the diode at the base of each transistor still stumps me.

[marcelomd]

Hi,
On your second clip there's some noise in the background, ticking. Very clear at the start and when you stop playing. Is this from the preamp?

Sounds very, very good. I'm playing with cascaded MOSFET stages. Same general idea as the vulcan.

Thanks

[fentom_lord]

Hey, marcelomd,
The preamp is still on the breadboard, so there's no shielding whatsoever, and because it's high gain, it picks up noise from the air and amplifies it.
HTH

[fentom_lord]

What's going on with the diode at the base of each transistor still stumps me.

In a similar thread of mine over at ssguitar, user Umlaut did some simulations and wrote:

"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"

Joe Davisson wrote on his website that they prevent hard saturation of the transistors. How that is accomplished is beyond me. I'm just a mere hobbyist with little knowledge of solid-state circuitry.

[printer2]

Some easy shielding is to put a metal sheet under the circuit and ground it to the circuit. Even a sheet of aluminum foil will help, I put an insulator over it, between the foil and breadboard. A thin sheet, even a pressed boar clipboard will work, as one example.

[fentom_lord]

Thanks for the hint, printer2,
That's a brilliant idea. I'll do it as soon as possible.

[Elijah-Baley]

With an extra gain stage it is really aggressive.
Do you want post the schematic, just for reference?

[fentom_lord]

Do you want post the schematic, just for reference?

No problem. I'll do it as soon as possible.

[fentom_lord]

Hi, Elijah-Baley, and everyone else who might be interested.
As requested, I'm posting a schematic of the guitar preamp from the 2nd recording in its current iteration. I'm thinking about adding some RC filtering like I would do with a high-gain tube preamp. You know, feeding 2 or 3 stages off a dedicated PSU node.



And here's Joe's design for comparison.
https://www.diystompboxes.com/analogalchemy/sch/v4preamp.html

I think this thing could take another gain stage if someone needed ultra-turbo-insane overdrive for some more extreme metal playing. I'll try that myself, but I'm out of space on my BB at the moment, so it's going to have to wait a while.

Enjoy,
M.

[antonis]

What's going on with the diode at the base of each transistor still stumps me.

Joe Davisson wrote on his website that they prevent hard saturation of the transistors. How that is accomplished is beyond me.

They act the same way as if they were placed between Bases and GND (AC)..
(they prevent BJTs from being driven too negative hence more close to distortion than fuzz..)

[marcelomd]

Hi, Elijah-Baley, and everyone else who might be interested.

Suggestion: Try a buffer after the volume control. Or at least after the tone stack, so it wont interact with whatever comes after.

[antonis]

Suggestion: Try a buffer after the volume control. Or at least after the tone stack, so it wont interact with whatever comes after.

Or move last stage buffer after Tonestack..

For perfectionists only:
(in case of buffer repositioning..)
Make last stage CE amp Collector resistor 10k and Emitter resistor 470R..
(retains bias and gain in the cost of current consumption..)

[fentom_lord]

Suggestion: Try a buffer after the volume control. Or at least after the tone stack, so it wont interact with whatever comes after.

Thanks for for the tip, Marcelo.
Initially, I wanted to put this preamp together just to see how it sounds because I wasn't able to find any samples of this particular design anywhere. Because, as Joe Davisson wrote on his Analog Alchemy website, it's based on the Vulcan circuit, I figured it would be similar in tone. I heard the bjt Vulcan on YouTube, I believe they were soggybag's videos btw, and although it sounded good, it's not a tone that I am after, so I wasn't expecting to use it myself. I decided to try it nonetheless and, boy, I was amazed! In fact, I liked it so much that I'm currently working on a switchable multi-channel preamp for my pedalboard with this circuit doing the dirty deeds. I just need a clean channel but that shouldn't be a problem. So, I was going to place an output buffer in the final build anyway.

(in case of buffer repositioning..)
Make last stage CE amp Collector resistor 10k and Emitter resistor 470R..
(retains bias and gain in the cost of current consumption..)

And feed the tone stack off the collector?

[antonis]

And feed the tone stack off the collector?

Yeaaappp..

Tonestack input DC level might be a V_BE (about 600mV) higher than previously (due to buffer absence) which, IMHO, shouldn't upset anything..

Of course, moved buffer should need its own bias configuration(*) (it's not directly biased from previous stage anymore..) and a DC blocking cap between Tonestack output and itself..

(*)IMHO, it should have to be of high impendance (piece of cake with extra one capacitor & one resistor) for not loading that high values Tonestack..

[fentom_lord]

Thanks a lot, Antonis. I think I understand.

[antonis]

Just an example of tone altering via a SPDT (ON-OFF-ON) switch..
(from metal to blusey..)