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Joe Davisson's Vulcan preamp

Started by Miyagi_83, May 12, 2024, 04:45:06 PM

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Miyagi_83

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.

Umlaut

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!

Miyagi_83

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?

Umlaut

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 :)

Miyagi_83

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

Umlaut

Well I breadboarded it early this week out of curiosity. A few observations:
- 18V instead of 9 does not make much difference.
- bias is finicky, get as close to 4.5V as possible (using 2 resistors in series or a trimmer).
- while the voicing of this circuit is not exactly to my liking, it is undeniably well designed: tried other coupling caps, bypass caps and so on, and it quicky went south. Only tried stuff for about 20min, though.
- snubber caps collector-to-base -> Nope. A 2.2n cap across the last collector bias resistor worked MUCH better to tame the highs.
- somebody at DIYStompboxes suggested using other diodes. I tried BAT85 and it sounded pretty good.

I will have to revisit it at some point during summer, as it definitely has a nice vibe. Hope this helps!

Miyagi_83

Hey, Umlaut.
It's great that you've been able to breadboard and play it. Thanks a lot for all your input. It's going to save me, and possibly others, some time.
My parts came in this afternoon and I'm looking forward to breadboarding it myself, but it's going to have to wait a bit. Hopefully, no longer than two weeks.
Oh, and I found some BC550's the other day. I might give them a whirl too.

I also have a couple of questions:
1. Did you use the 2N5089 transistors?
2. Now that you have some hands-on (and ears-on) experience, how to would you describe the voicing of this preamp?

M.

Umlaut

Yes, I did use 2N5089. I tried 2N3904 but it did not work as well :P
BC550 should give you a similar result to 89s though.
Bear in mind that I tinkered with it for less than 1h, so it was a bit of a summary test.
As for the voicing, I'd say marshall-ish with a raw, unfiltered edge to it. Into the right amp it can sound quite nice (think Black Sabbath, Pentagram, etc), but I prefer a bit more filtering and tone-shaping. While it sounds quite full and "amp-like" (not a term that I like, but...), and it has a nicer feel than your standard boss disto-box, I would still be partial to other multi-stage pedals like the AMT Legend ones, for example (check on FSB for schematics).
Oh, I tested it on the clean channel of my Peavey Bandit silver stripe, low input and bright switch off, which is an amazing pedal platform IMO.
Yesterday I was fiddling with a distortion circuit of mine which is quite similar in its topology and voicing, but using LND150 depletion mosfets. To my ears, they sound much more pleasant than BJTs in this kind of cascading gain stage circuits. I will probably post it at some point in the coming days, as I am pleased enough with it to share it  :)
Again, breadboard and see for yourself! lotsa fun can be had with this circuit, whether you tinker with the values or not.

Miyagi_83

Thanks a lot. I really appreciate your help and support. I'll keep you posted on my progress.
M.

Miyagi_83

#9
So, I did some breadboarding today to find collector resistors for my BC337's. I've found a set of 4 that bias very close to 4.5 VDC with a 22k in the collector.
I didn't bother to look for the 5th one because it's an emitter follower. Should I do it? Also, should I increase the value of the buffer stage's emitter resistor from 10k to 22k?
Next step is to put the whole thing together on the breadboard.

Umlaut

If you are talking about the last bjt, emitter follower indeed, I wouldn't go above 10k unless I wanted to purposefully introduce "cathode follower"-ish distortion.
FWIW I breadboarded the 3-stage version of this circuit, not the 4-stage one, and there was plenty of gain and still some wiggle room to push it with a TS without turning into white noise. YMMV :)
Oh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

Miyagi_83

Yeah, I meant the last stage, the emitter follower. I'll leave the load resistor at 10k then.
Speaking of the 3-stage BJT version, I kinda know how that thing sounds because there is a video of it on YouTube and, indeed, it does display the quality you described earlier. Still, I'd like to (and am going to) try the 4-stage preamp because I'm curious what it can do. There are no clips that I was able to find so I might become the first one to upload some :D
I started putting the circuit together on my breadboard, but I needed to take it apart halfway through because a lot of the connectors were overly loose, so I figured I'd retension them. Should have checked that first...
I might have some time to get back to working on it this weekend.

Miyagi_83

Quote from: Umlaut on May 22, 2024, 04:18:37 PMOh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

The 337's I tried today were very consistent, or maybe that's just my limited experience. Out of the 15-ish pieces I tested, one biased at 4.44 V, the five I selected are between 4.51 and 4.55 V, and the rest gave a reading of around 4.6 V. All that with 1k emitter resistor, 22k collector resistor, and the 4.7M / 1N4148 / 1M network connected to the base, as per Joe Davisson's schematic.

Miyagi_83

Ok, so here's an update.

I've put it together on my breadboard and performed a sound test. It is VERY promising. I might tweak the tone stack a tad, but, man, does this thing sound powerful!! Perhaps not an uber-extreme-downtuned-modern-metal kind of powerful but great nonetheless. Great job, Mr. Davisson!
I've made a quick recording at a baby-sleeping-in-the-next-room volume. I'll upload it later, on SoundCloud perhaps, and post the link here.

Miyagi_83

#14
EDIT: the link posted previously was strange. To say the least. Now it should be fine
https://m.soundcloud.com/user-996907905/joe_davisson_vulcan_preamp_bc3

Ok, here it is.
Guitar: LTD MH 50 with pickups from Dean Vendetta 2
Amp: single-ended EL91 Fender Champ-a-like
Speakers: some 1980s 12" marked Celetion (sic!)
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 of 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 caps have been scaled accordingly.
Enjoy.
Oh, and please forgive my sloppy playing 😅 I don't play as much as I used to...