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Joe Davisson's Vulcan preamp

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

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joecool85

For those looking for the link to the schematic: https://www.diystompboxes.com/analogalchemy/sch/vulcan.html

I'm curious what the diodes are for between the stages though.  Not sure I've seen that before.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Miyagi_83

Hi, joecool85.
Sorry about not posting the schematic or link. I should have done it in my first post. My bad.
Actually, it's this one
https://www.diystompboxes.com/analogalchemy/sch/v4preamp.html
The one you provided shows the 3-stage bjt Vulcan overdrive pedal and they sound completely different.
As for the diodes, I used 1N4148.

Miyagi_83

Update

So, I have been tweaking the circuit for the past couple of days and I think I've got it where I like it. Actually, there are a few versions that I'm fond of, so I'm going to make it a switchable two or three-channel preamp. Honestly, this design has enormous potential, afaic. I haven't tried it on 18V yet. That's still on my to-do list.
Now I'm going to need a nice clean channel to pair with it. Any suggestions are welcome.

Is it OK to post a schematic with my changes? On his site, Joe Davisson put the 'do not copy / all rights reserved' clause, so I wouldn't like to infringe his copyrights, annoy him or anything like that, hence my question.

edvard

Quote from: joecool85 on May 27, 2024, 08:56:13 PMFor those looking for the link to the schematic: https://www.diystompboxes.com/analogalchemy/sch/vulcan.html

I'm curious what the diodes are for between the stages though.  Not sure I've seen that before.

Per the description below the schematic; "The diodes in the bias network prevent hard-saturation of the transistors."

I won't pretend I'm smart enough to know how that works, but it sounds to be in the same vein of putting a diode across base & emitter to prevent zener breakdown events that degrade the junction and cause noise to build up over time.
See here: https://www.diystompboxes.com/smfforum/index.php?topic=132039.0

I wonder if those two arrangements might be considered general good practices for BJT-based preamp circuits?
 

Loudthud

#19
Generally, to prevent hard saturation of transistors (the kind that takes significant time to undo) you install a Schottky or Germanium diode between Base and Collector. I question whether it makes an audible difference in an audio circuit. The way this guy has installed the diodes, it makes sure any big negative input is able to degrade the Base-Emitter junction. 

edvard

Quote from: Miyagi_83 on May 29, 2024, 09:13:26 AMUpdate

So, I have been tweaking the circuit for the past couple of days and I think I've got it where I like it. Actually, there are a few versions that I'm fond of, so I'm going to make it a switchable two or three-channel preamp. Honestly, this design has enormous potential, afaic. I haven't tried it on 18V yet. That's still on my to-do list.
Now I'm going to need a nice clean channel to pair with it. Any suggestions are welcome.

I've asked before on other forums that if there were such thing as a "clean channel" pedal, what would it consist of, and most folks suggested either completely dry, like plugging into the power section with a tone control, or light-to-medium compression with no clipping.

QuoteIs it OK to post a schematic with my changes? On his site, Joe Davisson put the 'do not copy / all rights reserved' clause, so I wouldn't like to infringe his copyrights, annoy him or anything like that, hence my question.

He hangs mostly at diystompboxes.com, maybe pop in there and send him a personal message.  At the very least, I'd say draw your own schematic and highlight the differences, so you won't be straight-up copying.  Maybe include "modified from Joe Davisson's Vulcan" and a link to his page.
IANAL, so I'd ask him about it first.

Miyagi_83

#21
Quote from: edvard on June 01, 2024, 10:45:18 PMI've asked before on other forums that if there were such thing as a "clean channel" pedal, what would it consist of, and most folks suggested either completely dry, like plugging into the power section with a tone control, or light-to-medium compression with no clipping.


Thanks for the tip. There are a couple of ideas I've been looking at, like the Fetzer Valve with a tone stack, Rod Elliott's preamp, or Umlaut's Keelhauler. I'll start a new thread once I've found what I'm looking for.

QuoteHe hangs mostly at diystompboxes.com, maybe pop in there and send him a personal message.  At the very least, I'd say draw your own schematic and highlight the differences, so you won't be straight-up copying.  Maybe include "modified from Joe Davisson's Vulcan" and a link to his page.
IANAL, so I'd ask him about it first.


I sent him a PM this morning. We'll see what he says.

Miyagi_83

Here's a schematic of Joe's preamp tweaked to my liking. It's the way it is on my breadboard at the moment. To date, I have tried a few different combinations and this is one of my favorites.
Here's the original design
https://www.diystompboxes.com/analogalchemy/sch/v4preamp.html

As you can see, aside from different transistors and their collector resistors, I ditched two emitter bypass caps (3rd and 4th stages) and changed the values of the other two (1st and 2nd stages). I added bright caps here and there and changed tone stack capacitors. Also the TS potentiometers are higher value because these are the only ones I have. Oh, and my mid pot is 50k.
Many thanks to Joe Davisson for sharing his idea with the interwebs.
M.

Miyagi_83

Another update.
I have tried running this circuit at 18V. The tone is a little bit different, definitely brighter. It has a tad more headroom, although that might be my perception only. It puts out more volume too. Not by a lot, but noticeably more.
Note that I didn't bother rebiasing anything.
I also tried using BC550's. With these transistors installed, there's more juice on tap, more hair. I think I like it better with the 337's.
There's another thing I'm going to try soon, namely adding another gain stage. I'll keep you guys posted.

Miyagi_83

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

EDIT: Again, I'd like to apologize for the sloppy playing.

joecool85

Quote from: Miyagi_83 on June 12, 2024, 04:57:39 AMI 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

EDIT: Again, I'd like to apologize for the sloppy playing.

Sounds great!  Thanks for sharing!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Miyagi_83

#26
No problem. I breadboarded it to see how it sounds and shared my findings for others to have a point of reference because I had trouble finding any. Maybe my recordings will not get lost over time.
NB: I plugged the output of the preamp directly into the effects loop return of my DIY tube amp for this recording. Later I ran it through a buffer (a bypassed Boss DS-1) and it sounds different than in the sound sample.
EDIT: Of course, I can post the schematic if anyone's interested.

Miyagi_83

Here's the schematic for the iteration presented in reply #24. I plugged the output straight into the high-impedance input of a tube amp. The cable's capacitance is 530 pF according to my DMM.