Joe Davisson's Vulcan preamp

[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.

[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]

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.

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]

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]

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.

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.

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.

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]

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.

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.

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.

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