Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - Miyagi_83

#1
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.
#2
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.
#3
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.
#4
Thanks a lot for the reply, Loudthud.
#5
Hello, everyone.
First of all, I'm sorry if this question has been asked and dealt with before. I have done some searching, but haven't been able to figure out a straightforward answer. I'm new to solid state amplification. Now, to the point.
I've been looking at Rod Elliott's 40W power amp design and I don't know which components should be matched. I started reading teemuk's book, but for now it's quite a bit to digest, so I'm taking my time.
So, should I look for matched pairs of any devices found here?
The whole article:
https://sound-au.com/project215-p27-revisit.htm

Circuit diagram:
https://sound-au.com/p215-p27-revisit-f6.gif

Thanks for any input.
M.
#6
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.
#7
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.
#8
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.
#9
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...
#10
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.
#11
Tubes and Hybrids / Re: Mesa Dual Rectifier
May 22, 2024, 06:58:30 PM
Hey, Psabin8951,
I'm no expert, but a couple of things popped in my mind.
Which fuses does it blow - primary or secondary?
As I understand Mesa Dual Rectifiers, it should have a rectifier selector switch. Does the amp do that with solid-state rectifier selected?
Could it be a short on the primary side of the power transformer? 🤔
Like DrGonz78 said, a schematic and voltage readings would help.

You need to be super careful and you must know what you're doing if you ever decide to tinker with it yourself. As you probably know, there are lethal voltages inside that thing. If uncertain, take it to a tech.
#12
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.
#13
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.
#14
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.
#15
Quote from: Umlaut on May 21, 2024, 03:18:58 AMGK schematics are an extremely useful resource on how to bias and design jFet gain stages, btw. Most if not all of their guitar amps from the late 70s on (and a lot of their bass amps) feature jfets in one form or another.

So much to learn, so little time...  :)