15 years ago - Vulcan

[rankot]

I just tried to add some small caps in parallel with diodes and it brought no change, but I also noticed that my noise's pitch is actually not so high - it is somewhere around 90Hz according to my oscilloscope. And that noise is absent when guitar volume pot is low or at zero.

Also, voltage at collector of all three trannies is at 5.8V with default values for passive parts. All are 2N5088 with hFE around 350.

[Fancy Lime]

Hi there,

revised version:



I am now using BC550C transistors, which are usually easier to get than MPSA18 and I cannot hear a difference. Another reason for doing that is the tighter hFE specs on the BC550C, which makes consistent biasing easier. 2N5089 are a very close equivalent to BC550C and probably easier to get in some places but has slightly wider hFE limits. For the switching I went with BC337-40, although any BC337 should work. 2N4401 works as well and 2N2222 should do also. Probably even 2N3904. I changed the switching slow-down cap for Q4 (C24) to a smaller value after figuring out that the switching of Q2, which is then amplified by the gain circuitry, contributes to the switching pop. It is better now but not quite gone completely. Will keep on experimenting.

Cheers,
Andy

[soggybag]

Ooh it's sounding pretty good now. I just built up one of the boards from OSH Park. I made one small mistake, the ground on the volume pot wasn't connected to anything. That was easily fixed. I used the schematic on the first page of this thread.

I used 2n5088 transistors with gains of 430 hfe. I get about 6.3v at the collector at each. Which seems pretty good. If I understand it correctly we want to bias at about 3/4 the power supply.

At times I here a faint high high pitched oscillation. It comes and goes. Could be the power supply, I'm using the One Spot and there is no power filtering. Could something else this thing has a lot of gain!

[Crater Dylan]

I remember that "cutting ac feedback" from an escobedo schem. I still don't quite get it. The feedback resistor is divided by the miller effect so it appears smaller, does splitting it in two with a cap to ground make matters better, or worse?

Voltage divider bias is less variable because instead of having the base voltage determined just by base current and collector voltage (and the negative feedback between the two with beta), you have a somewhat stable base voltage to work with (still influenced by base current depending on the divider values).

I'm interested to know more about this building block, I've seen the vulcan before but haven't heard it. The diode is obviously throwing half of the wave in the trash, then that gets inverted and decoupled and the process is repeated. I guess there's no crossover distortion because there's only one diode, so no "coring out", just clipping and losing everything above -0,6V?

Anyway great thread everyone. Sorry for not quoting properly the messages I'm answering to.

[soggybag]

Version 2 of my PCB. I added power filtering and wiring for the LED.

[Ben Lyman]

Great circuit, almost 4 years ago I added a Rat filter knob and called it good. Here's 8 pages of material, including perf and Filipe's PCB layout plus a shitty 10 minute iPhone demo!
https://www.diystompboxes.com/smfforum/index.php?topic=116171.0

[soggybag]

Finally got around to building up these boards from OSHPark and confirming that they work. I made the project public. If you want to build a Vulcan, you can order boards from OSHPark, looks like $13 for 3 shipped.

https://oshpark.com/shared_projects/UgsLvqqP

[soggybag]

Picture of the latest build. Built and tested!

[soggybag]

Here's a link to a build doc:

https://docs.google.com/document/d/1RZGN90AS7-ltEiKo_XSCzGfeI8s1LfxNiioncr-u75I

[Steben]

Looks like a great preamp building block lay out for a SS amp....

[soggybag]

This sounds so good I had to make another! Some build process pics.

[bms19]

Considering doing it with BC108A, just for the look mainly ;-)
Ben