Joe Davisson's Vulcan preamp sound sample

[fentom_lord]

Yeah, changing coupling capacitors was among the first things I tried because the original circuit was too bass-heavy for my taste. I tried adding bright caps and lowering or even removing emitter bypass caps and that helped. Eventually, I found that just altering the input capacitor made a considerable difference, so I tried a few values and stayed with the one that gave me the best tone. I removed the bright caps and came back to the original coupling cap values of 22nF.

[Fancy Lime]

Great sounds from a fantastic concept. Maybe I can add a couple of my own observations from experimenting with the almighty Vulcan. This thing does not get the credit it deserves, which, ironically we all seem to agree on everytime it does pop up here.

1. You are using quite low hfe transistors. In my experience, this thing really shines with high hfe ones. The original suggested 2N5088 or 89, iirc. I tried those plus MPSA18 (same thing but even higher minimum hfe) and BC550C. They all sounded much better (aka gainier) to me than 2N2222 or 2N3904. Hfe does make a difference here and you may be able to get away with one less stage by moving to higher hfe transistors. Ymmv, as always. It's different, not objectively better, but well worth experimenting with.

2. The diodes make a difference, too. I liked BAT42 the best (about 1/3 of the voltage drop of a 1N4148). Ge diodes would definitely be the choice of the marketing department They'd call it a "Fully Analog Germanium Driven Tube Amp Emulator" or something. The choice of diode changes the bias a little and also the saturation behavior. Again, not better but worth exploring.

Cheers,
Andy

[fentom_lord]

Hi, Andy.

This thing does not get the credit it deserves, which, ironically we all seem to agree on everytime it does pop up here.

Hi, Andy. I read a lot of good opinions on the Vulcan, so I was surprised that there were no sound samples of the Vulcan-based preamp from Joe's site. That's why I decided to build, or at least breadboard it, myself.

As far as transistors and diodes are concerned, I did try it with BC550's but found the tone too hairy. Possibly, it's all a matter of adding some capacitance here and there, but, without that, I found the 337 tone more pleasing. As usual, YMMV.
I also tried using Schottky diodes, 1n5819 to be precise, but I spent too little time to develop an opinion.
Thanks for the hints.

[fentom_lord]

Some easy shielding is to put a metal sheet under the circuit and ground it to the circuit. Even a sheet of aluminum foil will help, I put an insulator over it, between the foil and breadboard. A thin sheet, even a pressed boar clipboard will work, as one example.

Today I put it on an empty chassis that I have lying around and tied the circuit ground to it. It's MUCH quieter now. Thanks a lot for the tip, printer2!