PDF
Electronics
|
|
|
|
Top Drive: A New 12AU7 Pre-amp Design Another 12AU7 pre-amp, a successor to "The Hood"! What makes this little tube powered pre-amp different though? Well for one it's voiced in a totally different way. The pre-amp sounds very much like a Vox/Dumble amp. Its got that great top end "jangley" character with that nice tight bottom end. It has great clarity and character all the way up the volume control. When the volume control is maxed out there is great "singing" distortion. "Top Drive" is very much like an old Vox (AC30 for example) in the sense that is has a single volume control. No master (gain) control, this means you need to crank up the volume for that great tube saturation. There is something about a non-master volume amp that just makes it special and sound different. Along with the volume control there is also a three band equalizer consisting of treble, middle and bass controls. I mentioned Dumble though, yes Dumble. What makes "Top Drive" unique is it has a footswitch-able gain stage in the form of a FET boost. With the high impedence FET booster driving the front end of the pre-amp you get tons of added gain, saturation and sustain. The distortion and break up are incredible, its like a pushed over the top Vox mixed with a smooth overdriven Dumble. The FET boost stage also has a volume control. Now this means you now have a volume control at the beginning of the circuit and the normal one at the end. So when you switch on the FET boost stage you now have a master volume amp! You can now dial in the perfect amount of gain without having to crank the volume. As well as the volume control for the FET boost stage there is a voicing control. The voicing control affects the FET's source bypass cap. A switch allows you to select bright, flat or fat boosted gain tones. This is all made possible by putting different RC filters in parallel to the FET's source resistor. What's nice about the bright control is it stays bright and clear all the way through the volume control's sweep, it's not like an old Fender bright control where the bright affect faded as the volume increased. Even more tones are added. A new power supply has been added which is a step up from "The Hood's". It provides a higher voltage and has more filtering. This means more output, clearer more defined tone and tight bass. The bypass switching is also got more complex and interesting. I have now designed a two switch external switching box using relays. It lets you bypass "Top Drive" totally in a true bypass manner. It also allows you to turn on and off the FET boost stage. This is really useful as depending on how you set the FET volume and normal volume the footswitch can then act as a clean/dirty channel switch. The switching system even allows one to use just the FET boost by itself. Both switches have activate LEDs mounted on the front of the pre-amp to tell you when which switch is activated.
Schematics:
Design, Build, Win? Completo! After a few weeks of building and debugging the "Top Drive" preamp is complete. This build went quite smooth, just took a long time. It was built in stages starting with the power supply. No issues with it worked fine straight away. The PSU is better designed than "The Hood's". It runs smoother, is more efficient and doesn't dissipate a ton of heat. Next I built the tube signal path. This also went smoothly. Only issue I was some 120Hz humming from the PSU. Some decoupling from a small resistor and some added capacitor filtering later and the problem was solved. The relay board then was wired, with it came lots of issues. Spent quite some time trying to find what was wrong with it. Eventually found a few tiny errors in the layout I had done for relays. Once a few traces were cut in my vero board and a few added wires later and the external relay switching worked flawlessly. Finally I hooked up the FET boost/gain stage. It went in simply and quickly. Biased the drain of the JFET up and was set. Wiring....hey when you have to fit so much into such a small space it's not easy!
The Sounds: First thoughts were "damn this thing has no background noise". Yep hardly any background, white noise, floor noise, hum whatever you want to call it. Anyways the tone is incredible. It is very tight sounding but with a nice amount of highs and clarity on the top end. With just the tube signal path (no FET boost) the tone can range from clean to great tube distortion. When the volume controls hits 2 o'clock the unit starts to break up and this great natural distortion hits you. It is not overly heavy, just the right amount of gain. I would describe the max gain as a early 60's British Vox or Marshall that is cranked. All the while the tone remains tight and clear. Even if the tone isn't spot on it can be tweaked by the simple but very effect three band EQ. The treble has a range from subtle and smooth to bright and very clear. The middle control offers a wide variety of tones. With the mids scooped the tone is clear and quite "twangy". Once you roll it up though the sound thickens up and has more "meat" and definition to it. Once the mid control is at about 3 o'clock the sound gets very fat and spongy!!! The bass control allows one to fine tweak the sound even more, it is smooth control that can really add depth. With the FET stage engaged things let loose! With the FET gain rolled up to about 2 o'clock there is a serious increase in gain. The sound becomes bigger chunkier and there is a lot more gain. Now with the voicing switch set flat the frequency response is well flat, nothing is boost. On the flat setting there is a little less gain, it's nice for just a pure volume boost without much coloration. This helps retain the character of the tube circuitry. With the voicing control set to fat boosted gain there is even more gain than the flat setting. The tone also becomes even bigger and the bass is boosted, the bottom end really stands out. The sounding is very tight and raw, it provides a great Texas blues type of tone. Finally with the voicing control set to bright well the sound is bright! Very clear and articulate, not piecing or shrill at all. It sounds pretty good with the bright on and some gain, kinda like a 70s Hiwatt. It sounds even better if the preamp is set clean, resembles more of a Blackface style amp. The FET booster can also be used all by itself. This means you can bypass the tube circuit with the footswitch and then turn on the FET boost with the footswitch so it's on all by itself, kind of a neat feature.
Overall a very cool unit. By far the best thing I have ever designed or built to this point. Has a wide range of tones and really sounds great when pushed hard. Soundclips: Ok I am not a recording tech an no guitar wizard. I managed to somehow
increase floor noise. Playing isn't the best but your listening to the tone
right? Haha | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
.jpg)
