5670 (2C51) Bass preamplifier This is a very nice and simple bass preamp I've made several years ago. I'd recommend it to anyone looking for a fat bass with good tone shaping possibilities. Contents: Introduction Schematics Problems and mods Chassis

Introduction

The whole thing started as some sort of Alembic F-1 bass preamp design, which is basically Fender's preamp with Fender TMB tone stack. I'm not sure whom to credit for the original design. My preamp features one twist which makes it very different from both Alembic and Fender. I've got two parallel channels with different tone controls. One is regular TMB (Treble-Middle-Bass) and the other is just the volume. TMB channel gives it a glassy scooped Fender tone, and the simple channel, with the voulme only, delivers fat middle-heavy crunch. Running them both in parallel provides a very interesting effect. First, double signal path makes everything sound fatter, sort of chorus without actually chorus effect. Second, Fender TMB is very nice, but I miss the meat you get when you drop the EQ, so here I get an option to mix that meat back. Al the modern bass preamps usually have some sort of EQ which drops the volume and scoops the middles, and that is the "modern" bass tone. So add the "vintage" overdriven tone from the other channel - and you can dial anything you want, from nu-metal to early 60's.

The last twist is the tube used. I've bought a pack of NOS General Electric 5670/2C51 tubes for $1 each, not really knowing what they are. As it turned out, these were some 5-star military-grade double triodes. They are used in some expensive hi-fi audio designs such as Millenia preamplifiers. These tubes are way better than 12AX7's as they have an awesome frequency responce. I've had no chance to compare them side by side with Telefunken ECC83 or 6SN7, but trust me - they're excellent. 5670 aka 2C51 is not interchangable with 12AX7 since it has a different pinout. The circuit values are suited for 5670's, but you can actually use 12AX7's with lower cathode and anode values, s.a. 100K load and 1.5K cathode. Today NOS 5670's are still cheaper that NOS 12AX7, so I encourage you to try them.

Schematics

5670 bass preamp schematics - PDF file, 458KB
5670 tube datasheet - PDF file, 271KB



So, you can see two parallel channels with common input and output. The power supply utilizes 3 small 1A 12V transformers. Two wired back to back to get a hi voltage and one is for heaters only. I guess it is possible to have one 4A transformer for both heaters and hi voltage with one smaller tranny to reverse the 12VAC back to 230V. Of course, you can have a separate heaters' transformer with 6.3V out or use a proper transformer with 6.3VAC and 250VAC outputs, I wired my preamp this way only because these were the transformers i've had at the moment. The ratings for the proper power supply transformer should be like 6.3VAC@0.7A and 250VAC@30uA. I guess any transformer from an old tube patiphone, tape recorder or preamp should fit.

Problems and mods

The TMB tone stack is a classic Fender affair and you can change it any way you like, there are hundreds of pages around the www about this subject. Pay attention to the volume potentiometers' values. Anything above 100K will result in wild squealing oscillations, so keep it low. Channel 1 is considerably louder that channel 2, where lots of gain is destroyed by the tone stack. I've got 50K volume and larger grid resistor on channel 1 to compensate for that.

At the beginning, I've made a switch to be able to run two channels in series. Like, four gain stages. As you may imagine, this was way too much - super saturated distortion and squealing oscillations. So I've kept it parallel only, and even this way you can have a nice distortion at maximum volume positions. If you don't want distortion, you can drop the master volume, it works fine without one.

Here's an original schematics of the circuitry -

5670 preamp - not final

It features the parallel/series switching and some different resistor values. I put it here just in case that if you wish to modify the final schematics you can see which directions my improvements went.

Chassis

I've bought a red steel box with a lock and used it for chassis. You can see the lock is still there - it must be hard to remove it, I haven't even tried. It's a very sturdy enclosure and I can recommend this type of lunch boxes. The steel is not too thick to cut through, but it's still durable and the finish looks good.



That's all, folks!