Friday, March 27, 2015

Idea for a clean preamp

EDIT April 2, 2015 9:38 AM
Updated version. Thanks for the feedback received so far!!!

Would love any feedback on this idea I had...

I was reading through "The Ultimate Tone Volume 5" by Kevin O'Connor and was intrigued by some of his ideas for three-stage clean preamps. Build it up, tear it down, build it up, tear it down, build it up, tear it down. Each build-up adds harmonics, each stage has different values for anode and cathode resistors, the third stage is a different tube than the first two. Each stage should add it's own unique character to the tone.

I was also reading a thread about tonestacks on and Merlin suggested a Bandmaster for treble and bass coupled with a Bridged-T for mids. So I threw those in there too to try out something different.

EDIT May25, 2015 8:40 PM
I ordered some of Merlin Blencowe's (The Valve Wizard) B9A Development Boards and decided they would make a great platform for prototyping this design. Here is how they turned out:

I really like how easy it is to do a layout and swap out components quickly and easily! I printed a full-sized image of the board on my laser-printer and penciled-in the components to plan my layouts and then made minor tweaks as I placed the components on the boards. In the top picture, the preamp's 12AU7 stages and Bandmaster tone stack are on the board on the right. The 12AX7 preamp's stage 3 and cathode follower with T-Bridge mid control and volume knob are on the left board. (You can click on the pictures to make them larger.)

I think these boards are very nicely thought-out and implemented. I love how you can solder either side so that changes can be made without ripping the boards out of your chassis. However, I feel that they are a little large (10 cm square) if your intention is to make these into a permanent install. For a simple one-channel amp like this, they would be just fine. But remember, you still have to fit your power supply and output tubes in there. If you are going to do a multi-channel amp with reverb and effects these are not very practical for permanent installation. However, I don't think that was the original design intent for these anyways. I think they were money VERY well spent for fast prototyping.

Those large 47uF/450V F&T filter caps will eventually be replaced by radial lead caps that will fit the boards a bit better. I just had those on-hand for testing. You can see I left the leads for them long on the bottom of the board so I can use them on another build someday.

Up next, wiring it into a power-amp... some time later this summer I hope! I am getting anxious to see how this will sound.

1 comment:

  1. Fixed a pretty piece of misinformation about what I put on the development boards... silly me... The left board of the top picture holds stage 3 and the cathode follower. there is no LTPI in there anywhere like I originally stated.