Fetzer Valve build report

[piano boy]

I have a marantz 2220 solid state stereo amp that had been thru a flood and only had one channel working. I wanted to use it to play my acoustic/electric guitar into a homebuilt cabinet at work or wherever. But when I plugged up the guitar to an aux input, I got very little volume.

Today I built a fetzer valve with a MPF102 on perfboard as a preamp. It does wonderful, giving me a very clean but not edgy sound into the marantz. I do have a lot of low frequency noise, but I believe that's because I don't have the FV in a shielded box. It sounds like 60 cycle hum.

I had some trouble figuring out how to measure 4.5 at drain. I thought I had a bad trimmer, and then maybe a too small trimmer, because I was getting a full 9 volts at the drain. Yet I was measuring a trimmer resistance of 80k to 0k as I moved the trimmer, so it was doing its job. I knew there had to be a voltage drop across the trimmer, and that it shouldn't be the whole 9 volts, so I had to be measuring the voltage wrong. Finally I connected meter ground to the ground leg of the 22uf cap, and started getting some voltage drops that made a little sense. From there I could trim to 4.5 volts; the source read in at 1.2 volts, and the gate at zero.

On first guitar use I got very little signal out (I was disappointed). After a lot of connection tracing, I decided to work with the last cap before the output. I had socketted it so I could mess with voicing as called out in the ROG build notes. I shorted this out, and for about 10 seconds had a lot of guitar...and then a lot of squeeling. I decided to check what I had in there against schematic value of 22n. I had a cap marked 473...oops. I checked the markings of some 22nf caps I just bought from Radio Shack, they were marked 223. A quick swap and the FV came alive!

I put the FV in front of a little gem next. It greatly increased the output volume, without introducing a lot of distortion except at the highest settings. The lil gem without the FV had a more "ringing" sound, chords seem to sustain longer. Not sure why that is...

Next I put the FV in front of a fender bronco tube amp. This bronco is really a vibrochamp hiding under another name. The hum and noise from the FV was just too much to use, going to have to work that out before I can do anything with this pair.

I'm really pleased with this circuit for what I built it for...a preamp for my acoustic guitar. Thanks to ROG for the clear layout and instructions.

[Peter Snowberg]

Very cool!

Congratulations are in order. Thanks for the report too!

[bwanasonic]

The Fetzer is a really cool and useful circuit. It should aslo be pretty quiet and hum free, so maybe boxing it will help that. Also your description of the FV's seeming to lessen sustain with the Little Gem made me wonder if you should try a J201. If you haven't tried it yet, try the AMZ Mosfet boost. It's properties might lend themselves well to your purposes, and it combines well with the Fetzer.

Kerry M