Little Box Full of Smoking-Hot Soul

I finished my latest amp.

This thing is called the Wingman. It’s a Fender Bassman preamp coupled with the output section from an amp called the Little Wing.

The Little Wing was created by an Internet forum dude who calls himself “Da Geezer.” It’s supposed to be a sort of small Bassman. It has two 6BM8 output tubes, and it dissipates 7 watts. I wanted more juice, so I built an amp with 4 tubes and two output transformers. Why two transformers? Because I failed to take the impedance into account. Going from two tubes to four, I messed things up, and by the time I knew it, I already had one output transformer on hand.

I mounted the transformers incorrectly. I got too close to the work, and by the time I pulled back and saw that the laminates on on OT were co-planar with the power transformer laminates, the holes were drilled. But there is a test you can do to determine whether this does any harm. You connect the PT to 120, and you hook a speaker up to the OTs. If there’s a problem, you will hear hum. I didn’t hear anything, so I knew I was okay.

I wanted to put a post-phase inverter master volume (PPIMV) on it. This is a potentiometer that allows you to reduce the signal from the coupling caps to the output tubes. It lets you run the preamp at full blast, without sending a huge signal to the output. In other words, you get the nice sound of a preamp working hard, without the incredible loudness it ordinarily causes.

Because I had four output tubes, I ended up with two PPIMV pots. This is probably unnecessary, but I wasn’t sure, so I didn’t take a chance. It seemed to work fine when I ran everything through one pot, but pots are cheap, so I bought two.

I machined the chassis out of 6-inch aluminum channel. I used the mill to make the holes. While I was doing all this, I realized I needed end mills exactly the right size for tube sockets, and I managed to find some on Ebay. Next time, the tube holes will be a wonder to behold.

I put the amp together, and it barely ran. I got oscillation and low volume. I fought with it for days. Finally I realized I had the output tubes wired up wrong. Each coupling cap has to go to both OTs, and I had it rigged up the other way.

When I fixed that, I found that the bass had a sound sort of like a synthesizer. I went through the amp, adjusting voltages. The Bassman uses a 355-0-355 PT, and this amp has a 275-0-275 job, so the preamp voltages were pretty low until I got rid of the Bassman-sized resistors.

I also had to adjust the cathode resistor on the output tubes, since I was running four tubes into one resistor. The Little Wing has one resistor for two tubes.

Even with all this work, the amp did not make me happy. The bass still sounded off, and the amp did not have the clarity I wanted, also, when I cleaned up the wiring, I got noise, including heater hum.

People said I needed a bigger PT, so I ordered one. The 6BM8 tubes suck a ridiculous amount of current.

I went back to the drawing board, and I reviewed some stuff I had learned about heater wiring.

The heaters on amp tubes run at 3.15 VAC. They generate 60 Hz radio waves. If you don’t run the wiring corrrectly, the amp picks up the waves, and you get hum. A well-known guru named Merlin Blencowe says you can’t have heater-wire loops around the tubes, which is the easiest way to do it. With 12AX_ tubes like the ones I used in my preamp, that means you have to run the wires across the tube bases. This is what I did. Stupidly, I would guess. A lot of people don’t bother, and their amps don’t hum.

It turned out I had gotten too close to the work again. I had made an obvious mistake. There are jumpers and resistors across the preamp tubes on this amp, and if you use heater loops, you can put the jumpers and resistors right down on the socket. This is what I did. Unfortunately, the heater wires were there. So I had a resistor lying ON a heater wire, and two jumpers running very close to other heater wires.

So I fixed that.

Strangely, it improved the amp in ways beyond the hum. Now it’s clear. The high-frequency noise is gone. It’s beautiful. Like a little Bassman, with less low-end grunt.

With the PPIMV, you can get all sorts of singing distortion. If you were quick enough, you would be able to play this thing without a distortion pedal. You can crank the volume and turn down the PPIMV to get loads of distortion with no volume increase.

It’s a very bluesy-sounding amp. Exactly what I wanted. It’s hard to stop playing it. It impressed me so much, I went back and stuck a PPIMV in the Bassman.

I’m not going to need the new PT, so I may use it to make a four-EL84 version of this amp.

Here’s the latest photo. I might actually put a cabinet on it. I got the normal and bright volume labels mixed up.

I may let my new church use this one. They have a cruddy old solid state Crate. I was talking to one of my friends–the new head deacon–and he said they might actually want to use me as a guitarist. That would be phenomenal. Lead is out of the question right now, but I might be able to do rhythm in a month or so.

I love the new amp so much, I’ve been working up arrangements and practicing more. It’s great for any tune that brings out the sound of the amp. I worked on “Secret Agent Man” today. Lots of grit and reverb. It may be corny, but I like it.

We’ll see where the amp business goes from here. I can’t wait to get started on the next one.