2W Trainwreck Express
Idea to build this amp came after watching an amazing YouTube video of Glen Kuykendall with his Trainwreck. I don’t need the power of original Express, so I went with a 2W version as posted on AX84.com.
I already had some NOS Raytheon 12SN7GTA tubes, so it made sense to use them and use 12.6V heater voltage for all tubes. Preamp tubes are JJ ECC83s and for phase inverter I got an Electro Harmonix 12AY7. Filter caps are all JJ (one axial and two cans), cathode bypass caps are Sprague Atoms and coupling caps are ERO/Roederstein MKT1813 tubular film caps. Caps in the pf range are silver mica and 1nF ceramic cap is MuRata. Signal resistors are all metal film, mix of nice brown Dale, few Xicon and Beyschlag. Power supply resistors are metal oxide. All plate resistors are rated 2W for lower noise. Output transformer is a 5WPP model from Musical Power Supplies. It’s supposed to have better bass response for hi-impedance tubes than Hammonds due to higher inductance. Power transformer is custom wound (as usual) by Trafomatic. It has a 250-0-250@70mA HV secondary and 12.6V@1.3A heater secondary and is wound on beefier 80VA core. All rectifier diodes are hefty ultra fast UF5408.
- 12SN7GTA output tube, should perform the same sonically;
- 12.6V heater operation. Preamp tubes heaters are wired to pins 4 and 5 with pin 9 left unconnected. Heater elevation voltage is applied to pin 9 of the PI tube;
- two 2x32uF can caps instead of a 47x20x20x20uF can cap (didn’t have those on stock when I ordered parts);
- Ferrite bead on the input lead, should filter out RF;
- 33uF 160V Panasonic radial caps for heater elevation and bias supply instead of 10uF, shouldn’t change anything.
- 25uF cathode bypass caps instead of 22uF. For electrolytic caps those are within tolerance range.
I like the chassis used on my Black Box Pre. Although it’s not particularly good looking, it’s very nice to work with and allows for each panel to be removed separately. I got the same box, just a bit taller for this project. Tubes, transformers and circuit board are mounted on a metal panel that’s attached to the chassis from inside. That way, you can only see jacks and knobs from the outside, top panel needs to be removed to access tubes and trannies and bottom panel covers the circuit board. That makes a compact build that’s easy to carry around which is important to me. Layout is pretty much the same as in the project file with a couple of changes that made sense to me since this chassis is smaller. Output jack and impedance switch are moved to the front of the chassis, fused IEC socket, power switch and indicator light are on the back to keep AC as far as possible from signal leads. Those ERO coupling caps have a line that (hopefully) marks the outer foil end. I connected it to the lower impedance (input) side to help with noise.
Heaters are twisted tightly together using a hand drill. It’s a great way to twist wires nice and uniformly. Just take two straight pieces of wire, twist them a couple of times and mount the twisted end into a drill head. Have someone hold the ends of the wires and use the slowest speed on the drill. Also, having 12.6V heaters means that current draw is halved compared to the original, so radiation from heater leads should be further reduced.
Preamp tubes’ middle pins are tied together and grounded to the closest chassis bolt. Since the middle pin goes all the way through the socket, having it grounded may help with shielding between two triodes.
Can capacitors are mounted using horizontal claws. This is a great way to spare the builder from drilling enormous holes needed for vertical mounting, when space permits.
Most of the leads that go to the circuit board are threaded through the closest unused hole as shown on the photos below. That helps with strain relief and improves reliability.
Shielded teflon wire is used for input jack to V1 grid and from volume control to V2 grid. It seems to help with noise level.
Click on a thumbnail to play the video on YouTube.
Click on an image to see more details.
It worked almost from the first try. The output transformer secondary on PP5W has the opposite orientation that expected, so NFB was actually a positive feedback. That made amp squeal like hell. I had to rewire NFB to go to the output jack sleeve instead of the 8ohm tap. That also means that amount of NFB would change when impedance is not set to 8ohm, but that doesn’t concern me since I only use it with 8ohm output.
I wanted to wait until I get used to the way this amp sounds before posting any conclusions. My usual reaction to a new amp or pedal is to not like it. It takes some time to dial in sound that I like and to get used to what it can offer. I’ll go over few different aspects that define how an amp sounds:
- Gain: it’s got more than expected. It gets dirty with volume control set pretty much anywhere, especially with humbuckers. I found that with amp volume control set at 30-40% it can nail some nice classic tones – Led Zeppelin with humbucker equipped guitars and SRV/Hendrix tones with single coils.
- Sustain: lots of it, especially with higher volume settings. After maybe 50% on volume control it only ads more compression and sustain. It’s nice for fluid sustaining solos, but to compressed for rhythm.
- Touch responsiveness: it can do clean-to-mean thing pretty well by using guitar volume control, especially combined with humbucker/split/parallel modes. With high output pickups and guitar volume maxed, amp volume control doesn’t do much as far as clean-to-mean goes, just adds crunch and sustain. In that situation I found that guitar’s volume control and pickup selector switch are more important, combined with pick attack.
- Tone: it is bright, but not as much as expected from reading on the web. It has to be bright to a certain degree to be able to sound good with lower input volume, otherwise it would be dull for clean and not sparkley. It actually bass that needs to be tamed. I keep bass control at 0-20% all the time. With higher settings it sounds flubby and muddy when overdriven.
- Noise: surprisingly, it’s got very decent noise level. No oscillations at any volume levels. Trainwrecks are notorious for being noisy and prone to oscillations, but I’m very happy with this build in that compartment.
Update (May 2010)
I’d like to add some bright switch observations. It obviously works well with some guitars and doesn’t with others, but it kept me from getting better tone from this amp for weeks. When you tune into decent settings with the bright switch on, turning it off makes the sound dull and takes some gain away. I would immediately turn it back on, but don’t give up and turn it back on before playing with other controls. To tame the harshness I was backing down on treble, gain and presence but foolishly kept the bright switch on (100pF position) all the time. That would make the sound a bit dull and still harsh. With my Ibanez the opposite seems to be working better – adding a bit of treble, gain and presence but turning the switch off. That makes the amp sound sharp but less harsh than with the bright switch.
Update (June 2010)
I had to add this update after trying the amp with my Strat because it’s a completely different beast! My impressions about this amp were that it had way too much gain when played with my Ibanez. It’s much more tame with the Strat. Transition from clean to mean is slower and has more sweet crunchy tones in between. This is another proof for how sensitive this amp is. Now I’d like to try it with Les Paul 🙂