). I learned of this forum after googling around for info about some of the different output transformers that I have including two A-3801's, which is what actually led me here. I decided to sign up for the forums after being impressed with the wealth of information about Trainwreck's as well as the friendly atmosphere.I've only been actually building tube amps for about 6 months now but finding a large stock of great Stancor and other transformers, chokes, cases and a few power transformers has made this an affordable and surprisingly relaxing hobby.
My first two builds were of my own design, a 2x6AU6/807 SE 20W amplifier and a 6AV6/2x6AK6 (in parallel) SE 2W amp. The 6AK6 amplifier also uses a 130-250V boost SMPS that I designed for use on mini/submini amplifiers (plans coming soon!). Both of these amps have their ups and downs and have been great learning experiences. But I've quickly found that the 807 SE amplifiers just can't hold up in a jam session against a drummer, bass player, PA, etc.. Even with 16-18W from the 807 through 4x12 cab, the only time I can really fit in with the mix is to put on some distortion.
The decision to actually build a Trainwreck (specifically an Express) was dictated by the output transformer (this is what dictated my last two builds as well funny enough). Finding a set of beautiful (and heavy!) Stancor A3801's was pretty much all of the "encouragement" I needed. The chassis and most of the parts I had on hand so this turned out to be quite an affordable build. I ended up spending $45 shipped for an 2T300 transformer (w/ 12.6V and 6.3V taps) and about $35 on a set of JJ El34's and some 8 and 9 pin sockets. The preamp tubes came out of my large box of new and used tubes that a teacher gave me a while back (somewhere around 500 tubes..). They were all GE brand 12AX7's that tested good or great on the tube tester.
So for the actual build, I decided after a bit of reading to build the A1a version of the Trainwreck as seen in the Blue Guitar pdf file. I figured that if anything, this would be a good springboard for a more personalized version of a Trainwreck. The fact that the Express is known for a wide range of great tones with just the volume and tone knobs was very attractive and seemed like a great fit for my style of playing.
The chassis I used was fairly unconventional for a guitar tube amp build (measures 12" long x 7" wide x 3" deep) but ended up getting used because it was free, was made of good quality aluminum and fit the transformers/tubes decently. I probably would have went with something larger if I wasn't used a toroidal power transformer but the low leakage made power transformer hum of low concern. The case did already have a bunch of holes in it because it was a recycled student project (I don't know what it was before) but I spent a bit of time planning out the layout in such a way that I could use most of the holes drilled in the case for something or another.
Building the amp took about three days, one for drilling the case and two for actually stuffing the chassis. The thing that I wish I spent more time on was planning out the layout and insides because the point-to-point wiring scheme that I used ended up getting pretty cramped towards the edge of the case. Also I was very lazy with the lead dress of this amp and it showed quickly when it came time to test the finished product. After eliminating a nasty motorboating problem associated with poor bypassing, I found that the amp oscillated uncontrollably and eventually the tube plates would start slightly red with the bias control "maxed" (i.e. grids most negative). The DMM showed that with the values posted in the A1a schematic, a bias voltage of only -12.5V was achievable. This was unacceptably low by a large margin and so after some napkin math, I ended up replacing the 220k resistor on the negative bias stage with a 68k resistor. This allowed for about -29.5V of bias on the tube grids, close enough for now.
So after fixing that little error (I think others ran into the same issue), the amp would turn on and idle decently, but an audible whine could be heard that could be changed by moving tonestack pots. I thought the issue was a grounding one, so I went about changing the amplifier from the chassis grounding scheme I originally (and skeptically) used to a star grounding scheme. This reduced a bit of hum in the amplifier but I was still having a nasty oscillation problem. The next step was to improve some of the wiring of the amplifier but alas, no great results here.
After the last two attempts failed, I spent a bit of time reading before stumbling across Gerald Weber's book and reading his great section on taming unstable amplifiers. The first thing that started showing some promising results was moving the shield of the V1a tube from the input ground to the 100k plate resistor. This killed allof the oscillations and noise in the first stage (I was using an oscope and tube test socket at this point) but couldn't eliminate the oscillations with all of the tubes plugged in. So I spent a bit more time cleaning up the wire routes and added grid stoppers to the first three preamp triodes. This helped to tame the oscillations but at the extremes of the treble, mid and presence controls I was still hitting oscillations.
At this point I'm getting close to ready to give up, but not before trying the "last resort" tricks that I was hoping I didn't have to do. The last changes I made were to move the presence control away from the other tone/volume controls and to add some negative feedback to the second and third stages of the preamp. The presence control was moved to the side of the case near the bias control which cleaned up routing in that area considerably. Then I added a plate-to-grid capacitor to the second stage and a plate shunt capacitor to the third stage. I was careful to choose a cutoff frequency well above what a guitar amp would actually deal with but low enough to swamp out those nasty oscillations. After these last tweaks, this amp CAME ALIVE
.The sound was exactly as described, extremely dynamic and very responsive to player touch. It goes from spanky clean to an almost singing distortion with just the twist of the volume knob. I've read some complaints from people about Trainwreck clones being overly bright but this was not a problem with my build through both a 200W monster 12" speaker and a super duper vintage 40's 12" speaker/enclosure that I picked up for $30 in Lockhart. With a humbucker equipped Ibanez, the sound went from punchy and clean to full on rock distortion that would make Hendrix jealous. I've never played a more dynamic amp, nothing responds to playing touch like this thing does. The sound with my 52 Tele reissue was exactly what one would expect, a bright yet sweet sounding clean sound. But the distortion with the tele just blew me out of the water, I've never heard this guitar sound hotter! I can't wait to play this amp at a jam session and onstage, I think it will have people turning their heads and covering their ears
.It's getting late and this post has already gone on way too long so I will wait until tomorrow to post pictures. This is probably the ugliest Trainwreck build you guys will ever see but I will still stand by it as a solid build that cost me very little in the way of money (but a lot in headaches this holiday!).
