While scouring antique shops for old suitcases (for another project — stay tuned), I came across this old lap steel. It’s probably nothing special, but I thought it looked cool, so I decided to pick it up and restore it to playability.
A re-amp box is basically the opposite of a DI — it converts a low-impedance line-level signal to a high-impedance guitar-level signal. For example, a guitarist could record his tracks at home at his own pace, then take the recordings into the studio and “re-amp” them through a proper amp/mic. It gives you the flexibility to experiment with playing the same perfect take through different amps and mics.
For a basic schematic, check out Jensen’s JTP-11P-1 BALANCED LINE LEVEL TO UNBALANCED GUITAR LEVEL CONVERTOR.
I recently finished building an amp for G, of X is Y. It’s a combination of several designs, but they all lead back to the Fender Bassman 5F6A. Ever since I built my first Bassman clone a few years back, I’ve been modding it and tweaking it, sometimes randomly and sometimes based on other people’s ideas. In fact, with the exception of one Matchless clone, I think every tube amp I’ve built has been some variation of that classic circuit.
For this latest amp, I once again started with the Bassman and modified it about halfway to Marshall Plexi specs. Then, I added a slight variation of Doug Hoffman’s Plexi Hot Switch Mod with separate gain and master volume controls. The result is a two-channel amp (albiet with shared EQ) that can go from Fendery cleans to Marshally crunch, with enough gain on tap for singing sustaining leads.
Specs:
3*12AX7 preamp tubes
2*6L6 power tubes
5AR4 rectifier tube45W (approximate)
Aluminum chassis in pine cabinet
Dimensions: 330x215x240mm (WxHxD)
Adam from Luwan Rock was at Yuyintang this past Saturday, and got some pictures of the amp in action.
This is the L-pad attenuator I made from the pile of parts in the previous post. I just put it in a simple beige box, so the owner painted it himself … I think it turned out pretty good.
There are a lot of articles on tube amp attenuators out on the web, so I won’t go into all of the technical details here. Basically, though, most tube amps sound best when they’re turned up loud and the power tubes are pushed hard. Unfortunately, most people buy much more amp than they need, and end up wondering why they can’t get a decent tone out of a 50W amp turned down to bedroom practice levels.
One of the most basic attenuators is the L-pad, which is just a simple resistive voltage divider that sits between the amp and speaker. In my experience, L-pads work well when you want to cut your overall volume in half, but they can get “fizzy” sounding when turned down too low. Due to the power levels involved, this requires some really big resistors — the wire-wound potentiometer in the picture below is about three inches in diameter.
Schematic:
In recent years, a type of attenuation called Power Scaling or Variable Voltage Regulation has become more popular. There are a few different circuits, but the basic idea is to use a large MOSFET to lower the overall B+ voltage of the amp. You can use a normal, low-wattage potentiometer to scale down the voltage on the fly while still maintaining the the relative voltages of each stage, which produces a more natural sounding distortion, even at low volume. I’ve seen several different variations, and a lot of discussion on the amp forums, but the schematic below is the one I’ve been building into all of my recent amps. This is a re-drawing of a circuit designed by Dana Hall, with my part numbers substituted in. For cathode biased amps, only the top half is needed.
Disclaimer: Tube amplifiers contain potentially lethal voltages, even when unplugged. If you don’t know what you’re doing, don’t mess with them.
Several people have asked me why I was wasting nearly a month waiting for samples of my new distortion pedal instead of just ordering a full run right from the start. This is why. Even after I sent the factory a handmade sample, full mechanical drawings, and a 3D rendering, they still managed to build the enclosure upside down — those four screws in the corners are supposed to be on the bottom of the box. Hopefully it won’t take them another month to fix the few minor printing problems and flip the boxes over.
After seeing the DC-30 clone I made a couple of weeks ago, another local guitarist has asked for a low-powered Plexi-style amp. With some careful measuring and custom transformers, I was able to squeeze everything into a 2U enclosure that fits in with the rest of his mobile rackmount rig.
I did all the hard work last week — drilling holes, fabricating the horizontal tube mount, etc. I hope to finish the tedious wiring work in the next few days and have it ready for fine-tuning next week.
A local guitarist recently asked me to perform the “Ruetz Mod” on his Rat pedal. It’s basically just the removal of one resistor, which smooths out the frequency response and lowers the gain slightly. While I was in there, I swapped out the terrible sounding TI OP07DP opamps for a couple of NOS LM308H’s. Of course, normal plastic DIP LM308′s sound the same, but the metal can package adds some undeniable mojo.
The Carlsbro PM12 is about the cheapest powered monitor wedge you can buy in China. If you play in a band here, you’ve probably used them in clubs and practice spaces … and if you own one, you’re probably used to lugging it to the repair shop every couple of weeks.
Last week a customer brought in his PM12 that had been blowing fuses nearly every day. Instead of taking it in for repair, he just kept putting in bigger and bigger fuses until they stopped blowing. Not only did the chip-amp (pictured below) explode, but the power transformer burned out as well. The fuse was still intact, though.
I finished wiring up the DC-30 clone amp today, and after a little tweaking it’s sounding good. I’ll test it for a couple more days before letting it go, so hopefully I’ll get a chance to take some video.
