Amplog
D-Type 1x12" combo

This is kinda like a "blog", except I'm writing about my latest amp build. Hence, "amplog." Today I'm writing about a build I just finished, a 20 watt 1x12" D-type combo. As many of you ampfans might guess, "D-Type" stands for Dumble. I've built this amp along the lines of a Dumble Overdrive Special. I've built a couple D-Type amps before and I wanted this one to be stripped-down to the tone essentials.

This project actually started last year. I had an Idea for an amp along the lines of the original Mesa Boogies, which were hot-rodded Fender Princetons. With this in mind I ordered a BF Princeton Reverb-style chassis from mojotone.com. I also ordered a BF Princeton-style cabinet from Will Dyke at Armadillo Amp Works. I asked him to make the baffle for a 12" speaker instaed of the stock 10" and to add 1" to the front-to-back depth for a slightly bigger cabinet with better bass. These pieces then sat around for a long time until I found the time and money to put it together.

My original idea was to cram a 50 watt tranny set in this little amp and do a Marshall master volume-type thing. I realized that I didn't really play any places where I needed 50 watts. I get enough complaints about volume now and a 50 watt combo wasn't going to help that;-) So, I reasoned, a 20 watt Princeton-style power section w/ 2x6V6 tubes was probably a better, more useful idea. Late last year and early this year I had the occasion to build a couple D-Type amps for other folks. I also thought back to the 1st D-type I'd built, a 20 watter that just sang! The cool thing about these amps is the volume actually comes-on fairly slowly. It's possible to get gooey gobs of sustain without much volume. This is more like what I needed for the music and clubs I'm playing.

I didn't want to drill extra holes in the face of the Princeton chassis. That left me with 6 control holes to work with. I've noticed for a while that nobody actually uses the channel switching featues of real Dumbles or the numerous clones. Folks tend to dial-up a sound they like and leave it there. I also got to thinking about the very 1st D-style amp I built, which was actually a mod I did on a friend's Traynor, which sounded badass! It was wired for overdrive only. The neat thing about that amp was you could dial-up any sound you wanted, from clean to scream. The clean tones, with the gain and drive down, sustained for days because the signal was still going thru the extra overdrive tube stages. Neat stuff! And, of course, the overdrive sound was supernatually smooth. Distortion with no fuzz! Wow! With all this in mind I decided to leave out the switching function and wire the amp for overdrive. This made the build a bit simpler and would still give me lots of tone options. Without O/D switching I woudn't need the O/D "level" control, only a master volume. I also figured I wouldn't need the mid control, bass and treble would be enough. So that gave me volume (labeled "gain" on my amp), bass (lows), treble (highs), drive, (master) volume, and presence. Also, I didn't really like most of the switches on a real D-Type (deep, bright, PAB, etc.) but I do like the mid boost (changes the treble cap, very cool-sounding) so I put a mid boost switch in the 2nd input hole. That gave me my panel layout and also defined the circuit I needed. I measured the hole spacing on the chassis and designed a new faceplate, which I had made by SignCrafters here in San Marcos on laser-etched aluminum. They did a great job, the faceplate looks killer and fit perfectly.

For the circuit boards I decided to go with eyelets instaed of turrets. Figured it would make part placement and swapping easier. I copied the filter cap board layout from several on the 'net and made my own main circuit board layout. Of course I made my own eyeletboards on G10/garrolite, 'cuz I make 'em better than anybody else;-) I did put in a couple turrets for the o/d trim pot. This all worked well and fit in the chassis easily. As I got around to ordering the trannys I noticed the tweed Champ, BF Champ and BF Princeton all use the same power tranny. I already had such a part from an old BF Champ build, so that got pulled and went in the Princeton chassis. Also, as luck would have it, I had a Deluxe Reverb output tranny hangin' around also. Made for 2x6V6s and bigger than the stock Princeton part it was perfect for my little D-Type. So there's my tranny set, let's go to work!;-)

Getting the chassis from mojotone, all punched and drilled, saved me a lot of fabrcation. I only had to drill a few small holes for mouting the new eyelet boards and the slightly larger output tranny. Building the amp went fairly quickly. Assembly and wiring only took a couple days. After a little bias tweaking the amp came to life! My favorite part! Now, on to the tweaking.

There were a couple tone issues at first. I had decided to use a tube rectifier because the power tranny was made for it and there was a hole in the chassis for a tube rec. The 1st rectifier I tried was a GZ34. Right away I noticed the B+ was really high for 6V6s, around 440vdc. Also the power tranny was running really hot. Upon looking at schems for the tweed Champ/Princeton the tranny was designed for I noticed these amps used a 5Y3 rectifier tube. I swapped-in a 5Y3, my voltage came down a bit (420vdc) and the tranny was running a bit cooler, so the amp was bit happier.

Next up, the master volume was really touchy. The amp got super loud right away with little/no room for turning it down. I had left-out the 1M master volume and was using the 100k "O/D Level" control as a master volume. I figured out the amp needed to see the load from both of these controls to work right. I put a 1M pot for the master volume, along with a 56k resistor to ground across the 1M pot. This simulated the "drive" control about 1/2 way up and made the amp start behaving. I got a much more gradual taper on the master volume and could dial any volume I wanted, from quiet to very loud, with little change in the tone. Now we can work with this!

Then there was a round of adjusting preamp voltages. D-Type amps really want to see the preamp tubes running at about 200vdc on the plates. This is a real sweet spot for these amps. After a couple changes to the dropping resistors in the filter section, and one plate resistor change, I got the magic voltages. The amp began to sing a bit more but still had the dreaded "cloudy midrange."

Now this a weird term. "Cloudy midrange." I'd never heard it untill I started working on D-type amps and reading the Dumble forum on the web. I thought it was "cork sniffer' crap. Untill I heard it. There's just this sound, actually a lack of sound, a lack of detail. You're used to hearing your mids all clear and present and you don't notice anything untill the mids aren't there. It just sounds...like the amp isn't "breathing" right. No sustain or detail. In D-type amps there's a couple things than can cause it. One thing, oddly enough, is Mallory 150 caps, D-types don't like 'em (Marshalls love 'em, go figure;-). Use Orange Drops or Xicons instead. The other thing, I found out, is the tonestack. I had wired the tonestack like a stock Princeton bass/treble setup. Turns out a D-Type tonestack is a bit different and it makes all the difference. Once I straightened that out the tone came pouring out. Clear, present mids and all. The mid control on a D-Type is a 250k pot wired as a variable resistor to ground. I didn't have a mid control in my build, so I wired a 100k resistor from the mid cap to ground. This is like the mid control 1/2 way up and works great.

The last tweak was to the filter caps. The amp was kinda heavy-sounding in the lows. I had 2x100uf/350v caps in series for a total of 50uf (caps in series divide, right?). I got to thinking to my self, "Self, you know, tube rectifiers don't actually like to see a whole lot of filtering in the 1st stage. All those amps this power tranny was made for (Champ, Princeton) have, like, 16uf for the 1st filter stage." So I changed the 1st filter cap to a single 22uf/500v cap. That was a good idea, the amp got much happier. The lows tightened-up and the whole thing just sang a bit more. Suh-weeet!

So now I've got the amp singin on my bench, time to put the cabinet together. In the great tradition of boutique amp builders everywhere, my wife does the covering;-) I drilled all the mounting holes, painted the inside black, and handed the cab over to Jes for some black Tolex. Getting Tolex to stick and look good is not easy but Jes made it look great! Another trick thing I did was to make the back panels oversized. They almost come together on the back, leaving a 1" space between them that acts like a "bass port". This gives the cabinet a sound somewhere between a closed and open-back cabinet. It sounds huge! Way bigger than any Princeton you've heard.

I had a neat idea for the speaker baffle. I got a piece of perforated steel from my sheet metal guys, cut to fit the front of the baffle, painted that black.Then I painted the baffle white. Then I got some black brush paint, dipped my brush, and dripped black spots and drips on the white baffleboard, like Jackson Pollack or something. With the black grille in place I'm thrilled with the finished look! Artsy, yet industrial. In another happy accident, the finished amp is the same width as my new 1x12" ported speaker cabinet. Together the 1x12" speaker cab and the 1x12" combo make a killer mini stack.

What's next? Well, for one thing I've got a gig next week and I'm dyin' to try it with the band. Also, some of you more observant-types might have noticed that a Princeton Reverb chassis has holes for four 12AX7 tubes and I'm only using three of them. The fourth tube is gonna be a one-tube reverb circuit that'll get added in the next week or so. I've got an idea how to hang all the parts I need on just a ring terminal or two. The controls for the verb send and return level will be on the back by the RCA jacks. I've just got to order a verb tank and verb tranny and I'll be set. It sounds big now, but with verb it's gonna sound HUGE! Look for video coming soon.

Update: 5/26/10
Installed the 'verb circuit! (see below) Features send and return level controls for the 'verb, located on the back by the RCA jacks. Works/sounds killer! Used the amp at a gig with my band, Mashed Potato Johnson. It rocked ass! Bigger and louder sounding than I expected. This one will go on the "menu" of stuff that can be ordered. A fun experiment that made a very cool amp!

Here's the D-Type amp with my band at The Triple Crown, San Marcos, TX
You can't see it real well (it's on the floor behind me) but you can hear it great!

Updater: This amp is FOR SALE!
Price: $1900.00
contact Ace for more info or to purchase.