|My DIY 5F6A "Bassman"||by John Edwards <firstname.lastname@example.org>|
My desire to build a guitar amp started about two years ago, when I "inherited" an old Williamson monoblock removed from an old home installation, which I thought I'd modify. I'd never done extensive electronics work before, beyond what it takes to keep my home 8-track studio up and running-in other words, I knew which end of the iron to hold, knew how to make up and repair cables, set up tape machines, etc. I had also restored a '59 tweed Deluxe that I found at a flea market a few years ago, so I had some (very) basic knowledge about what goes on inside a tube amp.
The Internet, and particularly all the helpful gurus on alt.guitar.amps and rec.audio.tubes, became my most important resource for information on this project. When I posted requests for schematics on the Williamson, one reply was, "Draw it yourself!" I took that suggestion to heart and it was a good first step. After corresponding with several folks, the suggestion came to build the 5F6A circuit, being relatively simple and generally regarded as "tone city". I scrapped my plans to cut up the Williamson and decided to start from scratch.
On to the details. The chassis is an aluminum box purchased mail order, and cut down 2" across the length of one side (it was 10" wide, and I wanted only 8".) To this, I added small sections of rack rails as "ears" for hang-mounting the chassis to the cabinet. The control plate is actually a brass kick plate for a front door, bought at a local home supply store for about $20, then cut by hand with a jigsaw, and the edges hand-bent to wrap around the top of the chassis. I wound up with about $40 total in the chassis, and did it this way because I felt $150 or more for a "boutique" chrome steel chassis would be overkill for my first project. Also, as I've modified and refined the amp, the chassis has would up with lots of mysterious "extra holes"
. The chrome "Tele-style" knobs look pretty good against the brass plate.
|1. Front view||2. Rear view w/back panel on||3. Rear view w/panel removed|
The power transformer was salvaged from an old 2 x 6L6 Lafayette tube PA head ($15). For the output transformer, I decided to spring for a Doug Hoffman 50-watt unit, since he has a great rep and his prices are better than most. The only problem was that his original OT had only 4, 8, and 16 ohm taps-no "true Bassman" 2-ohm tap, so the amp's original configuration had the two pairs of 10", 8-ohm speakers wired series/parallel for the 8-ohm tap. Lo and behold, late last year Hoffman added a 2-ohm tap to his 50-watt OT, so I had to bite and buy one. It was worth it! I've since added a switch to go between 2 speakers @ 4 ohms total to 4 speakers @ 2 ohms total. This lets me play at a slightly reduced volume while still hitting the preamp tubes with good signal level. The choke also came from Hoffman.
The circuit itself is pretty much a traditional 5F6A, with the addition of Kevin O'Conner's "ultimate" tube effects loop circuit from The Ultimate Tone. The amp didn't start out this way -I had dreamed up a convoluted switching scheme (channel switching, fx loop in/out, speaker switching, etc.) which used relays and diodes. The relays were noisy as hell, so I ripped them (and the 12V transformer) out about halfway through the project. I've got to give O'Conner credit for designing a superb effects loop -it works flawlessly, doesn't alter the tone, etc. Besides the effects loop, the only real deviations from the 5F6A circuit are the addition of a bias pot, a 3-wire AC cord, a terminal strip wired up to give me easy access for meter probes when biasing (transformer shunt method), and the aforementioned DPDT switch for switching one pair of speakers in/out (using the appropriate OT taps). The circuit is built on glass epoxy board with steel turrets, also from Hoffman. I highly recommend his stuff. Most of the effects loop is built on a small board separate from the main board. All caps (except in the bias supply) are Sprague, Mallory, or Cornell Dublier. Most of the tube sockets, retainers, switches, fuse holder, etc. were salvaged from other stuff or picked up from junk boxes at local flea markets and Hamfests.
The cabinet is built from 3/4" x 12" solid poplar lumber, bought from Lowe's. I didn't have the gumption to try and finger-joint it, so the corner joints are of simple tongue-and-groove construction with small braces glued/screwed into each corner. The finish is a pecan stain with three coats of clear polyurethane on top. I'm planning to build a matching 2-or 3-space effects rack. The baffle and back panels are made from 3/8" furniture-grade plywood sanded out to roughly 5/16" (Weber claims this is the thickness of the original Bassman baffles). For hardware, I decided on two "transverse-mounted" tweed-style leather handles on each end of the top. This seemed to make a lot more sense-one person can carry the amp using both arms rather than one-handed like an oversized "suitcase", and it also makes it easy for two people to carry it. I installed 2" hard rubber pop-out casters, and decided on chrome amp corners, more because the thing looked too much like "furniture" than for protection! The four speakers are Mojo MP10R HD's, attached with machine screws through the baffle to T-nuts on the front. The grille cloth is reissue oxblood/yellow stripe, and not being one to leave well enough alone, I decided to mount the cloth diagonally. (Great effect, but what a pain!)
|4. Detail view inside chassis||5. Back view of control panel|
So, how does it sound? It's hard to be objective, since I'm so pleased with myself for having actually succeeded in building this thing, but I think it sounds great (and LOUD!). I've done a good bit of tweaking to get hum and noise levels down (I'm still considering installing a DC heater line). Preamp tubes are a combination of Tung-Sol, Sylvania and RCA. Power tubes are JAN Phillips 6L6WGC drawing approximately 35ma. Single-coil guitars go into the "normal" channel and humbuckers into the "bright" channel. It gets a balanced, clean tone up to about half (a little less with humbuckers), then starts to roll over into warm overdrive. It's getting pretty loud at this point, so the speaker cut switch comes in very handy. Maybe the most impressive thing is the bass response. I guess Weber was right in attributing the Bassman's low-end response to the thin baffleboard (as well as the "floating" baffle mount, which means attaching only the sides of the baffle to the cabinet). With the separate treble, bass, mid, and presence controls, there's a really wide range of tonal possibilities. The amp is very touch-sensitive-you can dig in and it barks, but it's very polite and graceful with a light attack Bring the channel volume above half and it really starts to sing out (loudly!). I believe this amp would satisfy most guitarists playing rock, jazz, blues, or country. Not to belabor the O'Conner loop, but it really works like a charm. There's a send jack, send control, series/parallel switch (very important), return jack, return pot, and return level switch, allowing for a variety of configurations -my tube Echoplex sounds great through the loop, as do the several rack units and pedals I've tried.
Not counting the extra output transformer, my rough guess is about $600-700 for my total investment in the amp (remember that many of the parts were salvage, including the power transformer). For that price, I could probably have picked up a reissue Bassman, but I've heard too many disparaging remarks about them. I've played a couple of the boutique Bassmans in the $1500-$2000 range, and I believe this one would stand up well beside either of them. Don't misunderstand my point -there's no way this could be called "less expensive" after all the hours of work (and re-work) I put into it -I could never make a living this way. But it was a great learning experience, and I'm weighing my options on my next DIY project ...maybe a JTM 45?? (Anyone got a schematic? :))
Finally, I want to thank everyone who's given me great advice on this project. I won't name names because I'll leave someone out, and you know who you are. Thanks also to everyone who jumps in to help newbies like me in the newsgroups -even if I haven't corresponded with you directly, there's a good chance I've read your posts, and they helped make my project that much better. And, to those who host web pages like AMPAGE, Nate's Homebrew page, and all the others -invaluable resources.
CONTROL PANEL LAYOUT (viewed from front of amp) 1 3 5 7 9 11 13 15 17 2 4 6 8 10 12 14 16 18
1. Normal in jack
2. Bright in jack
3. Normal volume
4. Bright volume
9. Effects send jack
10. Effects return jack
11. Send level
12. Return level
13. Effects loop series/parallel switch
14. Effects loop return level switch
15. Fuse holder
16. Standby switch
18. Power switch