Caution: These are not DIY projects for beginners. High voltages are involved that can kill you. Do not attempt to build
these (or any other similar high-voltage) circuits if you don't know what you're doing. This site is not a how-to guide for newbies,
but rather a resource for experienced electronics hobbyists.
New - Updated La2a Layout and Documentation by Gambaholic:
There are 2 versions of each layout and there are layouts for the UA LA2A and the UREI 68' (both with a companion heater wire layout as you did). Each layout has a version where every component has a label or number. This was to enable the 'component placement' instruction doc. There is also a version of each without the labels which looks much better and less cluttered in my opinion. There are also some hi-res photos I took of the UA with the hood off so folks could see every component as laid out in the reference unit. There are also schematics of various LA2A revisions. (You can ignore the component placement doc as it was never finished.)
This build - based upon my La2a layout derived from the original Universal Audio Teletronix 1968 La2a schematic - is an accurate representation of the late 60's original, as well as the current reissue. Lead dress and component placement are exact. See photos. Build cost is around $800 USD.
A note about transformers: The La2a's original UTC HA100X (or A10) input and A24 output transformers do appear on eBay, but can be rather expensive, however, there are equivalents available from Sowter and others. The original Triad R-4A power transformer (earlier versions used a power transformer marked only PT894) can be found on eBay occasionally and at a reasonable price, but the Hammond 369JX is a fine replacement and features a universal primary.
La2a T4B: NOS and vintage T4B modules can also be found on eBay, but JBL still sells them, and they're much less expensive, around $90. It is possible to build your own, but due to minimum order requirements for the proper el-panel, as well the near impossiblilty of finding the octal socket metal enclosures, not to mention calibration, you're better off buying the T4B from JBL.
Video audio track (elbee3.mp3) features Martin OM-28 Marquis lead guitar crushed by DIY La2a Compressor
Note: La2a as a Mic Preamp:
The UA Teletronix La2a has enough gain to operate as a microphone preamplifier, providing there's sufficient signal from the source and you don't need phantom power - for example, a dynamic microphone on a guitar amp can be plugged directly into the La2a. Here's a clip I recorded June 2009 335lpmodsm.mp3 while doing some tests. The signal chain starts with a '68 Gibson 335 plugged into a cranked DIY Tweed Deluxe (with low pass mod) then a Shure SM-57 plugged into the La2a. Update: Here's a stereo clip with a Strat; the left side is the Tweed Deluxe (with the 1960 P12Q speaker) close miked with an SM-57 into the La2a and Pultec, the right side is an AKG 414/TLII about 2 feet off the back of the cabinet into the Redd.47 and MC76.
New: La2a as Preamp Demo Video 5/25/11
Chassis Work:
The chassis I used is the Hammond 525DGY2 from ACK Electronics in Atlanta http://www.acksupply.com/ where it was in stock. I think it was around $43.00. The chassis and panel are powder-coated 16 gauge steel - pretty tough to work but you can't beat the price:
Using a graphic program on my PC, I imported front and rear La2a photos, sized them, then added a transparent layer over each photo on which I marked the location of the holes for all the components and mounting hardware - adjusting for perspective. I then worked up mini layouts for the HA100X and A24, marking the central cutouts and mounting holes around their periphery, and placed them in the templates. Once all the other holes where sized correctly, I deleted the layer below (with the original photo) and was left with just a template for each that was pretty close to original specs.
I printed them out (each took 2 pages), then cut them out and taped them together to form actual-size panel and chassis templates. With some diluted Elmers glue brushed on the panel and chassis, the paper stuck right on and held. I followed up with some masking tape around the edges.
At this point, all that was left to do was drill out the holes marked on the paper stuck to both the chassis and panel:
All the holes were started with a small pilot hole to help with accuracy.
I used a corded drill (hand-held) and a vari-bit (expensive) for all the larger holes except the I/O transformers. The vari-bit works really fast and leaves a pretty clean hole - much, much better than hole saws. This part was no sweat at all - but be careful as both the vari-bit and material get very, very hot and small bits of very hot metal strips fly all over.
For the La2a I/O trannies I used bi-metal hole saws. These require some filing to clean up and don't make a perfect circle. Importantly, the hole saws make a considerably larger hole than their diameter, and are tricky to use on this material. You can also end up with an oval shape if you aren't plumb. This stuff was a little difficult.
The rectangular holes were done by drilling around the perimeter, then hacksawing the field away, then filing the remaining jagged mess into a rectangle. I cant believe these came out as good as they did.
The DIY La2a front panel template is set up for the Sifam meter and the chassis is set up for the HA100X. Also, the tube sockets do not have their mounting holes marked and the power tranny mounting is not marked. Use these templates at your own risk - they worked great for me, but you should definitely double-check before drilling.
Please be careful working on any material like this and be sure to wear safety glasses and gloves when doing so.
