Monday, October 20, 2008

Preamp: Prairie 10LS/1LB

I started out with the idea to make a tube DI box for the '63 Dano Baritone guitar I ordered. It turned out that the Dano was long delayed and the order was eventually canceled, due to some manufacturing problems. I continued with the project as a bass DI. I threw in a microphone input for laughs. The circuit evolved.

A starting point was the Altec 1566 preamp. I wanted a pentode input, so I chose the 5879, which is well known from Gibson guitar amps and is not too hard to come by. The input stage went through several revisions: Initially, it was diode-biased using two 1N4148s in series. In the original configuration, I also included a version of the pentode screen "squish" control from the AX84 Blues Preamp. In the end, the cramped layout (see below) meant that I had to abandon the squish, as the screen picked up loads of noise. In the end, I also dropped the diode bias in favor of a resistive network. The idle current keeps the screen and cathode a bit stiffer than bias and screen dropping resistors alone would have, but there's enough sag and cathode degeneration to make it interesting.

Instead of the parallel 12AX7 for the cathode follower I used a 12BZ7 which is very close in performance to two 12AX7s in parallel. This left the other half of the 'BZ7 for a gain stage.

I used two 6 VA Hammond 229 flatpack power transformers, which are really intended for printed circuit board use, but I mounted them on a turret/eyelet board. I used eyelets as a substitute for solder pads. There are a few unusual thing about these transformers (compared to more typical EI cores): They're semi-toroidal, so there's less radiated EMI; their voltage regulation is pretty high, so at low loads like this (or at open circuit) the output voltage is much higher than rated. I could have used a 2 VA model for the plate supply, but cost was nearly the same, and the lower-rated model had the same core and footprint. It was just 0.2" lower in profile.

Input and output transformers were 8:1 from Edcor. The input (MX8cs) was mounted on a small turret/eyelet board similar to the one used for the power supply. The output transformer (TPC104) came on its own PCB. The preamp circuit was built on a prefab turretboard from AES. The photos show earlier revisions of the project. When the squish control was removed from the front panel, the extra hole was used for a "thru" jack, which connects to directly to the instrument input when one is connected, or to the secondary of the mic input transformer when no instrument is connected.