Simple Tube Program EQ project

From the September 1947 issue of RADIOCRAFT magazine: an interesting ‘response equalizer’ project that looks to have great potential for use in the recording studio.  Download the complete article with schematics:

DOWNLOAD: ResponseEQ1947

You will need to download and read the article for complete details, but essentially this circuit provides the following functionality, with 7 controls:  4-position high-pass.; variable low-end boost with 4 selectable frequencies; 4-position low pass; variable high-end boost with 4 selectable frequencies; 10db overall gain with variable output trim.  And it does all of this with no inductors or transformers, and just one very cheap 6SJ7 tube.

To use this circuit in a complete studio-ready device, we will need to add several things:  First, an output stage capable of easily driving a 15k:600 output transformer.  I am going to try the very simple output circuit show above, taken from tubecad.com.

A 250-300v DC power supply (plus 6.3v filament power) will also be necessary; plus input and output transformers.  Once I get this all together, I will also need to figure out a good, silent bypass-switching arrangement.   Should be a good inexpensive tube eq project.   The overall application is very similar to the Pultec EQP-1A; this unit will not have as precise control due to the lack of inductors; but it will cost a heck of a lot less (likely $250 of parts verses $800 to build a Pultec).  Plus it allows one to boost and cut at different frequencies, which the Pultec does not offer.  Expect some build-notes, images, and audio examples here soon.