Pultec EQP-1a Documents:

DIY Pultec EQP-1a Layout | DIY Pultec EQP-1a Schematic | EQP-1a Filter Schematic | Parts List (XLS) | Pultec Manual (PDF)

Pultec DIY Photos:

Inside Amp Section | Inside Filter Section | Inside Front Panel | Rear | Fired Up | Painted and Racked | Rack | Rack Rear

Pultec Audio:

MP3 Clips | Rhythm Tracks

Pultec EQP-1a DIY Notes:

This is as close as you can get to a real vintage Pultec EQP-1a. Built from the original Pultec schematic with the layout designed around modern replacement parts (filter caps and trannys); and utilizing best practices regarding lead dress and component placement. Build cost is around $800 USD.

The Pultec's original Triad HS-56, HS-29 and Peerless S-217-D transformers would have been great, but sourcing them all is very difficult, primarily because of the rarity of the Peerless S-217-D output tranny. As the Peerless is so exotic, I didn't really care about sourcing the input and interstage trannys because without the S-217-D, the point seemed moot. On the plus side, modern EQP-1a equivalents from Sowter have a much smaller footprint which facilitates a 2u rack chassis if desired - and the quality is artisan. For those inclined, the original Chicago PCC-55 power tranny does appear on eBay with some frequency and modest pricing. For the inductor, I chose Cinemag's Pultec replacement CML-150T.

Update: It has been brought to my attention that the Hammond 270CAX power transformer is no longer made with a center tap for the 6.3V winding. To accurately float the heaters, you will need a transformer with a 6.3VCT. Earlier 270CAX transformers with 6.3VCT might still be available from some sources such as Angela Instruments. Sowter also has the proper iron.

Update: Another builder has used the new Hammond 270CAX tranny (and is not floating the heaters) and found that the output is rated at 100ma. This increase over the previous model (70ma) has resulted in B+ voltages that are spot-on with the original Pultec EQP-1a schematic. (My build is a little low (10%) in B+ and as a result is down around 2dB in output.) So, ideally, look for a power tranny rated at 100ma, 250-0-250VCT, 5.0 2A, 6.3VCT 2A.

Build notes: I separated the EQP-1a's amplifier section from the Pultec passive filter circuit by way of two separate turret boards. The amp section is on the rear panel. This provided a nice way to route the filament wires in the upper corner of the rear of the chassis to keep things quiet as well as keeping the HT stuff away from the passive.

One particularly interesting thing is that the 6x4 rectifier runs off the same 6.3V filament as the 12ax7 and 12au7 tubes, and the power transformer rectifier 5V winding only powers the lamp.

I ended up moving the interstage and input transformers to the left of the bottom turret board and the inductor to the right side. I think there are 3 grounds points: One at the OT lug. One at the input XLR, and one at the power tranny.

(I later trimmed those output transformer wires.)

The Pultec works great and is very quiet. I ran several sweeps and it's perfectly flat with the eq in and set flat. The eq curves look pretty accurate as well in all configurations, including gain.