Thursday, November 24, 2011

Thanksgiving update on the EQP-1A project

Now that the Grail amp has made its way to Switzerland, it's time to tidy up a couple of loose-thread projects.  First up is the final completion of the EQP-1A tube equalizer project,  intended as a companion piece to the Wombat Amps version of the LA-2A compressor I constructed for Black Iris Music

The main boards for this one have been complete for awhile, as has been the laser engraved front panel.  So the main job has been getting all the hardware mounted, and beginning the process of harnessing everything together.   Like the LA-2A, this project will also feature DC filament heaters, which, in conjunction with metal film resistors and a modern layout & ground scheme, will ensure virtually noiseless operation - or at least as noiseless as is achievable with tubes. 

Yesterday I had a chance to put together a DC heater circuit, based on the NJM2396F63 low-dropout regulator.  It's an interesting part, because it is designed to provide 6.3VDC -- and there's only one reason why anyone would want that oddball voltage... it's the standard for tube filaments. I'm not aware of any other application that require this, so I'm surprised there's still enough of a market niche to make these worthwhile to produce. 

Anyway, here's where things stand:

 Completed back panel 

Note that the power transformer is mounted internally, as are the tubes.  Visible are the Sowter UK input and output transformers.  The input transformer is a 3603e 600:600 ohm isolating transformer.  The output transformer, the 9530e, is Sowter's version of the literally impossible-to-find Peerless S-217-D found in the original.  It's a highly unusual transformer, as it includes an odd centre-tapped tertiary winding that is used to provide about 4% NFB to the input valve, as is visible in this schematic.

Back Panel insides

Here's a view of the back of the inside of the back panel.  I'm using the Hammond 270CAX as the power transformer.  The current plan is to use the 5V and 6.3V windings of this transformer in series in order to feed the DC regulator.  Note that the 6X4 rectifier tube is slightly unusual in that it employs 6.3VDC for its filament, rather than the common 5V -- all three tubes together will require 1.2A at 6.3VDC.  By my calculations (and accounting for power factor) this will push the VA rating of the 5V winding a bit.  However, the rest of the transformer will be pretty much loafing - the current demand for the high voltage part is very small indeed.  I therefore don't anticipate any difficulties, but in the unlikely circumstance that the transformer does get a bit too warm, I have a couple of fallback strategies... I left enough room internally for a separate filament transformer, as I used in the LA-2A.  Alternatively, it turns out that there is a 12.6V version of the 6X4 rectifier (called, you guessed it, the 12X4) -- just changing to a 12.6V heater instead of the original 6.3V would halve the current demand, and would be easier on the LDO regulator as well.

Mounted main boards

Finally, I got the main boards mounted in place, and the majority of the front panel controls wired up.  I made one small modification the larger main board... you'll notice a silver can visible on the right hand side.  This is the Sowter 9330e interstage transformer, which replaces the original Triad HS-29.  There was just enough room to mount it here, and the position turns out to work well with my layout.  Finally, the silver can (actually mu-metal for best noise rejection) on the filter board with all of the orange-drop capacitors is the Sowter 9325e inductor required for the EQ.  This build sure required sourcing a bunch of very specific oddball parts, so I'm very grateful that Sowter is producing these extremely high quality reproductions -- otherwise a faithful build of the EQP-1A would be next to impossible.

It won't be long now! Look for a wrap-up on this one soon!


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