Custom 6SN7 Extended Foreplay III! (an evolution, with photos)

[Mikey]

I finished up my 6SN7 Foreplay III this weekend, and here's the scoop!
Initial Impressions:

A new and improved chassis plate! The new chassis plate gives the kit a much higher quality feel, with more room for future modifications.

Finishing the Parts:

- The chassis plate is painted with a coat of clear lacquer.
- The transformer end bell is finished with satin black lacquer.
- The transformer lamination stack is painted with high heat flat black paint.
- The base is finished with Watco Danish oil.

Mechanical Assembly Notes:

The mechanical assembly was very straight forward. Since I plan on using this preamp with only one source, I omitted the selector switch and all of the associated input wiring. I chose to build the preamp with a nod toward the older style Foreplay, with the input and output jacks on the top plate near the tube sockets; this greatly simplified the wiring. Eventually, I will get some hole plugs to fill in the unused openings in the rear of the chassis plate.

The volume potentiometer is an inexpensive Alps 100K stereo pot, currently serving as a placeholder until I pull my Goldpoint stepped attenuator out of my Soul Sister preamp.

Mechanically, the only change required for the 6SN7

 

[Mikey]

- See the previous build notes in the post above -

Initial Impressions:

Nice circuit boards! The layout of the circuit boards, along with the multifunctional nature of the boards is very slick indeed. Kudos to PJ for the clever design. Also, the quality of the boards is a marked improvement, while the silkscreen and soldermask make assembly a breeze!

Can you tell I like the new boards? (grin)


Mechanical Assembly Notes:

The mechanical assembly was very straight forward. The assembly instructions were VERY clear and concise, with plenty of detailed step-by-step photos. If you take your time and pay attention to the manual, the build should be nearly foolproof!

Since this preamp was originally built to use 6SN7 signal tubes, I used a 6SN7 for the shunt regulator as well. Since the Foreplay III ordinarily uses an octal tube in this position, no mechanical changes were required. To wire the shunt regulator correctly, I printed out both the 12AU7 and 6SN7 datasheets and cross referenced the pinouts.

While I was under the hood, I also swapped in a custom 100K attenuator I constructed on an Elma 24-position switch...just for good measure.


All resistance checks and voltage checks were spot on, so I placed the preamp into my system and fired it up. While I will save the detailed sonic review until I have spent more time with the preamp, I can tell you this....it

 

[Mikey]

I performed this modification just to demonstrate what can be done with this preamp with a little bit of elbow grease and creativity. Of course, having all of the parts in my

 

[Mikey]

I've made a few modifications to my EFPIII preamp since the
last time I posted, so I thought I'd share them with the group:
- A new volume control takes up residence in the preamp.
This unit was custom made by Shallco, it's a detented
100K 'potentiometer' with 30 steps, 1.5dB per step.

- A more 'vintage' appearing volume knob and fascia plate
were added at the same time, lending to the retro appearance.

- The Solen output coupling capacitors were replaced with some
AudioCap Theta's, 3.0uF

- The bypass caps on the shung regulator board were upgraded
with a pair of Dayton film and foil caps.

Collectively, the capacitor upgrades and new volume control add
another level of refinement to an already fantastic preamp! If
you can shoehorn them into your chassis, give 'em a try!

Mike

 

[Wardsweb]

Very nicely done. I like the sleek layout and clean execution.

 

[2wo]

Yes, very nice. I am not surprised that Wardsweb liked it. 

great photos and I am always glad to read a thoughtful revue...John

 

[Grainger49]

Beautiful!  I haven't taken in all the text but will keep working on it.  I feel vindicated that you started with my cheapskate favorite RS $3 Alps stereo volume control.  And later I see you replaced it with a stepped attenuator.  Oh well.

This all says to me that I should get a real shop, first I have to buy a new house with space for a shop... sounds expensive.

 

[Paul Birkeland]

Very nice Mikey!  I am looking around to find a 308 to send to you...

 

[Mikey]

Very nice Mikey!  I am looking around to find a 308 to send to you...

Huh?  A Ferrari 308?

 

[Doc B]

Yo Mikey,

Beautiful work as usual. I see one more tweak you need. Put a 2uF 250V cap in parallel with each of the R4 resistors on the shunt reg board. That will reduce your noise floor.

 

[Mikey]

Thanks for the compliments everyone, I appreciate it!

Dan, I'll add those caps next time I'm under the hood, which will be soon.
I'm planning on replacing the volume attenuator with a custom remote controlled
unit that I'm designing together with John Chapman (from Bent Audio) and Shallco.

I've attached a few renderings to whet the appetite:

 

[Paully]

I converted my FPIII to 7N7, which is a 6SN7 with a different base.  I certainly felt the tone was warmer and more to my taste.  Glad you are enjoying it.  Certainly fun to be able to mess around with different tube options.

 

[ironbut]

I'm kinda late to this thread but Wow!
Really beautiful work Mikey!
I think that every "knob" should have a "skirt" to nestle into.
Keep us posted on the remote attenuator. I'm not really a huge fan of remotes themselves but being able to place the attenuator at the inputs and wiring up some cool up/down buttons (ala "The Jetsons") appeals to me. Capacitive touch switches, or snag a gestural touch screen from a gameboy?

 

[mingles]

Mikey, where did you source the "vintage appearing volume knob and fascia plate"?

 

[Mikey]

Mikey, where did you source the "vintage appearing volume knob and fascia plate"?

Mingles,

I'm fairly certain that I got them from a seller on eBay a long time ago...

Mike