Woo! Stereomour kit arrived today!

pRC

pRC

New member
The future summer amp is in da house, and I am very excited to start on my third bottlehead kit. That 2000 era foreplay lured me onto a happy path that still continues -- thanks, Doc and Eileen!

I'll post some pics of the build when I start making progress. Base will be walnut and ebony, top plate will be heavy copper plate, 12 x 12 with an extension on the back for a pair of triad chokes for the power supply. This amp will be driving a pair of field-coil Lowthers in Azurahorns with double 15 inch Augie OB woofers driven by separate amps below 160 Hz.

It may be a few weeks before I get into the wiring since we are also in prime bicycling season...

/pRC

 
As always, I know your build will be a thing of beauty.  I'm looking forward to it.

pRC was kind enough to offer to let me hear his Beta Seduction way back when.  He and I lived in the Atlanta area then.  I came, I saw, I bought and it has gone on since then.
 
Thanks, Grainger -

I have to say I am feeling a little guilty about replacing the top plate now that I see the real thing, but I want a little more space under the hood. The new aluminum top plates in the kits are beefy and I like the random brushed finish.

/pRC
 
Making surprising progress in between bike rides!

Base is 8/4 solid walnut with an ebony-topped section at the rear that will hold the Triad filter chokes in a separate compartment. Top plate is copper, copper binding posts have been in the hoard waiting for an appropriate project. Coupling caps will be Mundorf silver-oil...
 

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A couple of tips - You can order copper plate from McMaster-Carr or onlinemetals.com -- I just use the standard size of 12 in x 12 in, so no outside cutting needed. Online metals will also do custom cuts.

If I remember correctly this panel is 13 gauge, which is plenty thick for this use; I ordered several plates last time I bought and had this one already on hand. Circular holes are drilled with an Irwin step-bit, square holes are drilled out inside the desired lines and then squared up with a file and sweat equity.

Even being OCD about keeping the drill press surfaces clean, you will scratch up the copper a bit as you work (the back side has all sorts of scribing for hole placement before center-punching all the holes). I use a pad sander and fine sandpaper to remove scratches, then work up through fine scotch pads to get a semi-gloss finish. You can also polish with semichrome if you want shiny.

Unprotected copper will patina over time, which I like, but you may want to spray on a clear coat immediately after polishing if you want to keep it looking like fresh copper.

If you are placing pairs of jacks or binding posts that have a skirt, you can just drill one of the holes a tiny bit bigger than the other to give you some fudge adjustment for getting them exactly aligned.
 
My experience - the problem with patina is ... fingerprints! You can't touch it until the patina has formed some protection. I am told that horse piss accelerates the process...
 
Oh yeah, I've already accumulated a few since I got the scratches out - a couple of ways to combat them 1) a quick pass with simichrome will leave a coating behind that inhibits some prints, or 2) paste wax!

But really I'll be re-buffing with the light grit scotch pad after assembly anyway.

And another tip I forgot to add before -- copper work-hardens as it gets hot, so you want to watch your cutting speed with the step bits and back off the RPMs as the bit gets bigger in diameter, else you will find the bit just... stops... cutting. It's not the bit, it's that the cutting edge is moving too fast and heating up the metal.

And use your step drills from the back of the plate as much as you can - the cuttings spin around and make nice patterns around the holes, but the scratches aren't consistent enough to be an attractive decorative technique!

...and another picture - the OEM bottlehead top plate with the 12x12 copper plate. I have to say again that the OEM plate is a very nice part, beefy and well-finished. I went ahead and drilled a hole for the volume pot I will be leaving out of the build for now (this will have 2 inputs with a rear switch and be driven from a preamp) - I plan to cover the center hole on top with the square Bottlehead logo plate, up front and proud.

Thanks for the comments guys!

/pRC

aka, Randy Carter - industrial designer gone bad in Raleigh NC
 

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Won't be making much progress this weekend -

We'll be riding in the 'Blood Sweat and Gears' event outside of Boone NC. For you guys on the west coast, Boone is along the Blue Ridge Parkway and has three ski areas nearby, so that means mountains. We'll be burning up some calories and then drinking excellent beer to celebrate. Sunday is the traditional 'recovery' ride.

Getting way off topic, here's a picture of my ride - a RANS high-racer recumbent with 650b wheels (fat, fast randonneuring tires and disk brakes plus fenders). If I took all my commuter crap off the bike it would weigh about 23 pounds with the carbon fiber seat, but what's the fun in that. This pic is taken at the top of Georges Gap, a 2+ mile 6-8% climb.

 

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That is a cool ride. I've been thinking about trying a recumbent, as my bionic ankle doesn't like the traditional pedaling ergonomics much.
 
Made good progress this week and have the new amp working - had normal measurements at all points during checkout, with the exception of a bit high B+ due to choke and final cap changes in PS...

Changes to circuit:
* Bottlehead output impedance switch kit
* 2PDT coin-silver power switch - wired in parallel
* Triad CX-7 chokes instead of 270R in PS
* Mundorf 33uF film cap instead of 100uF electrolytic in final PS stage
* 4PDT coin-silver switch for dual inputs at rear of chassis
* Fixed 75k ohm input resistance - tantalum
* Mundorf silver/oil coupling and parafeed caps
* Mundorf 22 ohm power resistors in cathodes for 2A3s
* Caddock 249K resistors - 2A3 grid to GND

Still need to tune the hum pots, but the hum is low on the Azura/Lowther front horns and I though I would let it burn in a bit first with the Sovtek 2A3s; I do have some other tubes to try in the wings as well. Will also go back in and check for tank resonances at c2 in the PS (the 33uF film cap).

Thanks Paul Joppa and Doc B for a fun kit that sounds great!

Here are a few pics --

 
That is absolutely gorgeous work !  I knew there would be some vibration damping somewhere.  But that top plate will be hard to vibrate anyway.

Congratulations on another beautiful piece of work.
 
pRC said:
* Fixed 75 ohm input resistance - tantalum
Click to expand...
I'm hoping that's a typo.

This amp is looking pretty cool, it's also refreshing to see the amount of time being spent to get it right!
 
Caucasian Blackplate said:
I'm hoping that's a typo.

This amp is looking pretty cool, it's also refreshing to see the amount of time being spent to get it right!
Click to expand...

Oops, good catch - 75k ohm - fixed the original post!

Thanks for the note, and the helpful tips in earlier projects you posted on the Stereomour..

/pRC
 
Bonzo said:
Wow! What a beauty!
What did you use to cover the chokes? Looks very nice!

Ciao!
Click to expand...

Thanks! The choke covers are from an office supplies store, I bought them years ago. Stackable half-high pencil holders for paperclips, etc. - steel. I painted them with the same 'dusted'  hammertone in silver used on the power transformer bell.

Other crazy changes -
* Used Chris Ven Haus foamed EP insulated 20ga solid copper for about half the wiring
* Green wire for filaments for that old-school look, easier to suss out low from high voltage lines...
* JB welded 1/4-20 bolts to the underside of the top plate with chassis hardware to hold the top plate down onto the base. Top plate is pretty dead with all that contact area around the sides.

/pRC
 
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