Crack-a-two-a Build Log

[L0rdGwyn]

I am starting a build log for my recently purchased Crack-a-two-a and TwoQuiet attenuator.  This will be my second Bottlehead build, the first being the Crack + Speedball.  I was really happy with the Crack, loved it actually, so I decided to sell it and upgrade  FYI I'm no engineer, just a guy who likes to build stuff and knows how to use a soldering iron.

I'm going to be adding some boutique parts to the build, mostly for aesthetics, including new RCA jacks, tube sockets, isolation feet, and volume knobs.  Going to be about a month until I get my hands on the kit, so everything will be planned to a T by the time it arrives.  If it goes as planned, should be pretty sleek!  Also going to add some film output caps after getting used to the stock sound.  Never did this on my original Crack, so curious to see what impact it actually has.  I have a large selection of input and output tubes to roll when it is complete, as well as some Sylvania Gold Brand 6005 shunt tubes to try.

One aesthetic choice I am considering is tube shields for the 6AQ5's.  For the engineering-inclined, will tube shields have any benefit on the shunt tubes besides heat dissipation?  I know they are also used for RF isolation in driver tubes.

Thanks for checking out my build, I will post here as I go along!

[Paul Birkeland]

The 6AQ5s aren't particularly sensitive to noise pickup in this circuit, but there's no harm in shielding them.  Some of the anodized tube shields look pretty cool.

If you're buying different sockets, I'd recommend staying away from the teflon ones.

-PB

[L0rdGwyn]

Thanks, PB.  I'm picking up some vintage shields made for 6AQ5's, gonna polish 'em up nice and shiny and I've heard the horror stories with the teflon sockets, I'm staying away, most likely going with Audio Note ceramic sockets.

[L0rdGwyn]

So, while waiting for the Crack-a-two-a kit to arrive, I thought I'd work on another project in preparation for its arrival

Since I am moving from the Crack to the Crack-a-two-a, I decided its time to turn over a new leaf and roll some 6SN7's alongside my 12AU7's.  So, I needed an adapter.  I picked up a Garage1217 6SN7 to 9-pin adapter, having read they are the highest quality around.  I wasn't a fan of the exposed look of it though or the lack of insulation.  So, I decided to fashion a new base.

I used a rotary tool and diamond burr attachment to shave down the ceramic on the sides of the adapter.  I picked up a red phenolic octal tube base, cut out the base pins, and sanded the interior.  I then used some thin cyanoacrylate adhesive to bond the base to the adapter.  Voila!

A simple project but really happy with the results.

[adeep42]

NICE

[Paul Birkeland]

Yeah, not sure if it was you or someone else who posted a photo of that base elsewhere, but I was surprised to see exposed metal that one would assume connects to socket pins.  The voltages in the Crack aren't that spectacularly high, but that's certainly not always the case!

For a less labor intensive approach, silicone RTV from your autoparts store will cover that up and is non corrosive, but what you did looks a whole heck of a lot better.

[L0rdGwyn]

Thanks, was going for functionality and aesthetics, so the extra effort was worth it I think.  With the materials in place, only took 30-45 minutes.

[JamieMcC]

Nice idea maybe a circle of thin clear plastic like from a cd jewel case could be used to cover the exposed metal bits on the base.

[L0rdGwyn]

Received my vintage EBY 6AQ5 tube shields today, these are nickel-plated brass, going to attempt to polish them to a mirror for the final build using a Jeweler's Rouge compound.

Edit: added a pic after 20 minutes of polishing.

Edit 2: pic after polishing to chrome-like finish.

[xcoolhandx]

Well done Sir,subbed 

[2wo]

Nice....John

[L0rdGwyn]

We're getting close to Crack-a-two-a ship day!  To fill some time, thought I'd post here some of the components I'm planning on putting in this build.  As I said before, I recognize many of these "upgrades" are unnecessary and likely will not significantly improve performance of the amp, but I wanted to make this feel like a real statement OTL amp, so some premium parts are going in regardless.

Sockets - took me a while to get these together, but I am using Japanese Azuma gold-plated ceramic sockets.

RCA's - Audio Note silver-plated RCA jacks.  This was an aesthetic choice really, realized after the fact that getting nickel plated would have saved me the trouble of cleaning the sockets when they oxidize, but what's done is done!  Audio Note claims silver jacks have magical audio properties, so I'm gonna go with that.

Output capacitors - Mundorf MCap EVO Aluminum Foil and Oil.  Fitting these bad boys may take some effort, especially when the TwoQuiet attenuator is installed, but from my measurements it is doable.  I'll be building the amp stock and giving it some time before these go in.

RC filter capacitor on driver C4S board - Mundorf SUPREME EVO Silver and Gold.  Again, gonna build stock then put these puppies in at a later time.

Shunt tubes - picked up another relatively cheap set of shunt regulator tubes to roll in, General Electric 5-star 6005's.  As of right now, I'll have the stock tubes, the Sylvania Gold Brands, and these to roll through and compare.

Volume knobs - Audio Note 25mm Polished Chrome.  The top plate of the amp is going to wet-sanded and polished to a mirror finish, so I thought having matching chrome knobs would give the amp a real premium look.

Feet - Cardas Golden Cuboids with Dayton Audio spiked feet.  For the sake of convenience, picked up these myrtle wood cubes from Cardas, sold as audio component standoffs/risers.  I am going to sand them, stain them with the base, glue them inside the four corners of the amp, and install the spiked feet.

I'm also reconsidering the socket shields on the 6005's.  While they might look cool, they aren't going to really serve a functional purpose and might take the look of the amp a little over the top.

Will post again when the kit arrives and I start some test staining.  Yup, I am going to do a test board first, have a few different colors I'm considering 

[L0rdGwyn]

Time to update the ol' build log!

Been pretty busy these past few weeks (grad school and all that), so I wasn't able to make any meaningful progress until these past few days.

I was working on polishing the top plate to a mirror finish, got it 90% of the way there but was not satisfied with the result, so that has been put on hold while I gather some materials.  In the meantime, I've put together the base and started the stain.

Pictured you'll see my test board.  I really like General Finishes' gel stains and was deciding between either Georgian Cherry or Antique Walnut.  Decided to go with the walnut.  The base was assembled and I added my spiked feet combined with Cardas "Golden Cuboids" myrtle wood blocks as substrates.  Hopefully they hold up, putting Elmer's Wood Glue to the test, that transformer is no joke.

Finished the staining, really pleased with the look.  I was originally planning a second coat but with these gel stains it won't get substantially darker (in my experience), so just one will do.  Going to let it cure for two days then add the top coat!  I'll post pictures of the top plate when I get it right

[L0rdGwyn]

Well, I got my mirror finish.  It isn't perfect, but the imperfections aren't noticeable at a distance and I'm not sure it ever could be.

Here are some photos with the parts mounted to the chassis.  Had to sand the interior of the octal cutout to fit my Azuma socket.  All of the sockets are bottom-mounted.  Also subbed in some shiny hardware.  Base still needs its top coat.

[Paul Birkeland]

It sure is coming along nicely!