Questions for an upcoming Crackatwoa build

[larcenasb]

Firstly, I can't even describe how excited I am for this build! I've built a few Cracks over the years, but I'm finally moving up to the more sophisticated version. I've heard the Crackatwoa at Bottleheadquarters back in 2020, so I know what to expect. But before I build, I just have a few questions to clarify some things.

1) I want to test out different output caps, including some 2uF ones for preamp use. Is there anything inherently wrong or ill-advised about using leads with alligator clips to attach to the output caps, for easy swapping? (See 1st photo for my idea) I would zip tie the leads to adhesive mounts before they reach the terminal strips, so as to not stress the solder joint. And I'd use velcro straps again to make swapping easy. Also, I have a bleed resistor that Paul Joppa told me about a while back to use for safe handling.

2) Is the relationship of output cap capacitance to frequency response—based on impedance of the headphones—the same as the original Crack? (See 2nd photo) So, with my 600-Ohm AKG K240 Sextetts, I'd be fine going down to 33uF caps? Parts Connexion has a capacitor sale right now, and there are ASC 60uF 500V film caps for $4.49 (91% off), so I'll likely go with those. https://www.partsconnexion.com/ASC-81958.html

3) Lastly, I notice there are resistors going from the balance pot to the volume pot. What's that for? Does it make padding the pot with 75k and 33k resistors, like on the Crack, unnecessary, or is it still advisable to do to get more usable volume range?

Any help is greatly appreciated. Thanks so much!

[Paul Birkeland]

The resistors between the balance pot and volume pot are part of the balance control circuit.  You shouldn't need the padding resistors since you have a balance control.

I'm not a huge fan of clip leading coupling caps because clip leads can pop off, then you could potentially lose your ground reference and send DC to whatever is attached to the Crack.  I would at least use some hot glue to temporarily glue in some barrier strips or something so you have a solid place to put those caps.

Since there's so much space in the C2A, sticking to 100uF film caps is pretty easy!  As a bonus, you could also just use those for preamp duty.  There are plenty of solid state preamps with 100uF+ coupling caps sitting at the output. I would not use the ASC caps because they have radial leads, and I've designed the layout for axial caps. Inventory is pretty thin with the usual suspects, but you could grab these:
https://www.ebay.com/itm/331366732405?hash=item4d26ff1275:g:BcgAAOSw1vlUum3q&amdata=enc%3AAQAHAAAA4D5vCO%2BS8QiN63jnl852R2oWgn3CqGZUKh4Vnj%2BYz2SAayGwVwh9ULA%2Fp8p7pA8%2BsuPE7mEi9QTX%2B5LFW1WkJXzOREcmm8Z1DUsS1BdUXDfnzchqxVBbOb2xy5F5BWxsqxXH9o%2ByOUBdm%2FUtcoCLglHsfib8d%2B%2B3AuaUw2W%2B0Kq%2B4FmPwesdudTj5x6PP9XdLdLAbnkmDzRTXWYEX69HG%2BFy7Hij2qIo5qdaJe3gI2U%2FZWCQsxGbgVxaCVMjUZzomziU35fsPD7EwY8ZC%2FVwJmIwb49unjfQTKCf4cew5yED%7Ctkp%3ABk9SR66TiPjsYA

[larcenasb]

Thanks for your replies, Paul!

Sounds good, I won't pad the pot.

But the price of the ASC caps is so tempting! And I wanted to try out some Auricap XO 2.2uF caps for preamp use, since I have them. Wouldn't it be okay to solder a wire to one lead and route it to the other side? (Please see attached pic)

Also, are you the sole designer? I was going to make a rear badge for the amp. Is it okay if I add, "Designed by Paul Birkeland. Assembled by Lowell Bryant."?

Thanks again!

[Paul Birkeland]

You might also be able to just glue the ASC caps to the chassis plate, as they are rather large in diameter and that's a lot of gluing surface. 

Although I did design the Crack-a-two-a, lots of the tech that's in there comes from the other Paul as well.

[larcenasb]

Wait, can't I just use the zip-tie mounts? I was planning on using a velcro strap with them, for easier swapping.

And so I'll put, "Designed by Paul Birkleland & Paul Joppa", is that okay?

[Paul Joppa]

Most of our designs, especially the more recent ones, are collaborative to some degree. PB was clearly the lead designer of Crack-a-Two-a, and I'm comfortable with him getting the credit. "Design by Bottlehead" is a good option too.

[larcenasb]

You're both so selfless. Well, I think a Reduction will be my next kit sometime early next year, so I'll have a rear badge for that too, "Designed by Paul Joppa"

[larcenasb]

Talking more about it with PJ, it was decided that it's best to say "designed by the Bottlehead team," as they are very collaborative. I of course only suggested crediting them by name on my tags out of respect for what they do. I really appreciate their focus on the team as a whole.

I'm going to start painting the top plate and bell cap of my Crackatwoa later this evening, and am waiting for the Tried & True wood varnish to come in the mail. Super exciting times.

[larcenasb]

Hello all,

Going through the parts checklist, I have all the resistors, however, one pair is missing a color stripe. The manual lists a color sequence of "brown, orange, black, gold, brown" for the 13 Ohm 1/4W resistors. Mine are missing the brown stripe at the end (see photo). Is this okay and not a cause for concern?

Also, is it okay temp-wise to put some 80 mil butyl sound deadener on the underside of the top plate? I did so in my Crack kit because some of my 6SN7 tubes (via adapter) are a little microphonic but otherwise sound great. The sound deadener really helps to minimize the microphony when I use the volume knob. It's been okay for the 6 years I've had the kit, but does the c2a run hotter? Just making sure everything will be okay as I build. Thanks!

[Bourney]

By my reckoning a 4 band brown orange black gold resistor is 13ohm @ 5% tolerance, while a 5 band brown orange black gold brown is 13ohm @ 1% tolerance.

Not sure if the tolerance is an issue for this application. Probably not and your multi-meter will tell you how close you are to 13ohm. 13ohm + 5% = 13.65ohm. Less than an ohm.

The wattage may be more of a concern, I'm not sure how you can check that. Probably can guess it based on the physical dimensions.

[larcenasb]

Thanks for your reply, Bourney.

Gotcha, still 13 ohm, should be fine then. But if not, someone please let me know.

[Paul Birkeland]

The tolerance on the 13 ohm resistors isn't too critical.

[larcenasb]

Alright, sounds good. Thanks, Paul!