Bottlehead Crack

My ZMF Atrium arrived. Interestingly, while the 6XX seems to prefer the JJ input tube, the Atrium definitely sounds better with the cleaner Clear Top. What's interesting to me is that the CV4079s paired with the Clear Top sounds very solid state, like I go back and forth between the Crack and the RebelAmp now and they're more alike than different. And the Atrium sounds amazing on the RebelAmp, but the Crack opens out the soundstage, improves the imaging, and makes everything more beautiful. It's kind of incredible, I just had no idea tubes could do this.

 

Late to the party again. Tube rolling sounded like it was fun a decade ago, now with the state of the market it's just frustrating. TS 5998s for $300, TS 6080s and anything GEC nowhere to be found. Bummer. Really kills the value prospect.

Funny story, I just bought a cheap Raytheon VT-231 from some poor sod in Mykolaiv, Ukraine. If it gets lost in customs I'll just consider it a donation. Just doing my part to balance out the ratio of Raytheon products flowing between us and them

Anyway, got a bunch of mods coming. Looking forward to cracking it open again.

 

magic 8 ball says vtg siemens caps are too expensive now, but you could always go with muse ES cathode caps, and allen bradley resistors

 

Just an update on tubes.

So I received the following tubes as I noted having ordered previously:
* RCA 6AS7G
* Thompson 6080WA
* Brimar 12AU7WA

The Brimar I thought sounded a lot like the JJECC802 but better, a bit smoother and more pleasing. This became my default input tube.

The Thompson was a disappointment, honestly. Strong and clear but without the extra stage and magic of the CV4079s.

The RCA 6AS7G on the other hand sounded even better than the CV4079s in my opinion, just bringing everything those do but tweaking up the clarity and smoothness a bit. A small improvement, but an improvement nonetheless. One problem: they seem to have a defect where they produce a persistent buzzing in the right channel. When playing busy music it's not so noticeable, but play anything quiet and you can't not hear it.

However, in came a dark horse candidate. Over on Sonic Visions, @Lana had recommended a new production power tube called the Shuguang Treasure CV181-Z. And this thing, well... this tube doesn't seem like it's chasing the clarity and precision of the CV4079 or 6AS7G (which both get there without the harshness of the WA21). This tube is darker, more like the Haltron 6080 I tried that could give things a kind of extra touch of beauty and sheen without sounding thick and syrupy and maintaining the energy and expansiveness of the GEC and RCA tubes. Like you get the all the tube magic without any of the sense of sluggishness, everything still sounds clear and detailed. And it reigns in the treble a bit and prevents it from ever sounding messy to me, which as a treble-sensitive person is especially nice. The more I listened to it, the more I got addicted to the sound (especially after it burned in a bit--Lana warned it needs 50 hours to sound its best, or at least did for her). And the Shuguang/Brimar combo sounds amazing on all my headphones! Including the 800S which finally came back from Sennheiser repair (though the tribulations I had with Sonova customer support are another story). I heard that the Crack wasn't that good on the 800S from people online, that it didn't have enough power. I've not compared it with more powerful tube amps, but at least to my ears the 800S on the Crack, particularly with the Shuguang helping to tame the highs, just sounds epic.

In addition to that, given that tubes eventually wear out, I was worried about the inevitable prospect of any given NOS tubes prices launching into the stratosphere before they become completely unobtainable. Granted that I like the Brimar the best, but if I had to live with the Shuguang/JJ combo I would not complain at all, so that gives me a certain amount of comfort.

 

Double choked up on a Thursday afternoon. I doubt the second made much of a difference over a single choke, but it was only $13 and I knew I'd be able to make them fit, so might as well. They're grounded to the chassis via a fifth connector.

- 2x TRIAD C-7X chokes
- 3x Panasonic 100uf film output/last PSU caps
- 3x 2.2uf WIMA PSU bypass caps
- ALPS pot w/ thicc boi Connex knob and shielded/grounded Mogami input wire
- Schottky diode board
- Diode ground-breaker

The new PSU film cap is such a precise fit longitudinally that I had to file down the solder joint sitting against the wood of the chassis so it wouldn't contact. It sits higher than the others because I 3D printed a 6mm high ABS spacer to get it to clear the input cable running underneath. All 3 caps are stuck to the plate with 3M VHB tape which is holding up just fine to the heat so far after over a week of almost daily use.

Impressions:

Overall I'm chuffed. I gave up a Jotunheim 2 in favor of the unmodified Crack because of the extra life and character it offered at the expense of some of the technicalities of the Jot, but after the mods the Crack seems to have clawed a bit of those back. Unfortunately I don't still have the Jot, nor do I have any experience with anything TOTL so I don't really have much to compare it to, nor can I speak on any singular mod's difference as I did them all at once.

What I can say is that while the mods didn't "transform the sound" or turn it into a new amp or anything (which I'm glad for), they certainly did enhance things. I'd agree with the 15-20% figure mentioned previously here, which, given the going rate of the kit, made the <$150 outlay and the few hours of time worth it - the value proposition of the mods was consistent with the amp kit itself.

The most immediate differences were in the noise floor and the bass. Before, I got a bit of induced digital noise as I'm using ethernet over powerline on the same circuit. 100% gone, I think mostly due to the diode ground-breaker. The stock GE power tube that originally came with my kit is noisy, microphonic junk, but it's actually quieter now than it was before mods and would actually be acceptable to use if I didn't have a better alternative on hand. With new, high quality tubes, the ONLY noise is a slight amount of hiss at over 3/4 volume which is just fine with me. The quality of the bass though - complete transformation. It went from pretty blubbery and diffuse to actually punchy and far more controlled. I'd be eager to compare it side by side against a decent SS amp - I didn't realize how much I missed it.

Beyond that, not only does it retrieve a bit more detail just by itself, but it sounds more coordinated and lighter on its feet, so to speak - like it's better at doing more than one thing at once. The "sub-melodies" in the background (I don't know what the proper verbiage would be) that the stock amp would gloss over a bit (seemingly in favor of rendering the foreground music) are noticeably more refined now, like they're being played themselves by a completely separate amp. Perhaps this is what's meant when people say gear is more resolving, beyond just retrieving more details? Like I said - unfortunately I don't have any experience with anything TOTL to compare.

The tonal character didn't change all that much - I chose output caps that were not only decently-rated compared to the various base model audio-specific ones, but were reportedly more neutral-sounding as well so I could just listen to the tubes - and since that was always the stock Crack's strong suit, I'm glad for it.

I've paired it with HD650s and 600ohm DT880s, and am feeding it with an as-of-yet unmodified OG Modi Multibit, so my next step is to see how it's held back by the DAC. I listen to mostly electronic and metal like Tool, Heilung, and Amon Amarth, so I value coordination and dynamics over ultimate resolution, detail, and soundstage specifics, so I'm fortunate not to be too picky. I found it all to be worthwhile, and my upgrade-itis in eyeing the Lyr+ has been sated for now.

As for the tubes:

The jury's still out, but I'd say the Tung-Sol/Chatham 6AS7G I'm running is another across-the-board improvement over the stock GE 6AS7GA, even if the latter wasn't noisy and microphonic. With the rest of the amp in stock form this one tube change alone offered better tonal character and a noticeable improvement in layering - I didn't even know what that meant until I actually heard it. Worth the $75 I found it for. Plus it looks cool.

As for the Raytheon VT-231, I would not have paid the going rate of $150 for one of these. I was curious what the added space would be like, and while it's nice, it's not as much of a revelatory night-and-day difference others have described it as, and I think it gave up just a bit of the natural tonal character the stock Baldwin (Raytheon?) 12AU7 tube offers. It sounds like more of an even tradeoff so far - but then again, after a certain point I'm not too fussy or particular about soundstage width as long as the imaging is precise and solid enough, so maybe it's just not for me. I was lucky to find the tube for fairly cheap from a poor sod in Mykolaiv, Ukraine so I'm not out much. I'll do some more comparing.

Oh, and the Garage1217 6SN7 adapter is extremely high quality. Highly recommended. I 3D printed a TPU sleeve to cover the kinda-exposed contacts - it was a simple design, but I can provide the STL file if anyone would like to print their own. I think I need to make it a bit thicker first though.

 

I've been revisiting messing around with my amp lately and it brought me back to this thread to see what people are up to. I thought I'd take a minute to say what I've found that works. I've been using my amp as my daily sound box for a couple years now.

The 5998/421A tube makes a big difference. I still found a 6336A to be better if you feel like slapping in a teeny antek toroid or something to power the 5A heater. I liked the 6336A more than a 6528. These tubes sound better because the transconductance is MUCH higher than a "standard" 6AS7G/6080, which this amp seems to like. You get better bass and much better clarity and it removes the syrupiness.

A stepped attenuator makes a world of difference. I would get a ladder or series type to avoid the changing input impedance of a shunt. I was using a TKD black magic shunt pot for a while which introduced background noise at certain positions. I'm finding much better transparency using a 24 position stepped attenuator. they're also fun to build if you're so inclined. otherwise, goldpoint seems to be a reasonably priced albeit expensive option. 50k or 100k both work fine.

Driver tubes will make or break the amp. I run 6SN7's in mine and I've amassed what I consider to be every "great" VT-231/6SN7. I've found each one sets its own character for the amp, and here are a couple examples from ones I keep coming back to. My favorite is a 1941-1945 RCA grey glass. This tube widens the soundstage considerably and has the richest sound by far. Sylvania "bad-boys" (1951-1953 3-hole tube) are very non-fatiguing with huge top end extension and the deepest sub-bass I've ever heard. Tung-sol black glass round plates are extremely linear and refined. Ken-rad staggered plate "VT-231" tubes have quite visceral bass but not much else going on. Raytheon tall plate "VT-231" tubes are warm and very gentle and pretty sounding, but not much bass at all. MELZ 1578 tubes (good luck getting these at this point) are perhaps my second favorite tube and sound extremely lively.

If you want to do some cheap modding I would play with bypass output caps. I found the secret sauce for my amp was adding some random cheap vintage 1uf 160v paper in oil caps in. I would only do this after replacing the output caps with film types. I would also try messing around with this concept before spending a ton of money on tubes.

I also upgraded my crack to a crackatwoa so I can personally attest to how much adding shunt regulators (or basically just *having* a regulated power supply at all) gives this amp. I still haven't seen anyone add in any form of regulation to a crack in the original case. I'm quite confident that there's space for the bottlehead shunt regulators if one doesn't use those huge triad c7x's. If anyone's thinking about building a crack from the ground up you could check out the 21st century maida regulator.

Adding film caps to the power supply doesn't do much. Swapping the bias LEDs for the driver tube to schottky diodes or NIMH batteries or RC does a small noticeable amount but I didn't find it particularly special, and ended up swapping the LEDs back in for now. Maybe I'll revisit this at some point.

My last note is on background noise/hum. I've found that rectifying the driver tube to DC is a simple fix for most of mine. Outside of this I added a simple AC line filter, used shielded wire for all of my signal lines, and used a drill to twist my heater wires ultra-tight. And finally just for me since I use a 6336 tube I added 100R grid stopper resistors to fix some oscillation.

 

Nice review and yeah those MELZ tubes were some of the best I'd heard on mine. In combination with 5998 pure sonic bliss. 5998 are even crazier to find for a sane price these days though...

 

I mounted my choke yesterday and didn't see anyone else posting that hardware you need to do that, so I thought this might help some people. You need:

2x 10-24x1 3/4 screws
2x 10-24 lock nut washers
2x 10-24 screw nuts
1x Triad CX-7 Choke
1x 3/16" drill bit

You'll also need standoffs off approximately 32mm in length. I 3d printed my own as I couldn't find any at the hardware store, so here' a STL file I quickly made if you have access to a 3d printer. Get it here https://www.thingiverse.com/thing:6568666

Drilling through the steel is not nearly as bad as I thought it would be. I simply put tape over tube sockets and vent and drilled the holes top side up after measuring several times. Drill one hole at a time so you can verify using the actual choke that your spacing is correct. I'm getting 0 hum at this location. I also recommend using an old screw and *lightly* hammer it into the location you plan drill so that there's a small indent for when you start drilling.

 

Years and years ago when I had already built a couple cracks and was thinking of experimenting, I thought about getting four gigantic capacitors and mounting them horizontally sticking out from from the sides to make the case look like a monster truck...