FS: Bottlehead Crack amplifier Special Edition Factory Build Custom Finish
Will Ship To: Anywhere
FS: A Bottlehead Crack headphone amplifier, in excellent like-new condition. All CCS ("Speedball") upgrades. This was factory-built (not DIY). The finish is a custom copper hammer tone. This was a special edition amplifier, using an ECC99 input tube (nice!), with custom CCS boards to handle the ECC99's different operating points. About as nice a Bottlehead Crack amplifier as you will ever see. Price includes Paypal and shipping in US48.
note: Bottlehead's factory build price on these is +$340. Wood base clear coat is +$30 and hammertone finish is +$40. http://www.bottlehead.com/smf/index.php/topic,4783.0.html
Some people have asked about the ECC99 tube. Here's some information:
There's no particular reason for using the ECC99 tube, except that Bottlehead had the ECC99 available, and it is a very fine tube, representing the best of modern design (the ECC99 is not a historical model, it is a new model released by JJ/Tesla about ten years ago). Bottlehead suggested the tube to me, and I was familiar with it and thought it a good idea. In general it is an upgrade, to the stock 12AU7.
Here's a discussion of signal triodes by Lynn Olsen, who is quite knowledgeable about these, for use in his to-the-limit Karna amplifier. Note that Olsen groups the (new production) ECC99 with the best of the 1960s tubes, while he regards the 12AU7 as being relatively less attractive, with measurable higher distortion particularly in upper harmonics.
1) Select active devices for minimum upper harmonic distortion. Although some devices have reasonably low second-harmonic distortion, the ear is not very sensitive to 2nd harmonic. It's the 3rd and higher-order harmonics that create unpleasant "electronic" colorations. D.E.L. Shorter of the BBC Research Labs and Norman Crowhurst both proposed weighting harmonics by the square or cube of the order in to reflect audibility and annoyance-factor, and it's a shame their suggestions were never carried out. To this day, it's the 2nd harmonics that dominates THD device measurements, but it's the ones that are higher than that (even though they may be 20dB lower) that we hear. That's why a THD spec, without reference to the complete spectrogram, is essentially useless, and not only that, potentially quite misleading.
In the absence of a full spectrogram, try and find a 2nd and 3rd-harmonic spec for the device you're interested in. It should be at least 20 to 30dB lower than the dominant 2nd-order distortion term. Failing that, there are some devices to avoid if you want to minimize the proportions of high-order harmonics - pentodes, beam tetrodes, transistors, MOSFETs, and IGBTs. That leaves triodes, which are not all the same.
The triodes that were the most popular in the Fifties (12AX7, 12AU7, 12AT7, and 6DJ8) were not in fact the most linear available. At the time, commercial amplifiers used at least 20dB of feedback, giving no incentive to retain big, old-fashioned octal tubes like the 6SN7 (size mattered back then). The 6SN7 came out of a prewar family of medium-mu radio tubes, starting with the single triodes 27, 37, 56, 76, 6P5, 6C5, 6J5, and then, the dual-triode 6SN7, introduced in 1940.
The 6SN7, used in millions of radios and early black-and-white TV sets, was replaced by the 12AU7 miniature tube in the early Fifties. Although the electrical characteristics appear identical in a databook (Rp=7700 ohms, mu=20),the 12AU7 has quite a bit more distortion than it's octal predecessor. In fact, the 6FQ7 miniature was introduced as a direct-replacement for the 6SN7, carrying forward the low-distortion characteristics of the 6SN7, but saw very little use in Fifties hifi equipment as a result of higher cost.
Tubes in widespread use in the Fifties have acquired a "cult" status that has nothing to do with performance, or even sonics. Believe it or not, there were better tubes both earlier and later than the "Golden Age" favorites. The 6SN7 and it's predecessors have very low distortion, almost certainly because they were designed before widespread use of feedback.
The Space Age high-transconductance tubes of the late Fifties and early Sixties never saw use in commercial hifi equipment, mostly due to cost considerations. But commercial and aerospace-grade tubes were some of the best ever made for low distortion - and favorable distribution of harmonics. The 5687, 6900, 7044, and 7119 are at the head of the line for low output impedance, wide voltage swing capability, high current, and low distortion. As for modern tubes in the same family, I've heard good things about the Sovtek 6N6 and 6H30.
Speaking of exotic, for brave souls who aren't afraid of microphonics or 50 MHz oscillations, there's the exotic (and hard-to-use) WE417A/Raytheon 5842, and the truly over-the-top WE437 or 3A/167M. (Available in modern form as the Russian 6C45pi.) Beware of large sample variations (20% or more) in this family - it's very hard to get matched pairs unless you burn them in for 100 hours and then re-test for gain. It isn't that quality is bad - the same problem is seen in genuine Western Electric 437's - it's that ultrahigh transconductance specifications requires extremely close-tolerance manufacturing. By comparison, the more moderately specified 5687, 6900, 7044, 7119, and ECC99family is typically quite well matched, with variations of less than 5% from sample to sample.
For those that are still interested after all this detail, Olsen has more, including his personal lab test results. This is why I knew about and chose the ECC99 tube.
Device, Topology, and Harmonic Spectra
All of the foregoing applies to triodes — conventional RC-coupled, transformer, choke-loaded, SRPP, and active-load circuits such as mu-followers. It does not apply to: cascode-connected triodes, pentode, bipolar transistor, or MOSFET’s. This second group of devices do not have the simple square-law transfer characteristic of triodes; instead they have a much more complex exponential curve, and that translates into a much greater proportion of upper harmonics.
When you compare device specifications, take a close look at the ratio of 2nd to 3rd harmonic distortion for a basic single-ended circuit. Low-distortion triodes (6J5, 6C5, 6SN7, 6CG7, the new JJ ECC99, and direct-heated types) have much lower 3rd harmonic; for devices in the second group, the 3rd harmonic will equal or exceed the 2nd harmonic. Medium-to-high distortion triodes (12AU7, 6DJ8) fall between the two groups. (This is why the 6DJ8 is known for a "high-definition" transistor-like sound - the distortion spectra isn’t that different!)
Edited by nlaudio - 12/7/13 at 11:20am