Bottlehead

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Company name Bottlehead Corp.
Address PO Box 2786, Poulsbo, WA 98370, USA
Official site Official website
Email address bottlehead@bottlehead.com
Telephone number +1 360 662 1386
Fax number No fax number available

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General information

Bottlehead web site (December 29, 2009): Bottlehead Corporation's website is dedicated to do-it-yourself tube audio. We believe that knowledge is power, and that the best audio systems belong to knowledgeable do-it-yourselfers.

History

Bottlehead website (March 2001): The Bottlehead Technology Story Pretty much all the Bottlehead products incorporate some uncommon technologies which contribute to the exceptional sound. This paper describes the main ones.

Parallel Feed In parallel feed, the usual airgapped output transformer is divided into a DC current-carrying plate choke and an interleaved output transformer, coupled with a high-quality capacitor. The choke does not need to have interleaved windings, so it can be made to have a large inductance (good bass) and still retain a small capacitance (good treble). The transformer can be much smaller because it does not have to handle the DC current, and the smaller size permits it to have good high frequency response without excess interleaving and the capacitance that results.

Sonically, this means deeper, cleaner bass without sacrificing the highs.

Nickel Core Transformers Nickel cores are desirable because they have a much smaller magnetic hysteresis area, which means less distortion, and also because they have a higher permeability which means more inductance (better bass) with fewer windings (better treble). They are not really practical for air-gapped transformers because nickel alloys are easily saturated. In order to use them in conventional transformers, the size must be increased and - more important - the air gap must be increased so that the permeability advantage disappears. But with parafeed topologies, the interleaved nickel core transformer is able to fully deliver its potential.

Sonically, it gives significantly greater clarity throughout the frequency range, producing detail without the glassy, strained, or grainy character that usually goes with increased detail.

Direct Coupling The obvious advantage of direct coupling is that it eliminates a capacitor from the signal path. Since virtually all capacitors have audible sonic flaws, this must be an advantage. It requires a greater power supply voltage, which means only low-powered amplifiers and preamplifiers can be done this way if voltages are to be kept reasonable.

A second advantage not often appreciated in the literature is that the output tube can be overdriven without shifting the bias point. All conventional capacitor-coupled circuits will undergo a large shift in operating point when overloaded, even momentarily. This produces a “crunchy" sound, which is highly prized in a guitar amplifier but not much appreciated in an audiophile system.

Sonically, the result is a more graceful overload characteristic, and an overall cleaner sound with less apparent distortion.

Active Current Source Loading By loading a triode voltage amplifier with a high impedance, such as an active current source, it operates in a more linear region and produces less distortion. This is especially important in a driver, because the best sounding amplifiers appear to be those whose distortion signature is dominated by the output tube. In other words, the driver distortion should be less than the output tube's. This is surprisingly difficult when extremely linear output tubes are being used, such as the 2A3 and 300B. Very few small-signal tubes are as low in distortion.

A second advantage, not as widely appreciated, is that the current source provides a great deal of isolation from the power supply. A good current source will provide a high impedance over a wide range of frequencies including radio frequency interference. It is otherwise extremely difficult to stop RFI that is carried on the power line, because power supply chokes have a leakage capacitance and the large capacitors used will also have some inductance at high frequencies. Isolation from the power supply also removes the power supply capacitors from the signal current loop.

Sonically, this gives a clean sound with clarity and detail. It also gives a large improvement in bass impact and solidity.

Cathode-Coupled Driver The direct-coupled power amplifiers (Afterglow and Paraglow) both supply the driver plate from the output tube's cathode. This reduces the peak output power slightly, but it also makes it impossible for the driver to take the output tube's grid positive, so Class A2 operation and its attendant distortion and bias point shift cannot happen.

Sonically, this means instantaneous recovery from transient overload conditions (which occur much more often than most people think). A short transient overload may be completely inaudible by itself, but the attendant shift in operating point and the time scale required for recovery are clearly audible.

External links

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