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AutoStylin Magazine Tech Corner article, August, 2004

Though the legendary sound quality and high-end home audio pedigree of the Milbert BaM-235 mobile vacuum tube amplifier makes it at ease in even the finest home audio systems, its installation and particulars are as typical and tame as any other car amp.

Golden Sound, Literally

Earl Zausmer garnered much media attention several years ago, appearing on television, in hundreds of newspapers, and winning countless awards, including "best of show" and even "best sounding car on earth," with his show-stopping BMW540i. Among many other car-audio-firsts, Earl's car has two gold-plated Milbert BaM-235 amps. Many more pictures are available online at

The media exposure has been wonderful, but the exceptional, involved, champion image and aura surrounding his car perhaps projects an exception and not a rule regarding the rather commonplace system requirements, installation, and maintenance of a BaM-235.

The BaM-235 is actually warm, lovable and installer-friendly, and with little effort, it will benefit and improve the sound of nearly any car audio system.

Musicians love tube sound quality for good reason, and the 100% all-tube signal path in the BaM-235 lets you hear why, daily. Its natural sound and startling realism demand that even if you sell your car you'll keep your amp. The BaM-235 sports a long, proven track record of in-car reliability and a design pedigree that dates back decades into high-end home audio. Cost-no-object American design and build quality means the BaM-235 works well and sounds superb.

Two Small Differences

There are only two differences between installing a Milbert BaM-235 mobile vacuum tube amplifier and anything else: One is that it mustn't be mounted upside-down--it needs adequate airflow for cooling, just like any amp. The second: cold tubes take a few seconds longer to warm up gradually to listening levels, but the happy side-effect is an end to those annoying blaring-radio startups.


Its ten vacuum tubes consume about 5 amperes at idle --about the same as one low-beam headlight-- and about 20 amperes at full, constant power. With (naturally peaky) music the actual draw at full power is typically much less: A standard car battery can power the amp at normal listening levels for seven hours (we've used it at shows this way). For protection, the BaM-235 is fused and internally current limited to 25 amperes; however, this safety feature rarely activates outside of testing.


The tightly regulated and properly designed BaM-235 does not appreciably benefit from large external energy storage capacitors. Sure, having a bigger reserve never hurts, but the BaM-235 already has its own massive internal reserves, properly designed in the best place, right where the power is needed most.

Since 1986, the BaM-235 internally stores up to fifty joules of high voltage energy, which is regulated and recharged at supersonic frequency. This properly located, huge reserve audibly matters, and it's one reason for the BaM-235's unmatched sound quality.


The BaM-235 weighs about 20lbs. and it is a bit larger than a shoebox. It should be mounted to a sturdy surface. We provide neoprene well-nuts and mounting hardware that give a slight shock resistance while providing dependable mounting. Patented circuitry precludes microphonics (noise due to mechanical vibration). Tapping, hitting, shaking, jostling, dropping, or kicking a playing BaM-235 will not reveal even the slightest hint of microphonics. Impervious.

Thick metal chassis and sturdy base plate. Machined stainless fasteners that tighten without stripping out. Heavy duty barrier strips accept large lugs. Gold over Copper on durable Teflon chassis-mount RCA connectors deliver uncommon grace and sturdiness. Quality input level control adjusts sensitivity down to 50mV or up to ten volts. Chassis fuse in the negative line prevents the amp from inadvertently entering the car's ground circuit. Use #8AWG wire with a fuse near the battery in the positive line. Quality parts throughout. Teflon wiring. Meaty, better than mil-spec circuitboards. Conservatively operated tubes exhibit lifetimes that routinely span ten years or more.

Various output impedances are available: 16ohms, 8ohms, 4ohms, 2ohms, and 1/2ohm. All of which are unconditionally stable: You can (but probably shouldn't) disconnect the speakers or short them out for any length of time, even at full power, without damage. Dare that feat with any other amp.

While the BaM-235 excels in sound quality, musicality, reputation and renown, sheer listenability and enjoyment, high-end home audio design-pedigree, proven in-car track-record, and outstanding durability and reliability, it's also at ease in everything from the world's finest to everyday car audio systems. We believe the BaM-235 will improve the musical enjoyment and emotional impact of any car audio installation.


We sometimes hear complaints that "it's only 35 watts," or, "it's not powerful enough for a subwoofer." These mendacious worries never come from owners and are usually mustered by those who may not understand proper audio system design or who haven't experienced the indescribable clarity, immediacy and airy naturalness, for example in Earl's car.

The BaM-235's tubes clip gently (unlike the unforgiving harshness immediately encountered when an output transistor clips from being overdriven) and its sound quality is generally richer and more wholesome than that of transistorized amplification. Consequently, the BaM-235 can subjectively sound like a much more powerful amplifier than its objectively rated 35 watts-per-channel might indicate. Owners report this time and time again.

There is also the "efficiency" of the speaker to consider: Proper design and build (powerful magnets, a pliable surround, etc.) make for an efficient speaker. Ratings from speaker manufacturers can vary all over the place, and one has to be careful when comparing speakers to be sure they are measured under the same conditions. It's been our experience that the better sounding speakers often have higher efficiency and are generally more expensive to manufacture.

Sound Pressure Level (decibels @ 1meter)
(@ 1 watt input,
at 1 meter)
  75 78 81 84 87 90 93 96 99 102 105 108 111 114 117 120 Approximate
Power (Watts)
Needed To
Attain SPL
Volume (in dB)
Using Speaker
of a Given
78 .5 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384
81 .25 .5 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192
84 .125 .25 .5 1 2 4 8 16 32 64 128 256 512 1024 2048 4096
87 .062 .125 .25 .5 1 2 4 8 16 32 64 128 256 512 1024 2048
90 .031 .062 .125 .25 .5 1 2 4 8 16 32 64 128 256 512 1024
93 .015 .031 .062 .125 .25 .5 1 2 4 8 16 32 64 128 256 512
96 .008 .015 .031 .062 .125 .25 .5 1 2 4 8 16 32 64 128 256
99 .004 .008 .015 .031 .062 .125 .25 .5 1 2 4 8 16 32 64 128
102 .002 .004 .008 .015 .031 .062 .125 .25 .5 1 2 4 8 16 32 64
105 .001 .002 .004 .008 .015 .031 .062 .125 .25 .5 1 2 4 8 16 32

While the output of a speaker (measured in decibels, dB) for a given amount of amplifier power is termed its efficiency, over the years marketing has emphasized wattage rating at the expense of efficiency. Both values are required to convey maximum meaning. Without a companion efficiency rating, wattage rating alone becomes more a damage limit guide of maximum power handling capability. It's unfortunate that this has become the default. The chart shows the huge importance of efficiency.

Actually, neither criterion will tell you how a speaker sounds to your ear or how much you will enjoy listening to it with a particular amp. The fact that a car has 200 watt speakers and a 500 watt-per-channel amp is not a criteria for loudness or enjoyable sound. For example, Earl's car has only 60 watts (from two BaM-235 amps) across four speakers above 100Hz.

In listening to systems that range from exotic to factory stock, we sometimes use audiophile grade interconnects and speaker wires and home quality speakers. The BaM-235 coupled with audiophile gear and high efficiency speakers can be very revealing. It is not kind of, sort of, maybe sometimes, or slightly better, but the improvement is large and immediately apparent. Veils vanish and things not previously heard leap out from the music: The ambience, the many nuances and intricacies, the breaths, sighs and shimmers all become audibly apparent. And you feel the emotion.

Drumbeats that were perhaps homogenous thuds in SPL-competition systems are transformed, in that the type of drum sticks, the sticking or playing style, and even the drum head material itself becomes discernible. New layers are revealed. Duet voices become independently distinguishable and harmonious, instead of blurred. A woman's voice in a recording you've heard many times can suddenly make the hairs on the back of your neck stand on end. That's the kind of resolution, detail and emotional impact which you can routinely expect from the BaM-235. In any system, the BaM-235 helps you feel music, and that is priceless.

Incidentally, the BaM-235 provides its power with exceedingly low noise. For instance, you can connect an efficient horn speaker to the BaM-235 and put your ear right up next to it at idle but not hear hiss or noise. The power from the BaM-235 is pure, clear, and distinct. This makes it even friendlier to highly efficient speakers.

Alternatively, our TC-2R and TC-4R vacuum tube active subwoofer crossovers effectively double the power of the BaM-235 amp (or of any amp) by channeling half the energy in the audible spectrum (i.e., below about 100Hz) to a subwoofer amp and speaker that can best handle those low frequencies. This is because about half of the energy in the audio spectrum is in the lower octaves.

With volume controls for each output jack, either of our two crossovers allows a BaM-235 to excel at reproducing the upper half of the audio spectrum. The TC-2R controls volume for subwoofer and two high frequency channels, and the TC-4R offers two additional high frequency channels. More information is available online at

Material herein added and updated constantly; presented for inspirational and educational purposes per Fair Use.

Last modified 17 Mar 2017

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