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EM7 Amp, C4 Preamp, Single Cone Speakers.	(Click to enlarge)

This series based on the 6EM7 and 13EM7 encompasses 3 products denoted by their power rating in watts per channel.

• EM7-2.5 The original EM7 but with external rather than internal output impedance selection. This is the perfect starter amp for those curious about SE, no feedback sound. Plug and play. Cathode bias requires no adjustment when changing tubes. It simply takes care of itself. This is often called “Autobias”
• EM7-5.0 Same circuit as the 2.5 but with a larger, higher voltage power transformer. To achieve higher power the bias is easily measured and adjusted to get the best sound from any EM7 tube. This is often called “fixed” bias, which is a confusing term because you can vary it. It is more sensible to call this “adjustable bias” or more accurately “negative grid bias”. Since the EM7 is a very stable tube, you don’t have to check it often as the tube ages.
• EM7-12v Our top performing EM7 SE amplifier. Both the power transformer and output transformer are of increased size and power capability. This amplifier has more power, lower distortion and lower noise than typical 300B amplifiers.

Why the use the EM7 tube? The 6 and 13 EM7 are the same except for filament voltage, one being 6.3v and the other 13.0v. Since the filament winding is 13 volts, either tube can be used by reconnecting the filaments in series for the 6 and parallel for the 13. This highly evolved tube was specifically designed to be a vertical amplifier and output tube in large color televisions. The vertical output amplifier is a single ended, class A amplifier with a gapped output transformer, cathode bias and high current capability. To do it’s job it had to be both stable and linear. If not, you would be calling the service man all too often to re-adjust the vertical height (gain) and linearity (bias) on your TV set. The cathode is large and indirectly heated so there is no hum pot to null as in most Directly Heated Triode (DHT) amplifiers. The large cathode emits a lot of electrons thus increasing its current and power output capabilities. Low saturation voltage makes it more efficient than DHTs, which have large losses at maximum current.

Are the EM7-2.5, EM7-5.0 and EM7-12v Single Ended Triode (SET) amplifiers? Yes, all three are single ended. Furthermore, the EM7 tube is a real triode, not a triode connected pentode e.g., KT-66, KT-88, EL-34, EL-84.

Why is there only 1 tube per channel in the stereo amplifier, where’s the driver tube? It’s in the same bottle as the output tube. The EM7 is a dual triode where the two sections are widely different by design. The more familiar 6SN7 and 6SL7 are “twin triodes” where both sections are the same. The driver section of the EM7 is almost identical to the well liked 6SL7 driver. The output section, however, is quite unique. Where the driver section is capable of 1.4 watts dissipation, the output section is rated at 10 watts (that’s dissipation, not output but it does relate to output). The output section has a gain and output impedance almost identical to the 300B (mu=5, Rp=750 ohms). Rather interesting for a two tubes with 30 years between them.

How can you get 5 watts out of a single EM7 output section where a 300B only gets a few more but has a 40 watt dissipation and a much larger bulb? The EM7 is a very efficient tube that could not have been made in the 1930’s. During WWII and beyond there were many advances in tube design and tube materials. The now popular 6DJ8/6922 is another post war tube. While is was designed as a RF amplifier in a color television, it makes a great audio tube. We owe a lot to early color television and the tubes designed for it. The driver tube in the RM-245.1 is another popular color TV tube. Imagine what tubes might have been created if solid state TV had not come to pass. Incidentally, Sony was the first overseas company to buy a license from Bell Labs to make transistors.

Does this tube have long life? When I visited the Sylvania (Phillips) tube factory in Altoona, PA. I spent a full day conversing with the man who had managed the plant for the past 25 years. On the subject of reliability he stated “During the color TV era, our target lifetime was 10,000 hours minimum and we achieved that in many tubes”. Upon reflecting back to my days as a TV repairman I replaced a lot more horizontal outputs and damper diodes than vertical tubes. I made a point of noticing which tubes were original in the older sets and which tubes were replaced more often. I also noted which tubes stayed in my tube caddy and which were restocked often.

Is this tube easy to get and inexpensive? Yes to both. You can find them in small lots on eBay or get a matched pair from RAM TUBE WORKS for $60. If you decide on the eBay route you will have to buy several and match them for gain and current. I have a stock of several thousand and test them in lots of 100. The more available to match, the better the matches. 300Bs are also easy to get but several hundred dollars a pair.

What kind of speakers should I use with these amplifiers? There are many choices and efficiency is not the only thing that matters. Your preferred listening level is even more important than speaker efficiency. I have customers who listen as low as 65 dB SPL (Sound Pressure Level). They could use a very low (70 dB) efficiently speaker and be very happy. Customers who like to listen at 95 dB need a much more efficient speaker (95 dB). Radio Shack has re-introduced their analog SPL meter. It’s the most important thing you can buy to guide you on your journey in low power amplifiers. If you report your peak SPL levels and speaker sensitivity I can accurately tell you how much power you need and what amplifier will be the best for your application.

Are there any brands of speakers you would recommend? While developing these amplifiers I had the opportunity to hear them on many speakers. A member of Audio Circle brought his B-200 open baffle speakers, I had a pair of Omega 8 Grands, and some 1970’s Kenwood 4 way speakers with 15 inch woofers (they were a deal at the thrift store, had beautiful walnut cabinets and sounded pretty good). At the Stereophile show I heard a pair of Cain and Cain with 4.5 inch Fostex drivers and had the opportunity to hook up my EM7 to the ZU druids. These are all fine choices and are popular among current owners of EM7 amplifiers.

What speaker matches the EM7 most closely? I cannot tell a lie, ours. During the development of the EM7 I noticed several popular single driver speakers used the Fostex FE-103. I happened to have some in my garage and thought I’d better have a look at this driver. It is really quite remarkable and has a long history of hobbyist applications, i.e. it is very well known and for good reason. The cone is light enough and the magnet large enough to have high efficiency for 93 dB/watt. I tried them in a sealed cabinet and liked the mids and highs but the bass was severely lacking. I was afraid I would have to incorporate a sub woofer. I had a go at a ported cabinet of small size and was astounded at the difference. Now I had bass down to 50 Hz, but it sounded thin just like the open baffle and the speakers I had heard at the Stereophile show. These speakers sound thin because they start gradually rolling off below 300 Hz. There’s not much one can do about at fixing that at the speaker end without harming the efficiency and speed by making the cone heavier. Rear loaded folded horns can't do much in this region either. I gradually came to the conclusion that SET lovers were suffering with very thin lower midrange and saying “well I only listen to chamber music” In all honesty, my FE-103 was just as anemic.

Luckily I still had a simple passive EQ that I had made to solve a similar problem in my ESL. I had made it switchable for testing and listening purposes. The difference is incredible. When I switch it in for visitors they are astounded at what just 5dB EQ can do. No more thin lower midrange, no more thin sounding male voices, no more anemic cellos. This EQ is implemented with only 3 resistors and a capacitor per channel.

Is this like a conventional Bass control? No, not at all. Conventional bass controls start at 1,000 Hz, rise at 6dB per octave and shelve off around 200 Hz. This makes a mess of the midrange. No wonder audiophiles don't like tone controls. Our speaker EQ is rather special. It does nothing above 400 Hz and rises very slowly shelving at about 50 Hz.

Every single driver speaker I’ve measured has a similar slope starting around 350 Hz. This is why they all sound thin. This EQ is available at the output of our C-4 preamp or at the input of our EM7 amplifier. We have tried it with many single driver speakers and heartily recommend it if you own one of the previously mentioned speakers. It can be made switchable in/out or continuously variable. We can also make the starting point selectable by a rotary switch.

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