The EM7 Series Amplifiers
This series based on the 6EM7 and 13EM7 encompasses 3 distinctly different applications denoted by their power rating in watts per channel. We rate at 5% distortion while most other makers rate at 10% or above. In most cases, they don’t even show these specifications. We go so far as to have a published curve that you can see at: http://www.ramlabs-musicreference.com/emlframe.html
. Though drawn for the our 245.1 amplifier, the shape of the curve applies to the EM7 line also.
The EM7-2.5, 5 and 12 look the same from the front but have sets of connections on the back as to number of input and output terminals. We have a single thumb screw on top to allow entry with no tools at all. This makes biasing the 5 and 12 very easy. The 2.5 needs no adjustment ever.
EM7-2.5 The original EM7 now with external, easy to change output impedance selection. This is the perfect starter amp for those curious about SETs. Cathode bias requires no adjustment when changing tubes. It simply takes care of itself. This is called “auto bias, cathode bias or self bias”.
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 performance from the EM7 tube. This is often called “fixed” bias, which is a confusing term because you can vary it. Since the EM7 is a very stable tube you don’t have to check or adjust it more than every few months.
EM7-12 Our top performing EM7 SET 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. FAQs
..... Actually, I made them up, but ask a good question and see your answer here next time I update.
Why the use the EM7 tube? The 6EM7 and 13EM7 are identical except for filament voltage, one being 6.3 and the other 13.0. 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 this 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 the back of your TV set. The cathode is 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 (3 times as much) at maximum current.
Are these real 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, 6L6. When a pentode is triode connected the screen grid “catches” a fair percentage of the electrons coming from the cathode. It catches them before they get to the plate, so all your electrons are not arriving at the same time as they do in a true triode. The suppressor grid (3rd grid in a pentode) causes an additional problem as it turns back some of the electrons that are headed to the plate. Furthermore, to fit these grids into the structure the plate has to be moved 9 times farther out (numbers obtained from physical measurements on a EM7 Vs KT-88) In terms of ruggedness the EM7 is a champ. The top mica is twice as thick as the KT-88 and the glass is many times stronger. It took quite an effort to break one open to measure the dimensions. The design of the EM7 makes it one of the most efficient triodes ever made.
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 different by design. The more familiar 6SN7 and 6SL7 are “twin triodes” where both sections are identical. The driver section of the EM7 is 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=4, Rp=750 ohms). Rather interesting for 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 it was designed as a RF amplifier in a color television, it makes a great audio tube. We owe a lot to color television and the tubes designed for it. The driver tube in the 245.1 is another popular color TV tube. Imagine what tubes might have been created if solid state TV had not come to pass. Sony was the first overseas company to buy a license from Bell Labs to make transistors and the rest is history.
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 very few 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 to several dozen and match them for gain and current. I have a stock of several thousand and test them in lots of 100-300. The larger the batch, 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 good choices and efficiency is not the only thing that matters. Your preferred listening level is 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 (80 dB is about the lowest made) efficiently speaker and be very happy. Customers who like to listen at 95 dB need a much more efficient speaker (95 dB and up). Radio Shack has re-introduced their analog SPL meter. It’s the most important thing you can buy to guide you on your journey at any power level. If you can 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. You can do this yourself also. If your SPL (at 1 meter from your speaker) is equal to your speaker’s sensitivity then you need one watt. If it’s 3 dB higher you need 2 watts. Each time you add 3 dB over the sensitivity of your speaker you double the power required. Going in the other direction, for every 3 dB lower than your SPL at 1 meter you need half the power. If you listen to a 85 dB speaker at 75 dB SPL you need only 0.1 watts. Also measure at your listening position to get your average listening level preference.