Category Archives: 13EM7

Final notes on the 13EM7 based miniBlok amplifier

I have been twiddling with the resistors on the CRC smoothing circuit because they have been running to hot.  Since the transformers I used were for 220V primary, and I was building for 230V,  the standard R1 100R 1W was changed to 6K 2W to reduce voltage. 
For 13EM7 tubes I have tried full base RCA´s and coin base Sylvanias with no discernible difference.

*NOTE March 2011: later I changed the CRC resistor to 1.8Kohm. Now I am right on target with B+ and bias and it sounds hot on the Visaton Eminence openbaffle.

15 second exposure in total darkness with one second fill light

Here are two videos with a presentation of my amp:

Part One

 Part Two

Finished! the 13EM7 amp today!

Finished soldering yesterday night, and today after work I tested it, and fired it up on some cheap workshop speakers. Music at last! I ran some test sounds, which only proved that iPod is a lousy signal generator, or that my cables picked up a lot of noise. Inconclusive. Fixed a problem with the too high plate current. The LED were mounted with wrong polarity, Their resistor worked in parallel with the damping resistor to lower resistance. Fault found I reversed polarity and the plate current was back within specs Although still not correct for driving the LEDs, Any way I connected it to my Magnepan SMGa´s and Gallo ST1 subs, put Gwyneth Herberts “Ten Lives” in the CD player, track 2 “My Narrow Man”….Bliss…
Here are two videos that describe the inner workings of this amplifier. They are for infotainment only. please DON´T  attempt to build anything based on this description!

13EM7 dual mono

Within a month of completing the prototype circuit described below I was building an aluminum chassis for making a full dual mono single ended triode amplifier. I folded up the chassis from 2 mm aluminum, used 3 mm for the front and turned  transformer pots from 4 mm x 80mm diameter tube. A mechanic at work helped out by CNC machining covers for the tubes. After drilling and cutting all the holes and polishing I have now mounted the hardware. Originally my plan was to have all the transformers potted, but two of the covers came out badly, so for now I am content just  to leave the output transformers as they are and go on with the building.

In this picture all but two transformers and a socket are in place. I have rubber grommets  for every hole. From left to right in the picture;
Power switch, fuse and AC socket are at the back. 235Vac to 12.6Vac transformers are at the back (left). Next come 12.6 to 210V plate voltage.
the high tension power supply is planned to go on a PCB under these transformers in the middle of the amp. Star ground will be  on the bolt in the middle of the white reflection, between the output transformers. Then come the octal sockets. There is a ALPS 100K logarithmic potensiometer in the center of the front along with two red LED´s that will monitor the charge of the smoothing capacitors.

I am buying my components from, also a bit from Partsconnexion.
I use ruby gold capacitors for the power supply and 1N4007 diodes. From a local hardware store I got two cheap battery chargers rated at 2,4 amp 240v for the power supply, the first makes 13v for filament heating, the second is reversed and brings voltage back up to 230 or so. Since I am in a 240V county I don´t have to build the voltage doubler Nachbaur specifies, but in stead have to bring down the B+ voltage (for the plates), by tweaking the Capacitor, Resistor Capacitor (pi) filter. For understanding power supplies, I really recommend Duncan amps PSU DesignerII Its free and really cool software.

I went for a dual mono design, figuring it was best to follow the plans precisely. That was before PSU Designer. (PSU=Power Supply Unit) In retrospect I would rather build it with a common power supply to reduce parts count. The PSU was soldered on stripboard, experimenters printed circuit board (PCB).

The rest of the amp, is built by point to point wiring around the sockets. Which are octal by the way, I dont think Nachbaur mentions this.

I have a habit of raiding the local electronics shops dumpster and pulling out amplifiers and TVs. I used a couple of components from them as well as wiring from them. Mostly 18gauge 900v rated wire. A bit thicker for the filament. Remember to twist the filament wires tightly and keep them away from other the components. I kept the filament wires in the corners of the amp, and crossed other wires as close to 90 degress as possible to reduce hum, this is good practice. Another important thing is to orient your output transformers at 90 degrees. (Hammond 125CSE) Collect all the ground points at one spot for a star ground. This keeps ground potential even.
For input signal I used Solen SM series foil caps and a logarithmic Alps potensiometer, and then more Solen caps for the coupling capacitors.

This stoy continues with Final notes on the 13EM7

13EM7 Amplifier

After much surfing I decided on Fred Nachbaur´s Miniblok SET amplifier for my first foray into amplifier building. Se Fred´s website for a detailed description of how it works and what it is good for.

I decided to build the prototype on a wooden box from IKEA. that way I could keep a clean and safe exterior, and I could turn the lid over so it was just as stable to work on upside down. I ordered most components from Parts Express and Parts connexion. Found power transformers in some cheap local battery chargers, and ordered 13EM7 tubes and Hammond 125 CSE output transformers from Norwegian supplier Motron. Then it was just a matter of soldering everything together.

This is the finished circuit. I have not cut wires and component leads down because I was planning to transplant the design into a double mono chassis if I was pleased with the sound. I did a couple of stupid things here, like running heater negative  on the ground bus, and getting the wires to the output transformers right in among the 235VAC, but actually it settled down after 5 or 10 hours of burn in and behaved well with no audible hum on the 89db/w fullrange BiB horns I built to play them on.
Here is a view of the first serious listening session.

This story continues with the 13EM7 dual mono

Update 21.october 2011: Here is the frequency response measurement of the 13EM7 playing through the Tangband W4 TQWT. Measurement is done through a Panasonic WM61 into Mac Pro with FuzzMeasure. the signal is sent to the amp through a M-Audio Fasttrack. The Tangband has Fs of 80Hz, the output transformers of the 13EM7 amp go down to 60Hz.