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.

9 thoughts on “13EM7 Amplifier”

  1. Looks awesome – how did it sound? Also, the the original design called for 120V:12.6V transformers (or in our case for 240:12.6) – did you stick to that?



  2. It sounded better than you would expect for the price and size of speaker. Right now I´m using this setup as our living room stereo while the Magnepan´s amplifier is out of service. Bass rounds of from 80Hz, but the horn supplies enough bass that it fills out well. The speakers work with jazz and simpler pop arrangements. If I stick them both close to a wall, bass is amplified to the extent that you hear repetitious, unnuanced low tones. Some active filtering might be in order.
    Some people say the Mark Audio W4 bamboos are lacking in high tones, My hearing is to old to detect any trouble, and I think The W4 has a fine mid and high presentation, reasonable imaging and a presence that is interesting.
    This is a super cheap speaker I highly recommend, specially since there are so many full range 4-inchers to experiment with.

    The Miniblock is built as dual mono exactly after Nachbaurs plans. Since it was my first amp. I did it to learn by copying, not to experiment. I only tweaked the CRC filter resistor to give the correct voltage. I have played it a lot in the two years since I built it. Driving the Eminence/Visaton speaker it is noticeable that the output transformers only go down to 60Hz or so, and it might need another watt to drive these speakers to party dB´s.
    Here is a youtube presentation of the final design: http://taooftubes.blogspot.com/2010/03/final-notes-on-13em7-baset-miniblok.html
    Some time in the future I want to change the transformers to a single, conventional trafo, and play a bit with other operating points.

    Maybe I´ll do some measurements and post here in the weekend.


  3. In the beginning I considered a Notch filter centered at 4kHz, but after a few months of playing them I decided to play unfiltered.

    The cabinet dimensions are Width 7″ depth 9,5″ and height 45,5 ” The slanted inside board is 41,5″. I stuffed the pointed end of the speaker with polyfill down to past the speaker. and then some more in the bottom 180 turn, about 3″. The stuffing changes the sound quite a bit. so you have to feel your way and play them for a few weeks, adding or subtracting stuffing to get things right. For speaker driver I used Tang Band W4-1320SD

    Good luck on your build. I still have them and I am still happy with them although they are not my primary speakers


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