7193->6BX7 Push-Pull Aikido Preamp

I am making available to the DiyAudio community a dual mono Aikido circuit board design which uses a pair of 7193 (or 6J5?) input tubes and a 6BX7 output driver tube. Below are schematics (pdf file) and pictures. I will send the BOM excel file to anyone requests it. I will print, scan and put it in a future post. Although I am using this board as the first stage of my Moskido amps I’m sure that it can be used as a line stage preamp gain stage. A separate forum thread was started for the Moskido amplifier (http://www.diyaudio.com/forums/tube...brid-aikido-mosfet-amplifier.html#post4654659).

My latest 7193->6BX7 push-pull Aikido Moskido amp sounds fast, tight, rich, and nuanced – the best amplifier I have ever heard. Scattered throughout various audio forums are comparisons between the 7193/2C22 with the best available 6SN7s with the 7193 always sounding better. The first thing I noticed after replacing my 6SN7->6BX7 GlassWare Audio Aikido board was a more pronounced musical sustain and decay. I had been using NOS VT-231 tubes by Raytheon and Sylvania which sounded better than the supplied Tung-Sol 6SN7s. I designed custom PCBs to allow direct plug-in of these “mouse eared” tubes instead of using an adaptor. My second PCB version has just completed the 300 hour break-in period and I have never had a more satisfying musical experience.

The push-pull Aikido circuit is based on the schematic shown on Bob Prangnell’s Mad Scientist Audio web site in the DIY section. The R-sense and 6BX7 cathode resistors values were calculated using the complex formula from a write-up by John Broskie on his excellent TubeCad.com web site. The formula calculated 790 ohms for R-sense and 107 ohms for the 6BX7 cathode resistor. Because the 6BX7 needs about 530 Ohms cathode resistor to run at 20ma, I split the cathode resistor into a 107-110 ohm push-pull feedback resistor and a 390-420 ohm resistor bypassed with a 100uf cap. This circuit works and sounds great.

I tested the Aikido board using a signal generator and scope. The square wave at 20khz looks nearly identical to the input signal. The -3db point is way beyond 200khz unless an optional small cap is put on the output of the first stage. An input signal of 0.1volts resulted in an output of 0.77volts.

I used OshPark to make these PCBs. They make boards in multiples of 3 which cost me $62 total including shipping to the USA. I will send the Eagle CAD files for this PCB to anyone who wants them. These PCBs are 80mm X 100mm which is the largest allowable for the free version of Eagle PCB CAD software. I’m sure that someone who has access to pro Eagle CAD software can enlarge and improve the PCB layout.

Another plus is that the 7193 and 6BX7 tubes are affordable and available. I have a few extras which I can make available. This PCB should also work with the 6J5 which has gotten scarce and expensive. This project is of medium difficulty and should be attempted only by those who already have experience with at least one vacuum tube DIY build. I will add posts explaining more about this Aikido design in response to interest and questions.
 

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7193->6BX7 Aikido Preamp Schematic & BOM

I printed and scanned the Schematic and BOM for the 7193->6BX7 Push-Pull Aikido Preamp and attached them to this post for viewing. I can email the original files to anyone who requests them.

I am a believer in bypassing large caps with smaller film caps, however, leaving out these bypass caps may still result in a great sounding preamp.
 

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  • Aikido 7193 BOM Parts.jpg
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  • Aikido 7193 BOM Notes.jpg
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Aikido Filaments

Bob Prangnell's web site shows all the circuitry required for the 6.3VAC filaments including the floating of the voltage to 1/4 of the 300VDC supply to the Aikdio board.

The choke is wire wraps around a ferrite donut which Bob felt was important to ultimate SQ. His diagram shows 40 turns, however, this gave me too large a voltage drop with 18ga wire. I have about 15 turns. The number of turns can be used to provide the desired filament voltage.

Because I have two separate tubes for the input stage I have the filament connection polarity opposite between the two 7193s. This may contribute to the absence of audible hum because there may be some hum cancellation.

I will prepare a schematic of my Bias board for posting later on.
 
Hi Carlsor

Very nice project, while I was thinking and researching about making a new tube buffer or preamp (still cant decide) based on Aikido octal tube PCBs, I came accross with this thread. I was also planning using 7193 instead of 6SN7. I think this project is the real high-END for a preamp. I want to build this project also if you can help me :) I am not a advanced DIYer :(

First question: Can I use John Broskie's readily available Power Supply PCB for your Moskido project. If so which one should I use?

Regards..
 
Power Supply

Recepky,

Are you building a buffer, preamp, or Moskido?

I recommend the high voltage power and filament supply on the Mad Scientist web site which is not difficult to breadboard. The only change I made was to replace the 220 ohm resistor with a 10uH 65ma Hammond choke which further reduces ripple.

IMO a 7193 Aikido preamp is very High End and demands the best from the rest of your music system. The power supply must be high end too with Hexfred diodes.

This thread is intended to help DIYers at all levels, however, this should not be someone's very first project.