4S Universal Preamplifier for 12A?7 Valves
Mark Houston  
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4S Universal Preamp for 12A?7 Valves
The whole 4S (super simple single stage) series of valve preamplifiers took a slightly different turn with the 4S "Universal" design. No longer was the preamp design for a specific valve but designed to take on any of the 12A?7 valves. This added a whole new dimension to "tube rolling" with the 4s preamps. For me, I find that of the 12A?7 lot I prefer the 12AU7 valves. Here are photographs of two very different builds of the MKIV Universal preamp.
Photograph 1: MKIV 4S Universal preamp with 12AU7 - Twining Tea Box
Photograph 2: Super Simple Single Stage (4S) 12AU7 preamp with Genalex Gold Lion 12AU7
4S Universal Valve Preamplifier Schematic
The schematic for the 4S Universal Valve Preamplifier was designed by Matt and is shown below for reference. For full details of the preamp circuit design and performance, see Matt's project page, the 4S Universal Tube Preamp. 4S Universal Valve Preamplifier design works with the common 9-pin dual-triode valves 12AU7, 12AV7, 12AY7, 12AT7/12AZ7 and 12AX7.
Figure 1: 4S Universal 12A?7 Valve Preamplifier Schematic
On my 4S Universal preamp builds I'm using an output potentiometer (P1) with a 250kΩ audio taper. The gain with the different 12A?7 valves and optional bypass cap is shown in Table 1 on Matt's 4S Universal Tube Preamp project page.
Power Supply - 4S Universal Valve Preamp
In the first build I developed the idea of having the power supply and preamp on the one piece of circuit board. The power for this and many other works with an AC wall adapter (wall wart) and stepped up by using small stepdown transformer backwards. This trick allows you to avoid having to buy a large high voltage power transformer - allowing us to make nice compact builds. For the heater supply a 7812 voltage regulator is used to develop 12VDC regulated. The same power supply idea is used in the preamp builds shown. All preamp builds use component-2-compoent construction where possible.
Figure 2: Power Supply Schematic - 4S Universal Valve Preamplifier
Warning: This valve preamplifier project uses high voltage (250+ Volts). Contact with voltage potentials of this magnitude can cause serious injury or possibly be fatal. If you do not know how to build high voltage Valve amplifier projects or you are not comfortable with projects that use these high voltage levels, it is strongly recommended that you do not build this valve preamplifier. Follow your governing electrical codes for all wiring and connections.
Photograph 3: Power Supply and Preamp Circuit - 4S Valve Preamp
Photograph 4: The Black - Super Simple Single Stage (4S) 12AU7 Tube Preamp
Impressions - 4S Universal Valve Preamplifier
Right from my first valve preamplifier (two stage kit) I had used the 12AX7. When I started on my series of super simple single stage (4S) preamps, particularly the earlier ones I stayed with the 12AX7 valve. I was trying different 12AX7 brands in the various early builds. Some of the 12AX7 valves I tried: Conn Organ Company, Golden Dragon, Sovtek, Genelex, Mullard and even a NOS Mazda which I had bought from Tandy when in my early twenties. I thought the 12AX7 was the best valve for the job. Indeed the 4S preamp series were initially developed with the 12AX7 in mind. This didn't stop rolling in 12AU7, 12AT7 etc. but each time I went back to the 12AX7.
It was about at the third build of the 4S preamps, The Black, that I felt the preamps sounded a little strained, a littler aggressive. The bias point in these builds is only a few volts. Right from the beginning I was using a potentiometer on the output of the preamp. As Matt will point out, pots are noise, even good ones (I use Alps Blue Velvet at all times). If the pot is on the front end, any noise in the pot is also amplified through the preamp. The problem is then the source signal can overload the tube driving it out of class A and may contribute to an "edgy" sound.
Since I swapped the 12AX7 for the 12AU7 I have not looked back. I am intending to build another tube based phono preamp and for this a 12AX7 is required for better gain (12AU7 have lower gain, but can deliver more current). There is something very smooth sounding about the 12AU7 particularly so in the mid-range where the voice is. This tube treats voice and most acoustic instruments very well. Love your vocalists, then you will love the 12AU7. If you're a rock/pop man stay with the 12AX7, it is just a bit "edgier".
Photograph 5: Super Simple Single Stage (4S) 12AU7 Tube Preamplifier
The 12AU7 appears to handle big transients in music better and not make them sound distorted or strained. The dynamics of the tube appear to be greater but I think that it just soft clips a lot better. With the 12AU7 there is no end of brands. NOS and NIB, Russian, Chinese, American, Military there are many variants. Why not try them all? Comments and questions are welcome in the 4S Universal tube preamp thread on the Forum.