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4S Universal Preamplifier for 12A*7 Tubes

 Matt Renaud   USA Flag   To email Matt, type out the email address.

4S Universal Preamplifier for 12A*7 Tubes

Humble Beginnings

The basis for this tube preamplifier started out as a post in the Super simple single stage tube preamp thread on the website Forum. One of the members (see Mark's 4S Universal Valve Preamps) had suggested building a Super Simple Single Stage Preamp ("4S" Preamp for short) and there was much discussion concerning various tubes, gain, noise, etc. and several designs were presented using various dual triode tubes. Then the proverbial gauntlet was thrown down with the phrase "switch from 12AU7 to 12AX7". My answer was to design a universal line stage preamplifier that would work well with an entire range of tubes. Thus was the 4S Universal tube preamplifier born.

Assembled 12A*7 Universal Tube Preamplifier

Photograph 1: 4S Universal Tube Preamplifier - Assembled


The Electrical Design - 4S Universal Tube Preamp

The tube line level preamplifier design I came up with is a single circuit that can handle the following dual-triode 9-pin tubes with no changes: 12AU7, 12AV7, 12AY7, 12AT7/12AZ7 and 12AX7.

12A*7 Dual-Triode Vacuum Tube Pinout Diagram

Figure 1: 12A*7 Dual-Triode Vacuum Tube Pinout Diagram

Dependent on the tube used (all these tubes have the same pinout) and whether the cathode is bypassed, the peak gain may be varied by a large margin. Here is the circuit I implemented.

4S Universal 12A*7 Tube Preamplifier Schematic

Figure 2: 4S Universal Tube Preamplifier Schematic

The tube preamplifier circuit is actually very simple. It uses a reasonably sized grid stopper to control clipping and bias excursions, a pure restive load, and a high impedance grid resistor to avoid loading virtually any driving device. The volume control (P1 on the schematic) is on the tube output to help ensure that the noise figure for this Hi-Fi preamp is as low as possible. The input resistor (470k on the schematic) can be anything from 100k to 1M to properly load your input source. With these components (the output potentiometer is 250kΩ audio taper) and an optional 33µF cathode bypass capacitor the calculated preamplifier performance is shown in the following table.

Table 1: Calculated Performance - 4S Universal Tube Preamplifier

Calculated Performance - 12A*7 Universal Tube Preamplifier

Where:

  • ECo (V) - Cathode bias voltage in volts (Quiescet [zero excitation] value of grid voltage).
  • Av - Amplification or gain (v/v or unitless).
  • Av (dB) - Amplification or gain in decibels or dBv.
  • %D (i/p 1vpp) - Percent distortion with a 1v peak (2v peak to peak) sine wave input.
  • Ro (Ω max) - Maximum output impedance in ohms (ouptput impedance varies with potentiometer setting).
  • flo (Hz) - Low frequency -3dB response frequency.

As can be seen from the calculated performance, the peak gain can be varied considerably by choice of tube and the distortion (virtually all second harmonic) is very well controlled. This is a well behaved line stage preamplifier. The load line designs for the five different 12A_7 tubes are available on the Forum in the Load Lines - 4S Universal 12A_7 Tube Preamp post.


Tube Rectifier Power Supply Design

I decided that I wanted to do a simple tube rectified power supply for this preamp unit. Tube rectification works well in this application because the preamp draws only a few milliamps for both channels. At these current levels, the voltage drop in the vacuum tube rectifer is very small (~3.6v for this unit) and the filter choke expected inductance may be as much as twice its rated value. The design point for this supply, without applying these factors, is 1.2mV of ripple at 257V. This constitutes a ripple value of -107dB, in short a very quiet power supply. And, due to the factors stated, it is most certainly even better than it's design point. This is further borne out by the fact that the preamp is dead quiet in usage. This power supply is very well suited to the application. For more detailed information, see my notes on vacuum tube power supply design. Note, the capacitor following the 6CA4 tube rectifier must not exceed 50uF.

Power Supply Schematic - 12A*7 Universal Preamplifier

Figure 3: Power Supply Schematic - 4S Universal Tube Preamplifier

For solid state power supply ideas, see Mark's 4S Universal Valve Preamplifier builds.


Building the 4S Universal Tube Preamplifier

Deciding on the physical layout and build actually took some time. Up to this point, most of my tube builds have followed a fairly predictable pattern: wood base, aluminum top plate, polished finish, etc. For this tube preamplifier build I wanted to go for two major themes: Utility and Compactness. So I started with a stock enclosure from Pomona, the 4226 - PDF (24kB). This is an E size (8.3" x 4.2" x 1.74") die cast aluminum enclosure with a sheet aluminum cover and internal card guides. Although small, I was sure I could fit the entire power supply and stereo preamp in this enclosure. Instead of trying to run wires from the top plate to components mounted inside (which would make working on the box and sealing it up problematic) I decided to mount all of the components to the top plate. Then I could build the entire preamp and when complete, just slide it into the enclosure. Here is the layout for the major elements:

Control Diagram - 12A*7 Universal Tube Preamplifier

Photograph 2: Control Diagram - 4S Universal Tube Preamplifier

Warning: This vacuum tube preamplifier project uses high voltage in the range of 250 to 270 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 vacuum tube amplifier projects or you are not comfortable with projects that use these voltage levels, it is strongly recommended that you do not build this vacuum tube preamplifier. Follow your governing electrical codes for all wiring and connections.

The power supply components are all contained on the left hand side of the unit and all of the preamplifier circuit components are on the right. Internally there is a metal shield between the two sections which is held in place by one of the card guides molded into the cast chassis. One very important feature of this tube preamplifier build is the ground lift switch. The entire chassis is grounded through the IEC connector ground pin. The signal ground is isolated from the case ground by the ground lift switch. This way, if I am driving an amplifier in which the signal is grounded to the case, I can flip the ground lift switch and the preamp signal ground will be referenced to the amplifier signal ground via the audio cables. This avoids hum inducing ground loops when both chassis are grounded via the IEC connector. Here is a view of the component side with the preamp unit all wired up and ready to go into testing.

Underside - 12A*7 Universal Tube Preamplifier

Photograph 3: Underside - 4S Universal Tube Preamplifier

Things are fairly cramped inside. I could have gone for a printed circuit board approach, but I really enjoy point-to-point build techniques for vacuum tube amplifier projects. Besides this, I'm not sure I could have designed a PCB that would work with all the bulky items protruding into the case (inductor, IEC connector, tube socket, potentiometer, RCA jacks, etc.). Once the preamp build was done and all electrical connections triple checked, it was time to commence testing.

Photograph 3A shown below is a copy of Photograph 3 with some labeling that has been added for clarity with regards to the layout and grounding scheme of the tube preamplifier project.

Labelled Underside - 12A_7 Universal Tube Preamplifier

Photograph 3A: Labelled Underside - 4S Universal Tube Preamplifier

The labelled Photograph 3A shows which types of grounds are where and how the ground switch ties them together. You will notice that there are two wires running through the center penetration of the divider plate. The black lead is the high voltage rail coming out of the primary filter inductor and the green wire is the signal (and power) ground. These connect to points on the terminal strip under the final filter capacitor. There are two leads coming off the bleeder resistor. The following photograph (3B) shows how these leads connect to the terminal strip.

Terminal Strip Grounding - 12A_7 Universal Tube Preamplifier

Photograph 3B: Terminal Strip Grounding - 4S Universal Tube Preamplifier

As you can see in Photograph 3C below, the 100k plate load resistors for the two channels are connected directly to this tab as well. You can also see that the coupling capacitors connect to the other side of the plate load resistors at the plate pins on the tube socket.

Labelled Underside - 12A_7 Universal Tube Preamplifier

Photograph 3C: Labelled Underside - 4S Universal Tube Preamplifier

These photos (3A, 3B and 3C) should help make the grounding scheme that I used clear.

The 12A*7 tubes have two elements each with a 6.3v heater. In the tube these are wired in series with the central connection tied to pin 9. This means that a 12.6v heater supply is wired to pins 4 and 5 with no connection on pin 9. However, a 6.3v heater supply needs to put the heaters in parallel so that they each get 6.3v. This is accomplished by bridging pins 4 and 5, and then connecting the 6.3v heater supply between pins 9 and the bridged 4 and 5 pins. See the Photgraph 3D for the connections.

Heater Wiring - 12A_7 Universal Tube Preamplifier

Photograph 3D: Heater Wiring - 4S Universal Tube Preamplifier


Testing the 4S Universal Tube Preamplifier

Testing a preamp device of this type is really fairly straight forward. The first step is to simply power up the tube preamp unit and check all the major voltage points in the circuit. This is done with the unit open and sitting on a workbench. For this preamp build all voltages were as calculated to within a reasonable margin of error. For this type of device I like to test using a signal generator with adjustable output, a dual channel AC volt meter, and an oscilloscope. Here is a picture of the unit as I was running it through its paces.

4S Tube Preamplifier Testing Setup

Photograph 4: 4S Tube Preamplifier Testing Setup

The signal generator allows me to test the tube preamplifier unit at various frequencies and input levels, the oscilloscope shows me the input and output waveforms, and the volt meter allows me to directly calculate gain at any frequency without trying to get numbers off of the oscilloscope. So how did the 4S Universal tube preamplifier unit do? Here are the gain and phase plots generated from measured data.

Voltage Gain and Phase Shift vs Frequency

Figure 4: Voltage Gain and Phase Shift vs Frequency

This is very good performance. The single dominant high frequency pole here is due to the input capacitance of my test equipment. The tube input capacitance is at least an order of magnitude smaller and is swamped by the test equipment and the nest of wires on the table.


Impressions - 4S Universal Tube Preamplifier

Of course, all this test data is nice, but it really doesn't say much about how the preamp works and sounds. So once the preamp unit was deemed technically acceptable, it was time to determine if it was sonically acceptable. This entailed hooking up the preamp to one of my other units. I chose the 6V6 Lacewood amplifier.

In the past, when driving this amplifier with just my iPod, I was disappointed with the overall performance. Not with the sound mind you, but with the attainable power level from the amp. The iPod output simply wasn't enough to drive it to full power. So I inserted the tube preamp in front of the amp, turned the amp volume to max, and used the preamp control to adjust volume. Here is a picture of the listening setup.

4S Universal Tube Preamp with Lacewood 6V6 Amp

Photograph 5: 4S Universal Tube Preamp with Lacewood 6V6 Amp

In this picture, the iPod line out is driving the preamp, the preamp is directly connected to the Lacewood amp and it's volume is at max. First, with no input and both preamp and amp turned to max, the setup was dead silent. I could hear nothing from the speakers even with my ear right next to the speaker cone.

From the first moment I started playing music I was amazed at how the pair sounded together. There was absolutely no problem with signal level or power. And the added effective dynamic range really let the amp open up.

This preamp project was a great success. This is a preamp that can be adapted to the task at hand simply by swapping tubes. It also allows some tube rolling to compare various tubes in your stash. This is a great little unit to really bring out the most in your power amps. Comments and questions about the preamp project are welcome in the 4S Universal Tube Preamp Project (12A*7) thread.


12BH7 Tube with 4S Universal Preamp

 UPDATE  - 23 October 2014
Well, I have another update to the possible tube list for the 4S Universal Preamplifier. This is a special case because you can only use the 12BH7 tube if your heater supply is capable of putting out a full 600 mA into the filament. If so, the 12BH7 tube will expand your tube rolling capabilities. The 12BH7 tube is halfway between the 12AU7 and the 12AV7 from a gain perspective and is very low distortion.

Table 2: Calculated Performance - 4S Universal Tube Preamplifier (including 12BH7)

Calculated Performance - 12A*7 Universal Tube Preamplifier (including 12BH7)

About the Author

Matt Renaud is a career Electrical Engineer with over 25 years experience in advanced radio frequency system design. He has a passion for tube audio and teaching tube based circuit design to others through participation in tube audio forums and his own website, www.CascadeTubes.com. The majority of his designs are fully original and may be found on his website and on this forum. More DIY audio projects and articles by Matt: