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AK-9 Low Power Guitar Amplifier

By Sid of Stone Marmot

Dec. 7, 2008

I wanted a low power guitar amp I could crank up to get natural tube distortion without being obnoxiously loud. I also wanted something light that I could easily carry to more laid-back jams. I also had a small tube amp lying around that used a pair of 12AB5 output tubes.

The result is what I call the AK-9. Check here for the schematic. It has three selectable maximum output power levels: 9 W, 4.5 W, and 2.4 W. It is only 17.5 inches high, 15 inches wide, and 8.75 inches deep and weighs about 24 pounds. It has a 10 inch Weber Alnico Signature series speaker. It only has three controls: Volume, tone, and master volume. The master volume probably wasn't needed on an amp this small and is usually turned all the way up.

All resistors are ½ W minimum unless otherwise noted. All capacitors are 400 V minimum unless otherwise noted except for C14 through C19, which are 16 V minimum. Though I used an output transformer I salvaged from a 1960's Grundig tube stereo, the Hammond transformer shown in the schematic is approximately equivalent and can be used instead. The neon bulb power on indicator light that I have has the current limiting resistor built into the bulb holder. If your neon bulb socket doesn't have this resistor, add about a 100 kohm resistor in series with the neon bulb.

As you can see in the schematic, it only uses five tubes. One is a 5Y3GT rectifier. I stayed with a rather simple tone control which is similar to the Matchless Lightning, which has a great reputation for good tone. I did experiment with some more elaborate tone control circuits, but all I tried seemed to hurt the tone more than it helped the sound. Since I wasn't shooting for any existing sound, I decided to let the amp have it's own sound and keep the controls simple. Since I almost always have the master volume control up full, I may in the future convert it to a Vox-type tone cut control by adding a 4.7 nF capacitor in series with it.

I am usually switching between a 6 string and a 12 string guitar when playing live. So I usually build my amps with a switch that selects the input jacks so that I can leave both guitars plugged in all the time and just switch to the input for the guitar I am presently using.

The preamp has both sections of a 12AX7A tube in parallel. If you want to use a more elaborate tone control, you can use one of these sections for making up for the gain lost in the more elaborate tone control. Using only one section for the first stage will slightly reduce the gain of that stage and change the tone slightly.

I tend to design and build stuff out of parts I happen to have lying around. You can vary most of the component values by 10 to 20 % to use more standard values, such using 100 kohms for R9 and R10 instead of the 110 kohms I had in my parts bins, without changing the sound of the amp too much.

Notice I have 2 ohm resistors (R22 and R23) in series with the cathode of each 12AB5 output tube. This is so I can measure the current through each tube. You don't have to include these resistors, though keeping them will make it easier for you to match output tubes should you care to do so.

A pair of 12AB5 output tubes will put out about 9 W maximum. No one that I know of is presently making these tubes. But NOS (new old stock) tubes should be fairly easy to find on the net since there isn't much demand for them now. I've been able to get them for less than $5 each, including shipping.

The 12AB5s need a 12.6 V supply for the heaters. Power transformers with 12.6 VAC heater windings are hard to find these days. Since 12AX7As can also run on 12.6 V heaters, I created a 12.6 Vdc supply for all the heaters with a voltage doubler circuit on the 6.3 VAC winding of the Hammond power transformer. If you use a different transformer or change the load on the heater supply, you may need to adjust the value of R19 (3.3 ohms on the schematic) to maintain 12.6 Vdc across the heaters. The DC heater supply should also reduce the hum slightly.

I am reducing the power supply high voltage to reduce the output power in this amp. I achieve this by switching zener diodes in series with the center tap return of the power transformer high voltage secondary. The center position of my three-position standby switch is standby, with one side being on full power and the other side being on reduced power. A second toggle switch selects whether the reduced power is 4.5 Wrms or 2.4 Wrms. Table 1 shows the voltages I measure in my amp for all the power supply high voltage taps in all three power output positions.

Table 1 - AK-9 Guitar Amp High Voltages For Different Power Outputs

Power A B C D
9 W 279 Vdc 272 Vdc 247 Vdc 257 Vdc
4.5 W 224 Vdc 221 Vdc 201 Vdc 208 Vdc
2.4 W 168 Vdc 164 Vdc 150 Vdc 156 Vdc

These zener diodes are dissipating some power, so they need to have at least a 10 W power rating. They should also be attached to good heat sinks. I just used my aluminum chassis. Make sure you don't short them to the chassis when mounting them, except maybe the one side of the one zener that should be tied to ground.

Figure 1 shows the front of my amp. Figure 2 shows a close up of the front panel. Figure 3 shows the back with the back board on. I find that most open back guitar amps and speaker cabinets have smoother and tighter bass response if the back is about 50 to 75 % closed up.

Figure 1 - AK-9 Guitar Amp Front
Figure 1 - AK-9 Guitar Amp Front

Figure 2 - AK-9 Guitar Amp Front Control Panel
Figure 2 - AK-9 Guitar Amp Front Control Panel

Figure 3 - AK-9 Guitar Amp Back
Figure 3 - AK-9 Guitar Amp Back

Figure 4 shows the back with the back board off. I had an amplifier that already used the 5Y3GT and two 12AB5s which I just rewired for the power amp. I then made a separate chassis for the preamp and controls, with two cables running between the preamp and power amp. If you are building this from scratch, it is probably easier to put everything in one chassis. Be careful you don't get any of your tubes too close to the speaker magnet as the magnetic field will affect the operation of the tubes and change the tone. I learned that from experience with this amp, which is why the power amp is mounted sideways.

Figure 4 - AK-9 Guitar Amp Back With Back Panel Removed
Figure 4 - AK-9 Guitar Amp Back With Back Panel Removed

How does it sound? The guitar on the left and all the fill guitar parts in the song "Dreaming" are played through this amp. In "I Must Be The Change", the guitar on the right is played through this amp. No effects were used in either recording, other than some reverb and compression in the final studio mix of the recordings. The amp was simply turned up until it sounded right. The guitar in both cases is a Gretsch 6196T Country Club, using the bridge pickup. I find the best distortion sounds with this guitar occur when the master volume is full and the volume is between 11 and 1 o'clock.

I am not claiming this is the greatest sounding amp in existence. But it does sound quite good, is very small and light, and meets my needs. It may give you some ideas for an amp of your own.

Warning: There are some very lethal voltages in this amp. I don't warranty anything in this schematic or write-up and am not responsible for any problems that may occur due to mistakes in this information or on the part of anyone who uses this information. Build at your own risk. If you are unwilling to assume the risk, don't act based on anything in this blog.

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