The Analog Bros. Model 22 is our longest continuously produced guitar amplifier. It is a low power (approximately 20 Watts), single channel, tube rectified circuit using a pair of 6V6 output tubes in fixed bias push-pull configuration. Configurations are available from the basic version to options including reverb or FX loop and tremolo. |
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The origin of the Model 22 goes back to the mid 80s when we were working on the design of a recording amp. The primary design criteria were that it had to be able to produce an absolutely huge tone, but have a relatively low output power level. We were doing a small, no budget recording project with many of the guitar tracks being laid down in the control room while monitoring through headphones. Our first attempts were based on a completely reconfigured silver panel Princeton Reverb. What we found was that by driving the push pull 6V6 pair hard, we could achieve the saturated output tone that previously required very high SPL to attain. We’ve always loved the tone of the 6V6 (more than any other tube), and by careful design of the output stage and power supply were able to optimize the tone of this tube that had previously been used only in lower powered “student” type amps. The only exception to this as far as we know was the great Jim Kelly amplifier of the early 80s that used 4 6V6 output tubes and had many other unique and innovative design features. This project also led to the genesis of our Beta control. While the Princeton based prototype did use a post inverter master volume control to allow for lower output level with more pre-amp distortion, we were never satisfied with this or any other master volume configuration. Think about what a master does; in PA amps it was used to set a maximum output level while each channel with it’s own gain control would set the balance between all the inputs. In a single input MI amp, what was the point? The great tone of all the classic amps of the 50 through early 80s was obtained by pushing the output stage in conjunction with the drive from the pre-amp stages. By placing a attenuator anywhere between these two sections, you limit the contribution of the output amplifier to only making the pre-amp signal louder, not allowing it to contribute any harmonics of its own. This is great for a PA mixer, but counter-intuitive for a guitar amp. What if there was a method of altering the output level, harmonic contribution and damping of the output stage without resorting to tone destruction by lowering input drive? We thought on this for a while and worked up a variable feedback network control circuit. One of the many problems with commercially available guitar amps of the time was that most of them were just too damn loud. To get to the point where the input and output stages worked together to produce the full harmonic spectrum tube circuits are capable of, the SPL outputs became extreme. The cheap, quick fix was to install the ubiquitous master volume control. To the best of our knowledge, all of the major manufactures used only the simplest, worst sounding version master volume circuit. Some of the aftermarket amp shops (ours included) worked up superior sounding masters, but every one of them had the same shortcoming. They throttled the entire contribution of output stage. (Randal Smith of Mesa has built a very lucrative career on this “feature”.) However, we thought there was a much better solution. Design a true professional amp that was much more reasonable in output SPL by using a 6V6 output stage, use a pre-amp capable of great tone, couple them together with an optimized power supply and finally, add the ability to control the output stage with our Beta circuit. Why and what is Beta? Any amplification circuit can operate in two ways; open loop and closed loop. Open loop or free running will produce the most gain, but will also have frequency response aberrations and higher distortion. Closed loop or feedback refers to a sample of the signal taken from the output of the circuit and brought back to its input. Since the feedback signal is our of phase with the input signal, it cancels out any difference between the two. The benefit of feedback is to flatten frequency response and lower distortion. “Beta” is the Greek symbol used to denote the relative difference between closed and open loop gains. We had previously called this control “power amp gain”, but the term gain can have many meanings, and we felt it really didn’t accurately describe the function as well as beta does. Like everything else, the Model 22 has evolved. Many improvements and adjustments have been tried and some have stuck. One of the biggest improvements was to the Beta circuit. In its first configuration, it worked well, but had some side effects. The latest version is frequency compensated and better sounding, while still maintaining all of the adjustability of the original. Over the years, we’ve built the Model 22 amplifier in many configurations, from straight up to with reverb, FX loops and tremolo. Read below for the available configurations. The Model 22 is a single channel, passively EQ’d (low, mid and high frequencies), vacuum tube rectified guitar amplifier. The output stage consists of a push-pull, fixed bias 6V6 pair, producing approximately 20 Watts. Standard Features Include: Front Panel Switch defeat-able EQ. Engaging the EQ bypass will allow for passing of the unfiltered signal to the power amp stage. This will increase both gain and distortion. Optionally, we offer remote foot switch control of this function. Power Amplifier Beta control, as described above. Presence control in the power amp section to vary high frequency response, allowing for a smoother to more aggressive top end tone. A variable line level output (1/4" unbalanced) which is useful for slave amp’ing, direct connection to recording or reinforcement consoles, tuner output, etc. Optionally, we offer an additional low-impedance, transformer-coupled balanced output on a 3 pin (Pin 2 Hot) XLR-3-F connector with ground lift switch. Optional features: NOTE: either reverb or FX loop can be added, not both. "3 knob" all tube driven spring reverb. Our reverb circuit is similar to our parallel FX loop, as its input signal is derived early on in the pre-amp, precisely eq'd prior to being sent to the full length, 6 spring Hammond reverb pan, then actively mixed back with the dry signal post main eq. This architecture provides for a very full and clean delayed signal free of the "muddy and congested" sound that can result when playing at higher pre-amp gain settings. Additionally, the active mixing circuit does not pad down the dry signal path, allowing for minimum alteration of the dry signal. Full control of the reverb is provided for by adjustable drive, h/f cut and mix levels. All tube Active Parallel FX Loop: Our all tube FX loop topology is designed for maximum flexibility for use with any type of FX device, from stomp boxes to high-end rack products. The parallel circuit architecture allows for seamless integration of the effect into the overall sound with no degradation of the original signal. The loop's output is derived post first gain stage, pre EQ. This allows for a clear, undistorted signal being sent to your FX device. The signal is actively buffered prior to amplification via a high current-drive output stage. The output level is fully adjustable to match any possible load. The return path consists of a gain recovery amplifier, also fully adjustable. The wet signal is combined with the dry signal post eq by an active mixing stage. A front panel mix control allows for adjustment of balance between the dry and effected signals. Tremolo: Here we use solid state rather than vacuum tube circuitry. Using tubes for a “side chain” type effect where the signal doesn’t pass through any active component just doesn’t make sense. Some purists may disagree, but listening tests have proven our solid state tremolo indiscernible from any tube tremolo circuit. The space saved by not using tubes allows this feature to be offered in our products and offers better performance than the common tube circuits. The range of the LFO is far greater than any of the classic tube circuits, and the depth of the effect can be dialed in with greater precision. A unique Kevin O’Conner designed oscillator circuit first published in his volume “”Tonnes of Tone” was adapted by us to interface with our signal path. Speed, Depth and bypass controls are provided. The Model 22 head is available in the following configurations:
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