Innovation Reliability Quality
Unit 1a Chocolate Factory 2, 4 Coburg Road Wood Green N22 6UJ Tel 0208 829 8927
Single Ended Guitar Amps
Available with or without reverb
18w uses 6550 tube but can be converted to 22w by using KT88s. Plays like a 50w amp
Especially good for small gigs or rehearsal and recording. Uses 1 KT88 output tube.
  Contains extended preamp for high sustain guitar solos, for practice or recording. Overloads early in the signal. Uses 6V6 tube.
THE GUITARIST VIEW:. What is happening when a guitar amp goes into distortion? Why do musicians prefer the way valves distort compared to solid-state? As a musician I thought the subject wasn’t as simple as that. Even valve amps distort in a way that can be very ‘unmusical’ especially when presented with a range of frequencies, such as when playing any chords other than ‘power chords’."

Every rock guitarist knows only too well that you have to keep your chords simple when your amp is well into overdrive or you can quickly generate some pretty horrible sounds. But, this effect is still present even at much more subtle levels of distortion – and so dramatically affected tone even though more complex chords became possible at lower distortion levels. What if an amp could be designed to give the characteristically smooth valve break-up, but in a manner that was more musical to the ear?  This could improve the quality, or the musicality, of the amps tone for all styles of playing at all settings.

This is where Theos advanced knowledge of electronics and sound came to the fore. After a rather involved discussion about ‘high frequency inter-modulation products’ and the way the vast majority of valve amp output designs reject the more musical low order even harmonic distortion, the two of us started discussing designs that didn’t suffer these setbacks. The only products that came close to the criteria we were seeking were single-ended designs, class-A by definition, but these were pretty much all very low powered studio amps with very basic control features.




From these discussions evolved this amp range, which represents a complete ‘back to the drawing board’ approach to guitar amplifier design – and the results are simply stunning.

From the basic design approach, I added other requirements to my demanding specification. It had to be built to the highest standards of robustness and durability. It should have a high quality spring reverb built-in. Theo’s experience over the years of fixing many a design fault proved invaluable. Furthermore, unusually for such a highly subjective field, Theo has an amazingly objective approach to the amplifier design.
This approach resulted in an amp with a range of controls that enable the musician to dial in their exact requirements regardless of genre and instrument type. So rather than impose a sound on the musician, as many amps do, it enables the musician to blend different operating characteristics to achieve their desired sound.

The results are hugely rewarding. The amp seems to bring out previously untapped characteristics of each guitar you plug into it. Its like rediscovering your favorite instruments all over again. The sounds are very much ‘of the instrument’ but amplified in a way that bring character out of the instrument that other amps don’t even come close to. Its hard to describe, but you can hear it sure enough!"



Jonathan Crick



THE DESIGNERS VEIW: "Only low power single ended guitar amps exist, Fender Champ and Vox AC2 to name a couple.

At the end of the day, the ultimate judge of what is good sound is the player not the designer.

Therefore some people may prefer push pull outputs anyway (for different reasons) even with tubes which are just bias matched.

Then again, there may also be a completely unexplained and subjective reason why an amplifier without phase splitter sounds better to some people.

However, as an engineer, with some understanding (I hope) of what good guitar sound is, the following important factors influenced my decisions for the design of this range of amplifiers.  

*In single ended output stages phase splitting is not necessary, the output tube - or tubes in parallel single ended (PSE) circuits - produce the whole signal. So there is no discrepancy in the harmonic content within the same signal.

In a two tube push pull output stage, such as two 6L6 50w amp, the signal is split in two parts, one tube produces the top part of the signal and the other the bottom one. This is called phase splitting. Ideally, bias matching the two tubes is not enough; signal balance matching is required too. This is because when they are being overdriven each one (tube) must produce the same type of harmonics at any given signal frequency. When chords are being played, various intermodulation (IM) products are being produced. If the two tubes are not full signal matched, the top part of the signal will produce slightly different harmonic products than the bottom one. Depending upon the players taste, this may deteriorate the sound at high volume levels. That’s why signal drive balanced matched pair tubes such as Groove Tubes are so very well sought after. 

* Full class A - better sound continuity between low and high volume levels. For instance when you play a combination of heavy riffs and single string solos. No single ended stage could work in any other mode except full class


* A ‘Warm’ sound – this maybe because the low order intermodulation products created by the low frequency strings/chords tend to appear in the midrange, giving a certain blend of ‘warmth’ and ‘focus’ in the sound – as I read in an old hifi magazine article. This happens because of the relatively low primary inductance of the output transformer due to the air gap. See PREAMP page.

* The unique way in which the notes/chords tend to blend onto a smooth overdrive without losing their original definition. My inductor based equalizer design plays a big role in this too.

* The second and all other even harmonics are not cancelled like in a push pull stage.  In pentode mode the harmonics generated are a mixture of low, even and odd types, depending on the output tube and also how it is biased. High second harmonic distortion can make the signal sound musical and relatively clean even at high levels very near the point of overload. This is why for instance the 18w clean RMS model can sound as loud as a 40 - 50 push pull amp. Some even harmonic cancellation distortion takes place because of the driver, and some in the pre-amp stages. The amount of this cancellation can be controlled by using a master volume control between the driver and the output plus one or two preamp gain controls.

I have chosen the output tubes to be two parallel connected KT88s  for the 22W, two 6550s for the 18W outputs and one KT88 for the 10w output, and one 6V6 for the 3w output (a practice, or studio amp).

* The fact that there is no overall NFB and phase splitter makes an output amplifier with fewer stages, which gives it (subjectively) a faster attack

Sound/distortion diversity is still possible by including enough pre-amplification and appropriate gain/master control arrangements.  

Theo Argiriadis





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