The feedback control could also be called Clarity, Gain or Presence.In the world of electronics, vacuum tubes are almost obsolete.Nearly the last holdout, the cathode ray tube (CRT), is rapidly being replaced by the LCD and other new technologies.Despite this trend, the vacuum tube has seen a big revival in the field of guitar amplifiers, and to a lesser extent, hi-fi amplifiers.
Vacuum tubes and related parts have become more readily available in recent years as numerous companies have tapped into this market. The reason for the popularity of tubes in guitar amps involves the nice tones that are produced when tubes are driven to the point of distortion. For some background on this, follow some of the links on The Strat Monger. There are numerous solid state modeling amps that try to simulate vacuum tube amps with digital signal processing (DSP) techniques, but in the end, that method is never more than a simulation. One can spend a large amount of money and time building a tube amp from scratch. ![]() The difficult job of cutting chassis holes for the tubes and transformers is already done, one just needs to drill a few holes for the potentiometers and connectors. This project started with the amplifier from a Hammond M2 organ, chassis model AO14-1B. The output stage of this amplifier resembles a fusion between a Fender Princeton Reverb, Fender Vibroverb and ham radio transmitter. With 6V6 output tubes running at a 420V plate voltage, it puts out approximately 18 watts of audio power. The simpler is better philosophy was used in the design, multiple inputs with their own preamp stages were intentionally avoided to reduce hiss. The Hammonator amp has worked well driving both 12 and 15 guitar speakers. The Hammonator Model 1 amp is a simplified version of the Hammonator 2RVT circuit. Builders can start with the Model 1 circuit and easily add the Model 2RVT VibratoTremolo circuitry at a later date.There are a few unique features in this amp, and some slight deviations from the aforementioned simplicity goal. An optional fluorescent EM876HU6 magic eye tube (EM87 in action) is used for an output level meter, it is fun to stare at while playing. The EM87 uses a peak reading circuit that was inspired by this design then modified somewhat. There is a reverb send control (Dwell) that can be used to expand the variety of reverb sounds. Most Fender amps send only a full-strength signal to the reverb spring. By turning the reverb send signal down a bit, a less clangy and more spacey reverb sound results. With the feedback control turned all the way to the left (max negative feedback), the amp compresses the signal and the waveform peaks are reduced. With the feedback control turned all the way to the right, the sound is louder and less compressed and approaches that of the popular Fender Tweed Deluxe (5E3) amps.
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