Hawes Amplifier Archive by James T. Hawes, AA9DT

“Tube Sound” with FETs: Get That Feelin’

Art: Animated 
           tube cartoon   


•Psychoacoustics: The Tube Sound

•The Three Sources of "Tube Sound"

•Amplifier Design Effects

•Tube Effects

•Audio Transformer Effects

•Tube Effects, Table

•Achieving Amplifier Design Effects

•Achieving Tube Effects with JFETs

•Tube Effects with DFETs, Table

•Depletion vs. Enhancement

•Amplifier Design for DFET Amps

• Enhanced Solid Tube Tech

Art: FETs vs. tubes as two boxers

Psychoacoustics: The “Tube Sound”

The “warmth” of vacuum tubes: What is it really? “Warmth” is a psychoacoustic effect that we can hear. This article defines how an impressive tone can come out of speakers that connect to tube hardware. Plus, how to achieve a parallel eloquence with field-effect transistors (FETs). We don't promise to deliver the exact same effect. Such an assertion would be sacrilege to our tube brethren. But with FETs, one can produce a tone that is rousing, expressive, and inspirational.

Cautions and notices. A tube is a tube, and nothing else is. For tube sound from tubes, click this link to one of our tube vendors. Back to the ranch: This article refers to DFETs (depletion field-effect transistors). The DFET family includes both JFETs (junction FETs) and depletion MOSFETs. (MOSFETs are metal oxide semiconductor FETs).

Amplifier as instrument. To a hi-fi guy, an amplifier is a musical conduit: The goal is faithful reproduction. Yet to a guitarist, an amplifier is a musical instrument: The amplifier must reproduce the music and evoke the tone. By “the tone,” we mean the sensual effect of the amplification hardware.

Cartoon: Two views of amps, as two flags

Want to get that feelin'? Then your output transformer, capacitors, and resistors are crucial: As crucial as your guitar body type, string gauge, and pickup location.

The Three Sources of “Tube Sound”

Cartoon: Three sources of tube sound, as stairway

What is tube sound? “Tube sound” comes from three sources...

  • Audio transformer effects

  • Amplifier design effects

  • Tube effects

Audio transformer effects

Audio transformer sound may be the most important component of so-called “tube sound.” The transformer launches a midrange boost, slight ringing, and even harmonics.

Amplifier design effects

Tone. In guitar amplifiers, tone is king. A typical 1950s amplifier chains single-ended amplifier stages together with resistors and capacitors. The nickname for this style of amplifier is the R/C or resistance-coupled amplifier. Coupling components add their signatures to the output signal. If the power supply is compliant, peak signals clip or compress. The clipping is soft and clear, rather than harsh. Peak waveforms retain round tops.

Tube effects

Even harmonics. In a classic tube amplifier, a triode provides the “sweetness” or “warmth” of tube sound. The triode achieves this end by introducing even harmonics.

This “tube sound” table summarizes the most important characteristics...

Effect Type What Gets that Feelin'?
  • Sweetness. We also know this characteristic as “mellowness” or “warmth.” It's really a midrange boost. The transformer rolls off both the highest and lowest frequencies. Reinforcing the tone, the transformer rings slightly. The ringing may appear as even harmonics. Minimal negative feedback reduces the ringing and broadens the bandwidth slightly.

Amplifier Design
  • Completeness. “Old-fashioned,” single-ended amplifiers are the best way to achieve “completeness.” A Class-A, single-ended amplifier forms strong, even harmonics, particularly the second harmonic. Even harmonics lend “completeness” to the tone. The even harmonics achieve a multivocal choir effect: Each harmonic strengthens and harmonizes with the fundamental.

  • Bass response. All R/C stages lose some of the low frequencies. But assertive bass response results from high-impedance inputs and low-leakage capacitors.

  • Fullness. The input impedance of a tube circuit is quite high, typically in the hundred-thousands or millions of ohms. A high-impedance input is ultra-sensitive. It never loads down the signal.

  • Sustain (Enhanced decay time). Coupling capacitors discharge slowly into a high impedance. The slow discharge sustains the tone.

Tube Effects in DFET Amps

Musicians want a semiconductor that behaves like a tube, while producing a euphonious tone. Audio guru Nelson Pass proves that DFETs can do an admirable job. The table below summarizes how we produce the “tube sound” with DFETs.

Effect Type What Gets that Feelin'?
  • Sweetness (Midrange Boost) calls for a bandpass filter or an audio output transformer. The transformer favors midrange frequencies.

Amplifier Design
  • Completeness (Even Harmonics). With the DFET in a tube-like circuit, you'll probably obtain even harmonics. If not, add more local negative feedback.

  • Bass Response. FET amplifiers preserve bass response and bias, by stage coupling through non-electrolytic capacitors.

  • Fullness (Bass response) is excellent because gate impedance is high. With a high-quality coupling capacitor, the high impedance can deliver thundering bass.

  • Sustain (Enhanced decay time) follows from the high impedance input of each stage. Low-leakage, plastic capacitors between stages prolong the tone.

  • The Class-A circuit encourages even harmonics, particularly the second.

  • Tube-Like Tone. Out of the box, DFETs offer a tube-like tone: (1) Low-noise performance. (2) Overdriven DFETs distort softly, similarly to the way that tubes do. (3) DFETs clip less than bipolar transistors do.

Depletion vs. Enhancement FETs

Like a tube, a DFET is a depletion-mode device. A DFET's no-signal condition is “partly on.” The industry is phasing out depletion FETs in favor of more-efficient, enhancement MOSFETs (E-MOSFETs). Unlike a DFET, E-MOSFETs are normally off. But fear not: Like DFETs, enhancement devices share the high impedance and bass response of tubes.

Amplifier Design for DFET Amps

Block diagram

Block Diagram of R/C Guitar Amp

Standard Layout. A tube guitar amp has a standard layout. This layout produces the desirable sonic effects that we've described. A tube-like DFET design must also include many of the same layout features: The more, the better. These bullet points summarize the standard layout...

  • A high-gain, high-impedance preamp stage

  • Inter-stage coupling only through top-quality, non-polar, plastic capacitors (We recommend polypropylene capacitors.)

  • A passive tone stack

  • A recovery amplifier

  • A power amplifier (single or dual-ended)

  • An audio transformer

  • A dynamic loud speaker

  • Feedback from the speaker to the last preamplifier

  • A line power supply with a rectifier, filter capacitors and maybe a choke: Sag is useful compression.

  • Desirable for fidelity: A Class-A power amplifier

  • Desirable for even harmonics: A single-ended power amplifier

  • Optional: A pre-drive circuit

  • Optional: Built-in effects circuits (fuzz, tremolo, wah, reverb, etc.)

What to avoid. Apply global negative feedback only sparingly. (Such feedback introduces phase lag, especially with audio transformer designs. Phase lag adds undesirable high harmonics.) No polarized or electrolytic coupling capacitors. No op amps, Wilson current mirrors, or differential gain stages. No regulated power supplies. No digital anything.

Enhanced Solid Tube Tech

How can we enhance solid tube technology? The best way would be to install exotic, triode-like DFETs in the amplifier. At one time, such FETs were available from Japan. We call such devices short-channel FETs.

Nelson Pass. Solely for his own products, Nelson Pass commissioned SemiSouth to manufacture such FETs in silicon carbide (SiC). Unfortunately, today SemiSouth is history. Yet SiC devices from other foundries have reached the general market. For now, most available devices aren't short-channel devices.

Cartoon: Rainbow man

Ordinary DFETs have a triode region. This region is left of the pinch-off knee. Yet this range is outside the saturation region, where engineers operate the device in amplifiers.

One can experiment with devices in the triode (linear) region. The trick is to reduce the source-drain voltage below the pinch-off knee. For many devices, the power voltage would be only a few volts.

The curves (above-right) are for the naked tube or transistor. Adding a source (cathode) resistor to the DFET linearizes the output. (1.)

Drain curves for hypothetical DFET

Drain curves of hypothetical DFET. Notice flat, saturation region, right of current knee. Left of knee is “triode” (linear) region. (Click graphic to compare with triode curves.)

Straightening the curve. The source resistor improves linearity through negative feedback: That is, using the feedback voltage to dynamically adjust the bias. Bias adjustment also selects the drain curve. (You can see the bias voltages on the right side of the drain curve drawings.) The device's drain curve is the device's operating path. Using feedback bias for curve control is the same as straightening the curve, the path. A momentary bias change causes the drain current curve to jink up or down.


1. RCA, RCA Receiving Tube Manual (Harrison, NJ: RCA, 1940), 20-22. ▶Re: Negative feedback in traditional tube circuits. This material applies equally well to FET resistance-coupled amplifiers.

Google Search
Web www.hawestv.com

Copyright © 2016 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/amp_projects/amp_solid_tube/tube_sound1.htm
Webmaster: James T. Hawes
•Revision—June, 2022 •Page design tools: HTML, Notepad & Explorer