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DT50/DT25: Terminology

loop feedback amp tube class a/b bias dt50 dt25 pentode

DT50/DT25 Terminology FAQ

Headroom: how much signal “room” you have before encountering distortion, particularly saturation. If you have more headroom, the amp will feel like it responds to your playing subtleties better and you’ll have more dynamic range for expressive playing.

With less headroom, you get into serious distortion without playing as hard.

Topology: the layout of a “feedback topology” of the amp circuit referring to the voicing of the global negative feedback of the DT50. Changing circuits will change the voicing of the amplifier.

Class A: an amp design where current is flowing through the tubes constantly. Headroom is deceased, so tone becomes distorted sooner.

Class AB: an amp design where a pair of tubes work together by “trading” duties (one is conducting current while the other is in cutoff). Sometimes the term “push-pull” operation is used to describe this mode. Provides greater headroom and tone remains clearer as a result.

Bias: the voltage value between the control grid (grid #1) and the cathode, which dictates the power tube’s behavior in terms of distortion cutoff/saturation. There are two types of biasing methods: fixed-bias and cathode-bias (sometimes called self-biasing or auto-biasing).

The DT50 operates in fixed-bias when in Class AB. It switches to cathode-bias when in Class A.

Pentode: the power output tube is a 5-electrode object (cathode, control grid, screen grid, suppressor grid and anode/plate). In the DT50, when you select “Pentode” mode, you’re operating the EL34’s as pentode tubes.

Triode: a mode of operation where the power tube has 3 active elements (cathode, control grid and the anode/plate). Setting the front panel control switch of the DT50 to “Triode” alters the behavior of the EL34’s to act more like a triode, which reduces the power output and affects the high end as well.

Negative Feedback Loop (NFL): a certain amount of the amplifier’s output is fed back into the amp to reduce distortion and increase headroom. A higher amount of NFL results in a tighter response overall, which some guitarists may be seeking in their tone. A lesser amount of NFL can make the amp feel more “wild” and “unruly”, which can be also desirable to many guitarists. It depends on what you’re after and the context of what you’re after, musically-speaking.

Tonestack: the EQ-region inside of an amp where the Bass, Mid and Treble controls live.

Sag: the response of the amp to what is being played on the electric guitar, usually heard at the beginning of playing a note or chord that follows a rest. Sag will cause the amp to get a bit quieter, while also causing more distortion.