Descriptions of Amplifier Features

Following are some brief, non-technical descriptions of the various features of some Sound City amplifiers. If you have suggestions for additions or corrections to this work-in-progress page, please contact me.

Active Tone Controls

Each tone control in the Mark 4 "active" preamp, which was first available with the introduction of the 50 Plus, 120, and 200 Plus amplifiers in about 1973, acts as a sort of gain control for that tone control's specific range: i.e., Bass, Middle, Treble, and Presence. If all "active" tone controls are set to zero (0), no sound will be available from the amp even if the Volume control is set to 10. By the way, this is the characteristic test for an "active" preamp.

     Conversely, a "passive" preamp, which was standard in the Mark 3-and-earlier Sound City amp models (and in the DMI-era Bass 150 as well as in the Mark 4 50R PA, 50 PA, 50 PA Plus, and 120 PA), does not limit the output of the amp; i.e., if all "passive" tone controls are set to zero (0), the amp will still produce sound.

     According to the Mark 4 preamp's designer, the circuitry of the "active" preamp represents a "ladder circuit" where the signal from the Volume controls is "fed" to each tone control (i.e., each rung) in the ladder, and each tone control is essentially a gain control driven by its own ECC83 tube. Although scorned by some, this feature of the "active" preamp explains why the Sound City Mark 4 amps are very loud amps indeed.

Headphone Output

For a brief time, the Sound City 120 and 200 Plus provided a front-panel, high-impedance headphone output. On the 120 this output is located just to the right of the two non-attenuated instrument inputs. On the 200 Plus, this output is located just to the left of the two Slave inputs. On both amps, this output has a headphone symbol next to it.

Impedance Selector

This feature is located on a Sound City amp's rear panel and provides three selectable taps from the amp's output transformer: 4 ohms, 8 ohms, and 16 ohms. Please note that these are typical nominal impedance values; the actual impedance of your cabinet (i.e., the value you would measure with an ohm meter or digital volt meter [DVM]) represents what is called DC resistance. For a discussion of DC resistance, see the Measuring DC Resistance link on the Related Links page.

     Some people call this selector a dial and think you just turn it, but this isn't correct. The selector is actually a small cap that is plugged into a socket that is attached to the chassis. The cap should have two or three metal prongs on it that plug into two or three holes in the socket. Here are two types of impedance selectors: 1. a cap with a small window or 2. a cap with an arrow.

     For either type, you pull the cap out of the socket then push it back in such that 1. the window shows the impedance you want, or 2. the arrow is pointing to the impedance you want.

Instrument Inputs

The instrument inputs on the various sound City amplifers provide similar functionality and differ primarily in number. There are two essential types of inputs: Normal and Brilliant (see also Volume Controls following). Each of these types provides one Attenuated input, through which the signal to the preamp is reduced.

     The Attenuated inputs would be suitable for instruments with an active (battery-powered) preamp, while the non-attenuated inputs would be suitable for passive instruments.     The 50 Plus provides four instrument inputs: Normal non-attenuated (top) and attenuated (bottom) and Brilliant non-attenuated (top) and attenuated (bottom).     The 120 provides the same instrument inputs as the 50 Plus, but also provides two Slave inputs as well (see also Slave Inputs).     The 200 Plus provides the same instrument inputs as the 120.

     The Bass 150 provides only two instrument inputs: High (non-attenuated) and Low (attenuated).

Reverb In/Out
Available on some Mark 2, Mark 3, and some Mark 4 amps, these jacks provide the standard preamp loop functionality found on virtually all modern amplifiers, and are considered self-explanatory other than to say In = Return and Out = Send.

Sensitivity Switch

In brief, and relatively speaking, the sensitivity switch, located on the rear panel of the Sound City 50 Plus Mark 4 and 120 Mark 4 amplifiers, makes the amp louder in position 1 and quieter in position 2.

     The sensitivity switch affects the preamp by changing the cathode biasing point of the V3 ECC83 by means of switching a resistor in or out in series with the main biasing resistor of that stage.

     "Cathode bias" raises the cathode voltage above ground by means of a resistor. The voltage drop on the resistor causes the voltage on one side of that resistor to be at a different potential than the the voltage on the other side of the resistor. The cathode is at a positive voltage above ground, and the grid is at ground level. Think of the cathode being at ground potential, with the grid being negative.

     The sensitivity switch works by making this negative voltage less negative, with relation to the cathode, by means of taking one of the cathode resistors out of the circuit. Then if the bias voltage is smaller, the voltage from the previous part of the circuit seems greater and provides more amplification in the valve. On some SC schematics, this switch is labeled "GAIN."

Slave Inputs

These inputs are available on the 120 and 200 Plus Mark 4 amps and provide line-level inputs directly to the amp's output stage. This functionality allows the preamp stage of one amplifier (i.e., a Master) to use (or drive) a second amplifier (i.e., a Slave) as a dedicated power amp. Two slave inputs are typically available: non-attenuated and attenuated.

     Master-Slave functionality was very popular in the early 70s when the Mark 4 amps were designed and introduced by Sound City; however, the expressions Master and Slave have fallen out of favor for reasons of, perhaps, political correctness.

Volume Controls

Some Sound City amps provide two channels by way of a Normal volume control and a Brilliant volume control. In brief, the Brilliant channel provides more treble, while the Normal channel provides less treble, although there's undoubtedly more to it than just this.

     Each of these channels shares the same tone controls; i.e., the channels do not have their own, dedicated tone controls as with, say, a Fender Bassman amp's two channels, which each have their own set of Bass and Treble controls.

     The SC amp's channels can be linked (patched together) by attaching a guitar cable between the Normal and Brilliant inputs in one of a couple different ways. This linking allows both channels' volume controls to be used, but doing this might introduce hum due to load imbalances.

(FYI: The Bass 150 is a single-channel amp.)

To be continued...

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