|The F-2B contains two independant preamp/tone control sections. The amplification is entirely vacuum tube powered. It is normally used as a stereo preamp to interface stereo instruments to power amplifiers. Jacks are provided on the rear for outputs from each channel. In addition a Mono output jack is provided which combines both channels when the F-2B is used with a Mono power amplifier|
Circuit detailThe F-2B was inspired by the Fender Dual Showman amplifier. In the late '60's, we routinely added preamp output jacks to Dual Showman amplifiers and substituted an external power amplifier for the built-in quad 6L6's, most usually a MacIntosh 75.
The F-2B came about to provide the same preamp functionality in a rack-mounting package, as the Dual Showman was considerably wider than the standard 19 inch rack.
We took the opportunity to make a small improvement in the input jack circuit, while providing identical functionality. Plugging into Jack 1 alone gives full sensitivity, while Jack 2 alone is attenuated 6 dB. If signals are plugged into both jacks simultaneously, mixing resistors combine the signals and give isolation to the two input sources. In the original circuit, the mixing resistors are always in series with the grid of the first stage, and contribute a small amount of Johnson (thermal) noise. Our jacks have additional contacts which completely switch out the mixing resistors when using Jack 1 and the ultimate quietness of the tube is preserved.
As in its predecessor, the instrument signal is amplified by the first stage before any volume control. Without the wide dynamic range provided by the 300 volt supply, the first stage could be easily overloaded by large transients which are characteristic of electric instruments.
The tone control section follows, with a circuit attributed to Tom Walker, wherein the Bass and Treble controls contribute mostly boost and the Mid control provides cut only. While there is no absolutely flat position, the response is reasonably flat with the controls set at 2 - 10 - 2, with the bright switch off. The circuit can be described as interacting, such that the frequencies affected by each control is changed somewhat by the settings of the other controls. Additionally, when all 3 tone controls are turned to 0 then no signal is passed at all. These effects, while not suitable for a music reproduction system, have nonetheless proved highly pleasing for electrical instruments and have been adopted by a wide group of manufacturers.
The Volume control immediately follows the tone control section. The Bright Switch connects a capacitor around the volume control, the value of the capacitor being selected to bypass only the high frequencies. The effect of the Bright Switch is also interactive with the Volume Control, and there is no effect if the Volume control is turned up to 10, and the magnitude of the effect changes, depending on the setting of the control.
The signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used. While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.
The mono output on the rear of the F-2B provides a facility to use a stereo instrument with a mono power amplifier. The mixing resistors associated with the mono jack are only connected together when a plug is inserted, thus preserving maximum stereo isolation when using the separate Channel A and Channel B outputs. Another application is to connect the two channels in cascade, by a cable from the Channel A output jack on the rear to the Channel B input jack on the front. The power amplifier is connected to Channel B's output. The signal from Channel A is high enough to drive the first stage of Channel B into distortion. The tone controls of Channel A are used to emphasize those frequencies which will be most distorted, while the controls of Channel B shape the color of the resulting distortion. Channel A's volume control sets the amount of distortion, while the Volume Control of Channel B sets the output level (Master Volume).
The power transformer has taps so that the F-2B can be operated from 100, 120 or 240 volt lines (Mains). It is necessary to open the cabinet and solder the correct tap to the power switch to make the voltage change, however. The Line (Mains) input connector is the IEC (computer type) so the power cord is detachable. The layout of the circuitry and the lead dress to the front-panel components are carefully planned to reject external hum fields. In addition, an internal hum-balance control rejects hum from the AC heater supply. The package is a single unit standard EIA rack mount steel enclosure designed to survive on the road.
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