Here is a general description of the Bell 2200C amplifier:
Bell 2200C Mono Integrated Amplifiers
July 26, 2018
Description:
The Bell 2200C is a 20WPC (RMS) integrated monaural push-pull amplifier, of mid-to-late 50’s vintage with a full range of inputs and controls using 6L6 family output tubes (6L6GB and 5881 are specified but 6L6GC tubes may be used). 6SL7 tubes are used for the audio pre-amp stage and the phase inverter and 6SN7GTA is the audio driver tube. The phono pre-amp tube is a 6SC7 double triode. A 5U4G rectifier is specified and the power supply (unlike the 2200B model) uses R/C filtering throughout the B+ supply.
The amplifier uses NFB coupled from the 16 ohm output tap to the input of the driver stage combined with a tertiary cathode feedback winding, all of which provides quite low measured distortion levels up to maximum output power (< 0.5% THD+N measured 20-20kHz at 1W output and below 0.5% up to rated power at 1 kHz, both on a stock-unit).
Four, 8, and 16 ohm output taps are provided with a hum balance potentiometer in the 6.3VAC heater circuit, which is elevated to about +30VDC by the bias supply through the wiper of the hum pot. Overall bias level is fixed with the output tube cathodes measured around +30 VDC and the control grids operating around +2.5VDC, giving around -27.5V control grid bias levels and quiescent plate current draw of a little over 40 mA for each output tube (with used but tested good output tubes). Output tube current draw is balanced with a single DC BALANCE pot located under the chassis, but there is no adjustment for overall bias level.
Line level inputs on the SELECTOR control include TV, RADIO (with a level adjustment pot on the rear of the chassis), and TAPE. Low-level input is amplified through the 6SC7 phono pre-amp circuit and is selected in the PHONO position (with MAG1 and MAG2 inputs for high and low output magnetic cartridges and a CRYSTAL phono cartridge input jack). A low-cut RUMBLE FILTER position is available for PHONO inputs to counter excessive record or turntable rumble. A MICRO microphone input jack is located atop the chassis with a slide switch to select MICRO or PHONO. The microphone input operates through the second stage of the phono pre-amp.
An EQUALIZER selector has settings for “78 RPM,” “COL-LP,” “RIAA-ORTHO,” “FFRR,” and “EUR” equalization networks.
A “HIGH-Z AUX OUTPUT” jack is also located on top of the chassis with output from the 6SL7 pre-amp available (before the tone controls), presumably to allow the 2200C to be used as a pre-amp stage for a separate power amplifier.
There are tone control knobs for TREBLE and BASS. The on/off switch is ganged with the TREBLE control in the 2200C and the tone controls are active. Playing volume is adjusted by the “GAIN” control, distinguished from the “LOUDNESS” control.
A somewhat unusual continuously variable LOUDNESS control is provided on the front panel as well, demarcated in “-dB” and replacing the (more typical) “Loudness” switch found on many integrated amplifiers through the 50’s and 60’s (including the Bell 2200B model). In the zero position (fully clockwise), the control is not engaged, but as the knob is rotated counter-clockwise, a loudness filter network is engaged that progressively boosts bass and treble response compared to mid-frequencies while reducing the output volume level, thus maintaining the ability to reduce playing volume while boosting the low and high ends, as a more typical switch would do with “Loudness” engaged, as these curves show:
The curves are for “LOUDNESS” knob settings (working down from the top) of zero, -5, -20, and -30 dB.
The amplifier is quite sensitive, requiring <0.25 mV in the line level inputs to drive to full output power with the “GAIN” control at maximum.
At the time of this writing, no manuals or schematics can be located for the Bell 2200C model. Although a schematic for the B model is available, close examination of the 2200C model has revealed several
significant differences between the B and C model:
1. Power Supply—the C model power supply uses no chokes, unlike the B, and the entire C model B+ string is R/C filtered. The first three resistors in the C model string have different values, but the same wattages. Voltages appear to be similar at the key take-off points. The line fuse value is 2.0A slow blow in the C model and 1.8A SB in the B model.
2. Phono Pre-Amp—The C model uses a 6SC7 tube with the heater supplied from the 6.3VAC winding on the power transformer, compared to a 12SC7 in the B model powered by DC voltage generated in the output tube cathode circuit.
3. Loudness Control—as mentioned above, the C model uses a continuously-variable Loudness control, instead of a single-setting switch in the B model. In the C model, the overall volume is controlled by the Gain control.
4. Radio Level Control—the C model uses a 250K level adjustment pot for this line level input located on the back panel.
5. Bias and Grid Resistors—the C model power resistors in the bias circuit (connected to the ends of the DC balance pot) are 500 ohm 5W units compared to 250 ohm 4W units in the B model. The output tube grid resistors are 270K 1/2W, compared to 470K 1/2W in the B model.
6. Phono Pre-Amp Tube Plate Voltage--The plates in the 6SC7 phono pre-amp tube are wired differently in the C model compared to the B. The second plate (pin 5) take off is wired downstream of a 56K ohm resistor for total resistance of 56K + 27 K ohms = 83K ohms between this plate and 100V supply. This results in reduced voltage for the second (pin 5) plate at around 70VDC compared to the first (pin 2) plate at about 90VDC as measured.
7. On/Off and Loudness Contour Switch—The On/Off power switch is ganged with the Treble control in the C model. In the B model, there is a three-position rotary switch that has Power Off/Power on/Loudness Off settings. That is, the Loudness contour switch turns ON with the power and is switched off in the Loudness Off setting.
8. Equalizer Control Settings—The C model settings are: 78 RPM, COL-LP, RIAA-ORTHO, FFRR, and EUR. In the B model, these settings are: AES, LP/NAB, FLAT, FGN, and FFRR. Switch wiring and the configuration of the equalization networks are, of course, different as well.
Dave