Marantz Model 20B
```markdown
layout: technical-manual/model.njk
title: Marantz Model 20B
brand: "Marantz"
model: Model 20B
category: Tuner
years: 1965-1967
tags: [tuner, tube, fm, marantz, separate]
Marantz Model 20B
The Marantz Model 20B is an FM-only tuner introduced in 1965 as part of the company's professional-grade "separates" line. It was designed to pair with the Model 15 power amplifier, and it shares that amp's blue-backlit meter and chassis styling so the two look like a set. The 20B succeeded the earlier Model 10B tuner. It used a superheterodyne circuit instead of the 10B's complex and costly heterodyne design, which made it a more commercially viable product, and it still aimed at the high-end market.
The tuning mechanism is the interesting part. It uses a dual concentric shaft. The outer shaft controls the primary tuning through a precision 4-gang capacitor, and the inner shaft operates a proprietary "M-X" (Multiplex) filter for noise reduction. The circuitry is entirely tube-based, with 13 tubes, including two 6AU6s in the IF stage and a 6CG8/6FQ7 as the ratio detector for FM demodulation.
Specifications
| Specification | Detail |
| Type | FM Superheterodyne Tuner |
| Frequency Range | 88 MHz to 108 MHz |
| Tubes | 13 total (6CG8, 6AU6 x2, 6U8, 6BH6, 6EA8, 6DT6, 6DR7, 12AT7 x2, 6AQ5, 0A2) |
| IF Stages | 3 stages, 10.7 MHz |
| Usable Sensitivity | 2.5 µV (IHF) |
| Capture Ratio | 3.0 dB |
| Selectivity | 40 dB (alternate channel) |
| Audio Output | 2.5 V RMS (100% modulation) |
| Output Impedance | 10 kΩ (High), 600 Ω (Low) |
| Harmonic Distortion | 0.8% (mono) |
| Semiconductors | 4 silicon diodes (in power supply) |
| Power Consumption | 65 Watts |
| Dimensions (W x H x D) | 16-1/8" x 5-1/8" x 13-3/4" |
| Weight | 28 lbs (12.7 kg) |
Historical Context
Marantz built the Model 20B from 1965 until approximately 1967, at the peak of the tube era and during the rise of high-fidelity FM broadcasting. It was a no-compromise tuner meant to pull the most out of the FM band, and it sat between the ultra-expensive Model 10B and the more consumer-oriented models. It also came shortly before Superscope acquired Marantz in 1966, which shifted the company's focus toward integrated receivers.
Design Features
The 20B's main technical features are its tuning system and its filtering. The dual concentric tuning shaft handles both main station tuning and adjustment of the proprietary "M-X" notch filter, which is designed to reduce high-frequency noise and interference from multiplex stereo signals. The front panel has a large, blue-backlit tuning meter that shows signal strength and center-of-channel tuning. The chassis is built on a heavy-gauge steel frame, and the critical audio stages sit in aluminum shield cages to prevent interference. The power supply uses a solid-state silicon diode rectifier, which is more reliable than a tube rectifier.
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