Mega Rare Reference Quality Hifi Graaf GM-OTL Vacuum Tubed Class A Amplifier with Russian 6C33C Tubes unrivalled in the Hifi World

This is unrivalled simple!

Supplied with the reference level
6C33C Military-grade Tubes


The performance is nothing short of world class just check the reviews

This is a beast to say the least weighing in at 23kg

I've had this from new and is roughly 10 years old

This will be fully re biased and all voltage calibration for peak performance

The Graaf GM 20 OTL is a highly regarded, Italian-made, 20-watt per channel, Class A, output transformerless (OTL) vacuum tube power amplifier known for its transparent, dynamic, and detailed sound. 

Key Features and Specifications

The GM 20 is notable for its fully differential, balanced, and DC-coupled design, which contributes to high immunity to noise. It uses 6C33C tubes in a bridge circuit for the power stages. 

Specification DetailOutput Power20 W + 20 W RMS into 8-16 ohmsFrequency Response1 Hz - 350 KHz (-3 dB)Input Sensitivity600 mVInput Impedance50 KohmsTotal Harmonic Distortion (THD)< 0.4% at 1 WWeight22 KgDimensions (LxBxH)35 x 44 x 22 cm

Performance and User Experience

Sound Quality: Reviewers praise the GM 20 OTL for its "unrivalled transparency, speed and dynamics," offering a good balance of clarity, fluidity, and warmth. The OTL design allows for an immediate and intimate sound, with a palpable sense of "presence".

Aesthetics: The amplifier is often described as "exquisitely beautiful," featuring a high-gloss black chassis and chrome tube cover racks.

Speaker Matching: Due to its OTL design and 20W output, it performs very well with efficient or specific speaker loads like electrostatic speakers, which can present tough but manageable impedances for this type of amp. Some users recommend using zero autoformers to protect speakers from potential tube failure.



The Graaf GM 20 OTL amplifier specifically uses the 6C33C as its main power output tube, which is a robust Russian military-grade tube known for its high current capabilities. 

Primary Tube

6C33C: This is the designated power tube for the Graaf GM 20 OTL amplifier. It is a Russian-made, military-grade triode, often referred to as a "workhorse" tube, and is central to the OTL design of the amplifier, known for its ability to deliver high current with a low voltage power supply. Due to their specific use in OTL designs, these tubes can be rare, and owners often keep spare matched pairs. 

Driver/Gain Stage Tubes

The amplifier also uses smaller signal tubes in its input/driver stages, which are important for bias and offset regulation: 

EF184: This is a pentode used in the input/driver stage.

E88CC (or compatible equivalents): Used in conjunction with the EF184s for the amplifier's balanced operation and offset regulation. The designer has noted that a PCC88, which is the same tube with a 7V heater, can also be used as a substitute and some users find they outperform the E88CC in the GM-20. 





Reliability: The output tubes (6C33C) are noted as being rare and potentially difficult to replace, though the amplifier is built with care to ensure stability over time. 

The Graaf GM 20 OTL (Output Transformer-Less) vacuum tube amplifier is a Class A stereo power amplifier designed for high-fidelity audio reproduction, delivering



20W+20W20 W plus 20 W

20W+20W

RMS into 8-16 Ohms. Renowned for using 6C33C tubes, it features a wide frequency response (



1Hz−350kHz1 Hz minus 350 kHz

1Hz−350kHz

at -3dB), minimal negative feedback (6dB), and a weight of 22kg. 

Key Specifications of the Graaf GM 20 OTL: 

Power Output:



20W+20W20 W plus 20 W

20W+20W

RMS.

Technology: Vacuum tube Class A, Output Transformer-Less (OTL), using 6C33C tubes.

Frequency Response:



1Hz−350kHz1 Hz minus 350 kHz

1Hz−350kHz

(-3 dB).

Input Sensitivity/Impedance:



600mV600 mV

600mV

/



50kOhms50 kOhms

50kOhms

.

THD (Total Harmonic Distortion):



<0.4%is less than 0.4 %

<0.4%

at



1W1 W

1W

.

Damping Factor: 4.

Negative Feedback: 6 dB.

Signal-to-Noise Ratio: Linear 85 dB, A-weighted 95 dB.

Dimensions/Weight: Approximately



35cm×44cm×22cm35 cm cross 44 cm cross 22 cm

35cm×44cm×22cm

(



L×B×Hcap L cross cap B cross cap H

𝐿×𝐵×𝐻

) and 22 Kg.

Power Consumption: ~450W (idle).