Test Report on: Ampeg G212 Amplifier
Date: During February 1975

The Ampeg 212 is a combination amplifier suitable for professional usage.

On subjective tests with a guitar, the amplifier exceeded our expectations and gave a very good sound, controlled by extremely effective tone controls. The reverberation sound was pleasant and undistorted and had good tonal characteristics. The tremelo was also good and did not transmit a 'thump' to the speakers but it did cause a slight loss of output volume.

The performance was better than one would expect from an amplifier of its physical size, and controls were easy to use and obvious in function.

Construction & Layout

The cabinet is constructed from ¾in. plywood and covered in a soft fluffy material not usually used for this type of application. There are large chrome plated metal corner caps on the cabinet giving very practical protection to the unit.

The control panel is sufficiently large to allow well spaced controls with reasonably sized knobs recessed adequately to protect them from accidental damage. The front grill is on its own frame and has ample clearance for the two 12" Altec-Lansing loudspeakers to execute full movement of their cones. There is a handle on the top of the cabinet which is too far from floor level and cannot be used to carry a unit of this weight. There are no handles on the ends of the cabinet, which is where we think the handles need to be positioned.

The amplifier chassis can be removed for servicing by removing four screws from the end-cheeks of the cabinet. On the top panel of the chassis is a circuit diagram, which will save service engineers a great deal of time, and reduce repair costs significantly. The whole chassis forms an electrostaticly screened box which ensures that it neither receives or transmits interference.

The output transistors are on an adequate heatsink and are protected by a perforated metal cover. The electronic components used are of well known (American) brands and are mounted on phenol bonded paper printed circuit boards. This material is not as good as glassfibre but is commonly used to good effect in this type of equipment. High-powered resistors are mounted on their leads well above the boards to avoid burning them, although the support wires are thick, some support spacers would have been advantageous in equipment intended for frequent 'humping'.

The quality of soldering was good and the wiring neat and well planned. The electrolytic capacitors and mains transformer were of ample size and should give a trouble free 'work-life'.

The 'Folded-Line' reverberation delay line, manufactured 'by the beautiful girls of Wisconsin, U.S.A., under controlled atmospheric conditions' (quote from device's label) is mounted within the screened chassis enclosure and did not give feedback at any time during our tests, at any Volume level.

High frequency 'roll-off' (protection from fast transients) is included in the output in order to protect the loudspeakers. The output stage is D.C. coupled so no troubles from noisy electrolytics will be experienced. This arrangement also gives good rejection of mains-borne noise, however, we did not carry out any exhaustive tests on this aspect of the equipment.

Power

116 watts r.m.s. into 8 ohms at 1 KHz. This was measured at the onset of clipping (0.5% T.H.D. 30.5 V r.m.s.).
Power Output at 1% T.H.D. gave 124 watts r.m.s.
Power Output at 10% T.H.D. gave 149 watts r.m.s.

Sensitivity

All figures were measured at the onset of clipping and a measuring accuracy of ±5%.

Tremelo

With Depth Control maximum 80% modulation; the peak level is only 70% of normal non-tremeloed signal. Maximum Speed 11 Hz, Minimum Speed 1 Hz.
The modulation (±5%) is triangular waveform.

Tone Controls

These measurements were made on Channel 2 and Channel 1 was checked to confirm that it behaved in the same way. The measurements were only very slightly different.

Reverb

An attempt was made to measure the reverb first echo time by shocking the Input with a square edge; however a reliable measurement could not be obtained.

Background Noise

Measurements taken with tone controls level and both volumes at maximum.
Signal to Noise Ratio -71.6 dB.
The noise becomes far greater when treble boost and ultra boost are added.
In the extreme case with ultra boost and full treble on Channel 2, the signal to noise ratio measured -48.5 dB.
Hum from the mains is well below noise level.

Output Protection

The manufacturers instructions warn about loading the amplifier below 8 ohms. We loaded the amplifier with 4 ohms and wound up the signal and were unable to detect the presence of any overload protection system. Having no instructions from the manufacturers about output protection we decided not to subject the amplifier to short circuit tests.

Capacitive Load Test

A load of 1 micro farad was placed across the 8 ohm load and we found no sign of instability using any level of input signal. The conclusion drawn was that there would be no instability problems under normal operating conditions.

Test Equipment

Ferrograph Model R.T.S. (by courtesy of Advision Studios)
Advance OS25 with calibrator.
Resistive loads 2% tolerance, 4, 8 & 16 ohms.
Power measurements are correct to ±10%.
Sensitivity measurements are correct to ±0.5 dB.
The test equipment was also used for the Marshall Lead 100.