Vintage

Over a half century out of its five-year guarantee period, our sample of Bryan's Mark III Model 500/700 combo was still going strong. Specifications from the era are limited and the 2x13W power output needs to be viewed with some caution as this was rated into 15ohm, not the 8ohm standard of today. However, the 2x6W/8ohm achieved here is largely in line with what might be expected, and the 2x10W/4ohm rather better still. Unusually, output under dynamic conditions was less than its continuous power [see Graph 1], ranging between 2-4W from 8 to 4ohm even with a high 5% limit on the acceptable distortion. Harmonic distortion was also rather higher than the 0.25%/10W quoted by Bryan Amplifiers Ltd back in the day, this figure also referenced to an easier 15ohm load. Into 8ohm, midrange distortion increased from 0.9%/100mW to 1.9%/1W and 2.1W/5W. Distortion also increased significantly at low frequencies to 8.5%/20Hz and high frequencies to 4.4%/20kHz (all re. 0dBW).

Otherwise, the Model 500/700 combination matched or exceeded its other specs, including its 40Hz-20kHz/±1dB frequency response, with bass/treble controls set to 12 o'clock. In practice the roll-off was pronounced above 20kHz, falling to –3dB/25kHz and –40dB/100kHz, while sub-bass extended to 14Hz (–6dB), albeit with a max +1dB 'boost' from 40Hz-200Hz [see Graph 2, below]. The 'Rumble' filter [dashed trace] kicks in a little earlier at 23Hz (–6dB) while the HF 'Scratch' filters almost exactly meet their 4kHz/6kHz/9kHz (–6dB) roll-offs. Finally, the combined 500/700 amps' gain is +31dB (Aux input) and although the A-wtd S/N ratio is a little below par at 75dB (re. 0dBW), allowances should always be made for vintage pieces. PM