This beautifully presented amplifier, from John Howes' vintage collection, was a joy to test. Pye's own specifications have the HF5/8 labouring under a mere 0.3% THD at 1 kHz/5W output, the amplifier climbing to a 'max power output' of 8W. Bearing in mind that the diminutive EL90 beam tetrode valve is itself rated at 4.5W in a single-ended configuration, this was always going to be a challenge but the HF5/8 still mustered a full 6W/8ohm at exactly 1% THD. There is really no advantage under dynamic conditions but I was able to coax closer to 7W/8ohm by relaxing the THD ceiling to 2% - into lower impedances still the amplifier dropped to 4W/4ohm and 2W/2ohm output [see Graph 1, below]. Distortion is a little higher than Pye may have thought at the time, ranging from 0.1% at 0.5W, 0.2% at 1W, 0.45% at 2.5W and 0.85% at the rated 5W. Pye gave no indication of load, so it's likely the 15ohm 'standard' of the day may have elicited slightly more favourable results.
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