ECC88 is not an SQ valve, only E88CC. The high gm of both means low shot noise, when referred to the input, but not necessarily low flicker noise. A valve may be low noise at 45MHz, but still noisy at 1kHz.
ECC88 provides good linearity at middling gain, combined with low impedance. This is an attractive combination of features for many circuits. It does not mean, as some seem to imagine, that any random circuit can be improved by swapping the designed-for valve for an ECC88.
ECC88 provides good linearity at middling gain, combined with low impedance. This is an attractive combination of features for many circuits. It does not mean, as some seem to imagine, that any random circuit can be improved by swapping the designed-for valve for an ECC88.