I've found alot of modern ECC83's rather microphonic. Whether this is a problem or not depends on the model of amplifier, where the valve is mounted, the gain of the circuit, etc. The guitar amp valve specialist retailers can supply microphony-tested valves to order. A mullard pre-amp valve is going the way of a certain other sought-after audio valve, with the colour of the ink and the batch code determining how many 20-pound notes extra you'll have to cough-up.
