LF Lift in the KT88

[Matthew Snell]

I'm happy to report that the Low Frequency Lift that Noel Keywood reported in the review of the KT88 has a perfectly satisfactory solution. John Caswell has worked his usual magic and (although he won't tell me why ) has found that increasing the value of the first stage coupling capacitor (C1) to 1.0uF produces a frequency response curve that stays flat pretty much all the way down to 1Hz; there is still a slight lift - but innocuous, and nothing to worry about. I am very grateful to John for sorting this out for us, and thanks also to Nigel (Vinylspinner) for providing us with a loan KT88 which enabled us to double-check everything. I've run this mod. for a couple of weeks now and I'm completely happy with it - thanks for your patience!

I think it fair to say that in most situations, this is a fix to a problem that doesn't really signify - but it's an easy one to do and I'm a firm believer in "if it CAN be right, then it SHOULD be right".

For the perfectionists among you who want to do this modifcation, there is no need to remove the existing C1 (0.1uF) - just install a new 1.0uF capacitor in parallel with it to give a final value of 1.1uF. For those of you who have bought the KT88 since February 2011 (i.e. since I took over World Designs) I'll be happy to supply replacement / supplementary capacitors free of charge. Apologies, but I can't do this for earlier buyers - but I can, and will, give a decent discount.

Thanks to all for your input on this, and John: I owe you!

Matthew