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

[John Caswell]

Whilst on this subject I note that in Noel's review of the Icon Audio MB845 MkIIM that there is a lift in the freq. response of about 2.5dB at 5Hz so perhaps it is not uncommon.
I am going to guess that it is optx related as the two amps I tested had different optxs. Nigel's was fitted with Morite optx and peaked at about 4-5 Hz whereas the demo unit was fitted with our new (very very good) transformer manufacturer and peaked at about 6-7Hz. In both cases the lift was substantially reduced with the capacitor change as documented above.
I remain convinced that there is nothing to worry about.

John

[bob orbell]

Thank's for the report Matthew and John, I will be adding the new capactors to mine today, sure I have two someware. BOB

[Phil Y]

Hello John and Matthew,
Just a thought, had you considered doing a similar test of frequency response on the KEL84 or does the different circuit mean that it would not be relevant to that amp ?

Regards, Phil.

[John Caswell]

Hi all,
Well no I hadn't, and of course it is quite likely that there is nothing amiss.
Every KEL84 I have had in didn't show any anomalies and Noel's review of it stated that response rolled off below 50Hz measuring -1dB at 16Hz so I don't think that there is anything to worry about.

John

[Phil Y]

OK, if Noel has tested the 84' (in presumably the same way) then I agree, no need to check again. I was just thinking that it made logical sense to perform the same check accross the entire WD amplifier range.

Thanks John.

[bearded fiddler]

I'm baffled by the sheer number of capacitor types available. Would this one do? http://www.benl.ebay.be/itm/WIMA-Imp...item484b46b6ac

Jan

[colin.hepburn]

Originally Posted by bearded fiddler

I'm baffled by the sheer number of capacitor types available. Would this one do? http://www.benl.ebay.be/itm/WIMA-Imp...item484b46b6ac

Jan

If the cap is for your amp
1. Contact Matthew snell of WD here http://www.world-designs.co.uk/contact.htm
he will sore you out a good cap see here
http://www.world-designs.co.uk/sax.htm

[John Caswell]

In response to PhilYs comment "I was just thinking that it made logical sense to perform the same check across the entire WD amplifier range".
No it doesn't really, a lot of the amps have been going for 15 years or more and it is really impractical and possibly verging on the ludicrous to do this, especially if it is, say, an output tx problem, as we couldn't do anything about it anyway.
I repeat, I remain convinced that there is nothing to worry about.

John

[Richard]

This is good news, well done John and Matthew The whole episode seems a bit strange as I've never seen such an LF response. All the valve amps I've checked rolled off noticeably below about 20Hz and I can only think this one had a small lift due to component variation and it being designed to maintain LF extension as far as possible. Anyway a good and simple fix

I used to have a very good low frequency sig gen went down to 0.2Hz and routinely swept down there as it was my primary sig source. When I got a low distortion oscillator which went to 9Hz I sold the sig gen as I'd never found a use for the ultra LF response so it was gathering dust and was a £200 machine even secondhand. With hindsight the fact I'd never found a use for it was its use

I'll get another sig gen as and when one comes as they're cheap these days and their distortion isn't a problem for sine testing down there.

NOT playing devil's advocate here but I agree with Phil that it would be a good idea to check Kel84 unless John is referring to a later test than the Dec 2000 one I have. If it too is using TXs from the new supplier it would reassure buyers and take John only a few minutes to check all is well.

As a potential benefit it would then be good to see Noel revisit Kel84 in a new review as it has had a few changes over the years inc new mains TX and pcb with better grounding I think.