About my KT88 Tubes


I am fairly new to analog,tube audio equipment so I thought I'd use this thread to gain some insight on a tube situation that I am experiencing.
My Cary 120S amp uses 8, KT88 power tubes and came with 8 matched Electro Harmonix tubes. Recently, I started loosing tubes from both sides. I would replace them and rebias everything and everything sounded good. I started thinking probably these tubes were getting near the end of there useful life so I chose to replace them all with 2 matched quads of "Gold Lions" which I have heard, were superior. Upon installing them, I am noticing that my top end is very shrill almost to a tinny sound. Do these tubes have a break in period? Does anyone else have experience with these tubes?
markus1299

Showing 1 response by newbee

Well I've not used any "Gold Lions" or EH's for that matter. I usually use SED 6550's or KT88's, but that is not why I'm posting.

I have a different Cary amp and did quite a bit of research into Cary amps in general, but mainly as to tube selection and biasing, especially since I'd noticed with some Cary amps that owners were complaining about early power tube failure. Including myself. Initially I lost a 6550 in a set biased at Cary's recommended setting. It was the only SED power tube I'd ever lost thru sudden failure.

It seems there is some consensus that Cary's spec's are optimized by maximizing power tube output, thus creating sudden pre-mature failure, and the settings had little to do with maximizing tonal qualities. In fact I found in a discussion Dennis Had commenting on his preferred bias setting on some of his amps and they were markedly different that were recommended in the manuals and produced sound with his preferred tone.

As a result, I dropped the bias setting on my 6550's from 50ma per tube to 45ma per tube and have had zero problems since. Could it be a coincidence? Sure. But I think not.

There are a lot of folks who have posted very favorably about the Gold Lions, which I've never heard. But from comments from folks I trust I've concluded that they sound far more like 6550's or even KT90's than typical KT88's (some of which tend to have a warmer tone).

If I were you I'd lower the bias setting on the tubes, as I have, and give them a good 100 hours to break in. The highs might just be less shrill and the bass develope appropriately. You won't damage anything by doing so, and you might be pleasantly surprised. If the tubes are still 'shrill' look for a brand which has a reputation for more 'warmth.

FWIW.