Gold Lion KT88 Issue

[Loren42]

I recently purchased a matched quad set of these tubes to replace a quad of Rubys where one of the tubes had failed. There are two tubes per channel.

The failed Ruby simply stopped conducting current, so I sprung for a set of Gold Lions.

When the amp is first turned on and the delay circuit finally kicks in, one channel of the amp produces a moderate hum for about 3 seconds, then it promptly goes away. The amp plays fine after that initial hum. Bias appears normal.

I swapped tubes between the left and right channels and the problem also follows the tubes, so I suspect that one tube has a problem.

Any thoughts?

Thanks in advance.

[the_manta]

Maybe one KT simply needs a bit longer to fully heat up and conduct properly. But 3s is too long for such a anomaly. Another option would be a heater cathode insulation failure. When not heated, the isolation failes and when heated up, the heater expands in the cathode tube a bit and it's gone. Last thing would be anomalies in the building up of the depletion region due to mechanical issues or gas inside the tube. It doesn't matter how you turn it....there is a failure in the tube.
I suggest to simply return the tubes where you bought them. I never bought any KT88 and when I would take the cheapest (because I can return them when they don't reach the specs) but when I hear the name "Gold Lion" I assume these weren't the cheapest KT's. So just return them
Regards

[TubeHead Johnny]

I've heard rumors the New Sensor manufactured KT88's bias can drift wildly during warm-up. You said the amp is correctly biased and sounds fine. Is there a way to monitor the bias current as the tubes are warming up? It sounds like the mentioned tube is drawing too much current during start-up, causing the hum. Before returning the tubes, see if there's a way to prolong the delay circuit to 10 seconds or longer.

[Loren42]

Originally Posted by TubeHead Johnny

I've heard rumors the New Sensor manufactured KT88's bias can drift wildly during warm-up. You said the amp is correctly biased and sounds fine. Is there a way to monitor the bias current as the tubes are warming up? It sounds like the mentioned tube is drawing too much current during start-up, causing the hum. Before returning the tubes, see if there's a way to prolong the delay circuit to 10 seconds or longer.

Thanks. The delay is currently set to 55 to 60 seconds. I would think 55 to 60 seconds would be more than long enough.

I only watched the bias with a digital meter and it did not vacillate wildly, but the affected side does tend to drift a little on one of the tubes, so that has me thinking that the tube is flakey.

[TubeHead Johnny]

I really wouldn't worry about it too much Loren, because ALL tubes drift during warm-up. That's why they tell you not to do any bias adjusting until the amp has warmed up for at least 15 minutes.

[Loren42]

Originally Posted by TubeHead Johnny

I really wouldn't worry about it too much Loren, because ALL tubes drift during warm-up. That's why they tell you not to do any bias adjusting until the amp has warmed up for at least 15 minutes.

I did wait until the system warmed up to adjust the bias and then rechecked every hour or so to fine tune it.

It seems like only one tube will vacillate after it warms up. The others show a very steady reading.

I'm measuring the bias current by looking at the potential difference across a 10 Ω cathode resistor. So, it is the actual difference across that resistor that fluctuates.

[TubeHead Johnny]

If the bias badly fluctuates even after a significant run time, then it does indeed sound like you have a bad egg. I had the same problem with a JJ KT88.

[Loren42]

Second tube has now gone bad. This one is microphonic when you tap lightly on it. It makes a cracking noise and it is definitely the tube.

I would hope this is simply a bad batch, but two tubes out of four is not a good indication for quality.

Has anyone else had or know of recent quality issues with Genalex KT88?

The Chinese Rubys at least lasted a year of more before one went belly up. The Gold Lions haven't made it two weeks before 50% of them failed.

[TubeHead Johnny]

What is the current setting for each tube? At what plate voltage? If they are biased very hot, they will fail quickly.

[ixe13]

Hi,

Last year, I bought 3 Gold Lion KT88 for my tubelab Simple SE.

On checking the Bias after a little while, on one of them, the bias kept slowly rising and rising...

I had bought the third one as a backup, so I decided to use it instead and at the same time see if it could be a problem developping on the amp itself.

Well, would you believe it, that third one got real noisy after a few days...

Popping sounds you know...

Then I was really begining to think that it was the amp, did some tests and it was the tube that was bad, wow!

I decided to use 6550's instead, the Tung Sol ones, well, got pretty surprised how good the amp sounded with these, I'd say as good than the kt88. To my ears anyway. A bit brighter, but in a good way

they got only about 100 hres on them so far, I dont listen only to that amp, got the bug and build a few so far ,

They're very stable so far. Also I use EDCOR OPT on the Simple SE.

I must say though that I use 4 Gold Lion KT66 on a pair of QUAD II , and these are Rock solid with more than 300 hours on them. The bias didn't move at all. Well, 1Ma on one of them. Impressive.