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Biasing SE KT120 tubes

I'm quite baffled by these tubes and what their actual dissipation rating is. They are spec'd as a 60W tube, but when I bias them over 40W on my newly build SE monoblocks I start seeing signs of them red plating on one side of the tube. At 50W they start getting some serious looking red plating. I biased up some cheapo muzishare branded KT88 tubes at 90% in the same amp and they don't show any signs of red plating. Obviously in a SE amp biasing the tubes cold has a major impact on the power they produce, I built this KT120 amp to try to get more power than my KT88 amp, which it does (and sounds great) when biased up but I'm afraid it's going to kill the tubes.

I have a quad of these KT120 tubes and they all behave this same way. Is this normal and are these tubes really just KT88 tubes in a bigger bottle or?
Are the tubes heating up, and then going into thermal run-away?
What is the DC resistance from g1 to ground?
Are you using Self Bias and bypass caps? . . . (I bet that you are not)

Is there a specification for the maximum g1 return resistance for self bias?
Is there a specification for the maximum g1 return resistance for fixed bias (or fixed adjustable bias; I bet you are using that).

If the filament voltage is correct, and all the above is correct, then I have a guess . . .
The tubes are bad (low quality).
. . . Just like stevenrotterdam said.
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Do not feel bad about delaying to use those tubes; it happens to all of us.

3 or 4 years ago, I purchased a Marantz CD player with balanced XLR outputs.
A few months ago I finally designed and built my first balanced amplifier, followed by the second mono-block amplifier.
Because of my delay, I can not purchase another identical new CD player (I want to build more balanced amplifiers to use in other rooms).
So many projects, so little time and energy.
OK diving deeper into this. I ordered a new pair of KT150 tubes from a "reliable vendor" to try to get some headroom.

The only connections to the output tube right now are the plate lead from the OT which has 235DCR, the screen from the UL tap via a 1.2K screen stopper, the cathode to ground through a 0.1% 10 ohm resistor. The grid is fixed bias with 50K of total resistance from grid to ground.

526V B+ to the transformer
503V at the plate, that's 97.8ma
17.8V across the screen stopper, that's 14.8ma
1.12-1.13v across the cathode resistor for 112-113ma

That's 56W of a 70W rated tube or 80% bias. Well within all the published design specs.

One of the new tubes is showing some red on only one side biased as shown. The other shows more red (an alarming amount) on one side at only 100ma on the cathode. The second tube also looks slightly crooked to the base and the internals look "tilted" as well. These were sold as a "matched pair"...

Below is the data sheet that comes with the tube. In the average characteristics for A1 single tube (shown below) they are running the tube at a lower voltage but much higher ma load. I'm still 100V below the rated max. They state the minimum "normal" operating point is at 60W calculated off this data they supply. Either of these tubes would be red plating at 60W. Clearly they are saying on their datasheet they intended for these to be run in class A at 80%+ bias. So it's not a case of my running these in a way they weren't designed to be run. The KT120 data sheet shows the same sort of operating points for Class A use, that I can't even get close to with 4 different tubes without them red plating.

So is this just a case of really bad QC? Given all of these are red plating on only one side, if the cathode isn't centered in the tube between the plates, I would suspect more current would flow to one side than the other causing one side of the tube to red plate and the other look fine?

Yeah I get I can turn the bias way down and cool them off, but the whole point of these SE monoblocks for me was to make aprox 2X the power of my 300B amp to drive some less efficient speakers I want to play with. Looks like with one of these KT150 tubes, I can run it at 112-113ma, make 18W at 2% THD but it's still lightly glowing red.

Was it too much for me to expect the tubes to perform to their listed "average characteristic" operating point in the data sheet? I've just never seen an output tube behave like these have. Just really bummed after spending a lot of time trusting this data sheet, that it's looks like I'm going to end up with a glorified KT88 amp at just a couple of W more than my 300B.

Wish I had seen this before designing an amp around them... I should know better I guess than to trust anything out of russia given my experience with their camera gear in the past.
I uncounted the same asymmetric red anode issue when the new re-issue coke bottle 6550 came out, the problem then was serious anode outgassing that rapidly swallowed the mirror getter to grey. Over a couple of weekends hard playing basically saw the end of them but the operating conditions were well within the boundaries. That put me off the brand, even though I got them through a reputable supplier who swapped them. The grey getter is the dreaded sign of near-end of life. Poor material quality control, lack of pre-annealing vital metals, done either too short or being skipped, and worst where´s the manufacturer test soak quality control ?
Now with the sanctions, this is retro for us tubers and the supply shortfalls of various power types. It seems those tubes in the bottom of the suppliers mismatch box are appearing. Quality Beware when so-called matched pairs or quads are asked for. The loss of trust and confidence.
Bear in mind the availability situation in Europe is bad to impossible with one manufacturer quoting unpromised 8 months delivery with enormous backlogs, if that. That has changed my future design consideration. So, rethinking if I was designing a SE amp, my obvious tack now would be a parallel pair configurations of an established line.

Interestingly, I find my EH KT90´s (now 7 yrs in service) didn´t display any reddish anodes when previously hammered at 55W. B+600V in UL fixed bias.
Like other types they are also in short supply and currently de-rating them at a modest 35mA quiescent where they will have a longer life.

Sorry to hear about the quality problems with KT120's, but this thread showed up in just the right time: I've been saving up some money lately to buy tubes for a SET amp using a pair of LL1688/80mA transformers that I've owned for 15 years or so. The idea was/is to primarily use 300B's but also add a pair of octal sockets for KT88/90/120, either triode wired or UL with CFB.