I have a Tubelab SE for 300B. It's a rather new build but I have been using the tubes with another buil for a while.
Today both tubes went in runaway. I have to admit that I was squizing them a bit, 400 V on plate with 90mA, buy it lasted maybe 20 seconds, I switched ti off.
Then I checked everything on the circuit, and it looks like it works correctly, but when I start to pull up the current in the 300B the bias at the beginning raise slowly, then all of a sudden jumps to 100 - 120 mA.
Now the question is: did I damaged the tubes, or I can have hope that is something else ?
Thanks,
Davide
Today both tubes went in runaway. I have to admit that I was squizing them a bit, 400 V on plate with 90mA, buy it lasted maybe 20 seconds, I switched ti off.
Then I checked everything on the circuit, and it looks like it works correctly, but when I start to pull up the current in the 300B the bias at the beginning raise slowly, then all of a sudden jumps to 100 - 120 mA.
Now the question is: did I damaged the tubes, or I can have hope that is something else ?
Thanks,
Davide
Hi Davide,
I am new here, but just from my little knowledge, what were the vacuum tubes looking like? Red? Pink/purple between the grid wires? I read something way back about the grids being able to run away because of biasing (covered already above) or because of a mismatch with the grid leakage resistor to the tube (apparently there has to be a resistive load to the grid). Make sure that the lower Rg1 value on the manufacturer's tube data sheet is used and then checking that value against what resistor is really there.
If I can find the link (it was either here or at diyaudioproject.com) then I will leave it here.
Do you have pictures? Do you have access to a tube tester to see if your tubes are damaged? And have you tried swapping out tubes (do not do this if it might cause damage to another set of tubes)?
Here is the link. I knew I had it saved as one of my vacuum tube bookmarks. As a beginner in wanting to make a first amplifier, I think I have saved at least 100 links covering various problems from blue smoke to no sound from diyaudio.com alone (which has probably kept (or scared) me from starting my projects for the previous six months 
).
http://www.diyaudio.com/forums/tubes-valves/156720-what-just-happened-plate-glowing.html
I hope that this one applies and best of luck.
).http://www.diyaudio.com/forums/tubes-valves/156720-what-just-happened-plate-glowing.html
I hope that this one applies and best of luck.
So, the good news is that we have music again. The tubes look fine, I had them tested with tube tester and with an amp in a shop. If you are in Tokyo visit Hino Audio, They don't speak a word of english, but they are so nice, that you manage to find your way.
Anyway, I changed the mosfet, but I am not sure that were the cause of the problem. Now I have a lower B+ and a more conservative bias and everything looks fine.
The tubes are Golden Dragon 4-300BC, should be kind of good. I had been using them at 390 V 75mA for a couple of year. Maybe mat B+/bias was really exaggerated.
Is it possible that the tubes kind of recovered during the time I did not use them ?
Thanks,
D.
Anyway, I changed the mosfet, but I am not sure that were the cause of the problem. Now I have a lower B+ and a more conservative bias and everything looks fine.
The tubes are Golden Dragon 4-300BC, should be kind of good. I had been using them at 390 V 75mA for a couple of year. Maybe mat B+/bias was really exaggerated.
Is it possible that the tubes kind of recovered during the time I did not use them ?
Thanks,
D.
Well, that is good news that nothing really went South. Did you manage to try the tubes in the amp you had them in previously? You might be able to tell sonically, or visually (though it is very subjective).
I am not sure about tubes recovering -- the only recovery they can do is cool down (the inards can't do much of that very efficiently in a vacuum either).
Keep an eye open now and then and check the bias after a while in case of drifting form things like the tubes themselves or drift in any of the electrolytic capacitors.
Take a picture of the amp, I am curious to what it looks like.
I agree, and I have had this problem with older versions of the Shuguang 300B and Valve Arts tubes, but rather hoped that they had fixed the issue by now. (Grid not extended sufficiently beyond the plate structure to allow cut off at plate voltages much above 350V or so. Once you can't actually cut off plate current you have lost control of the tube.)
OT
You always have to see things in prospective: yesterday I was crying for the potential loss of my 200 $ tubes.
Today, just in front of my eyes, my two years old son pushed one of my expensive speaker, that felt on a 52 inches LCD screen. I managed to catch the speaker before the floor, but not before the screen
Results: scratches on the speakers and LCD in pieces.
AAAAAAHHHHHHH !!!!!
Sorry, I needed to vent.
D.
You always have to see things in prospective: yesterday I was crying for the potential loss of my 200 $ tubes.
Today, just in front of my eyes, my two years old son pushed one of my expensive speaker, that felt on a 52 inches LCD screen. I managed to catch the speaker before the floor, but not before the screen
Results: scratches on the speakers and LCD in pieces.
AAAAAAHHHHHHH !!!!!
Sorry, I needed to vent.
D.
Not to worry. My son at an early age smashed my laptop computer accidentally. So I went out and got a new one. It was a great upgrade.
So you see, in your situation, an upgrade to an 80 inch is now in the cards, a set of 4 larger speakers (one pair to use and one pair for backup). This means another amplifier also for the backup speakers, new inputs, and a new computer (so you can put your thoughts on diyaudio)..
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