Hi! I am quite new to the valve stuff. Now working on LANEY VC30, and I treated it with new matched quad EL84 from JJ. They are runnin' 36mA per tube with 280V across.
So i should be on the far cool side. What is puzzling, is orange glowing spiral around cathode and infra-termometer reads nearly 200celsius on tube. I chcecked all screen resistors, according to schematics, they should be 100R, actual values were 82R, and then substituted them with 220R and measured 0.8V across. I let it run, out of the box for 5 minutes, an it looks like my problem is gone, but after mounting it back orange spirals are back. Any hints?
So i should be on the far cool side. What is puzzling, is orange glowing spiral around cathode and infra-termometer reads nearly 200celsius on tube. I chcecked all screen resistors, according to schematics, they should be 100R, actual values were 82R, and then substituted them with 220R and measured 0.8V across. I let it run, out of the box for 5 minutes, an it looks like my problem is gone, but after mounting it back orange spirals are back. Any hints?
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Probably because they are JJ. Nasty valves. I always fit Sovtec (6p14P) or Svetlana (6BQ5) as they are more rugged and last longer plus I set the current at 30mA to give me class AB. I see the VOX AC30 then in about two years if gigged most nights.
Too much current and the output transformers fail. (Stock fault).
Too much current and the output transformers fail. (Stock fault).
800 mV across 220 ohms means a screen current of about 3.6 mA. Times 280 V means slightly more than 1 watt of screen dissipation - well within the limit of 2 watts, as from J/J's and older datasheets. Hence, I wouldn't be too worried.
Best regards!
Best regards!
Is there a powerful speaker in the cabinet? Perhaps the magnet deviates or concentrates the electron flow in some strange way and the screen gets too much of them, overloading a zone of it.
Yes! That cheks out, output valves are less then 8mm from speaker magnet, and one valve, that is furthest does not glow its screen. I have seen some guy putting different speaker in the VC30 because of the clerance. It will be pretty tricky to measure screen current (and pointless, it is obviously too much), with amp in the box. This problem is definitely co-caused by JJ design, because EH's don't do it. I guess i need to buy another matched quad from different manufacturer. Thank's for cracking this nut, I feel stupid for not seeing the causality.
OK, no need to feeling stupid. Sometimes things are at our view, and we can´t view. This happens.
ITOH I believe that changing the brand of the tube will not cure the problem, the only one will be apart away thwe tube and the magnet.
Good luck.
ITOH I believe that changing the brand of the tube will not cure the problem, the only one will be apart away thwe tube and the magnet.
Good luck.
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