Pushing output tubes

[drewl]

So after replacing the output tubes on a 50w marshall biasing at about 1volt is around 30ma.
Running into a load to test it, it easily develops 3,6 even 10 volts or 200 to 300ma, and one of the new tubes started arcing like it was going to implode!
I tried a different set and they seem alright, but with alot of people either cranking their amps or using an attenuator how many new tube mfg's can take this abuse?
or am I just being paranoid?

[Amp Kat]

So after replacing the output tubes on a 50w marshall biasing at about 1volt is around 30ma.
Running into a load to test it, it easily develops 3,6 even 10 volts or 200 to 300ma, and one of the new tubes started arcing like it was going to implode!
I tried a different set and they seem alright, but with alot of people either cranking their amps or using an attenuator how many new tube mfg's can take this abuse?
or am I just being paranoid?

Well I guess your talking about EL-34's as some Marshalls like the JTM-30,60 and some 50 watters use 5881's which is a shorter tube than the 6L6 and much tougher as far as plate voltage. If you want to do a torture test as far as voltage try one on the 5881 Sovtek and you'll see that it will take more voltage than it's specs show. The thing that kills tubes however is not voltage but a combination of voltage + current in the form of wattage or tube dissipation %. An EL-34 in push pull mode and fixed bias has a max rating of 800 volts which you hardly ever see and of course being biased for correct dissipation would last as long as it running at 500 volts biased correctly. The EL-34 also draws much more screen current and if it's a factor for wear I'm not sure but you would think it is. The current production tubes surely aren't as stout as the old Mullards or several other NOS tubes but if you treat them right they will last a pretty good amount depending on how long and how loud you play. I like the JJ's and the Svet-C-Wing EL-34. The only tube I really don't like overall is the EH6L6 which is a Sovtek WXT I think as it gives me problems alot and archs and causes serious problems. One more thing about arching is if you see it arching I would turn it off immediately because it can do some serious damage to not only circuit components but can fry your output tranny in a heartbeat.

[wizard333]

I've found that if you are going to be cranking the amp, you need to bias a little cooler than you would otherwise. If you get near the standard .7x(plate dissipation/plate volts) and you watch what happens when you crank the amp, you end up pushing way too hard and exceeding the tube's ability to handle it. I find that an amp that is going to be pushed like that all the time sounds better and is much more reliable (and forgiving on the tubes) biased between .5-.6 rather than .7.

I guess the point of that is, you could be biasing way too hot for an amp that is going to be used that way. Try .55x(plate dissipation/plate volts) and see if that doesnt work better.

[wizard333]

By the way, if you want a modern tube that can take a beating well, go with JJs.