Tube life ?

StingRay85

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If you slam the amp with pedals and basically create square waves for hours on end, I don't think the tubes will last too long. I've seen Xf2 Mullards turn red with every chord on a UA Ox box, they are not made for that
 

sdn25

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If you slam the amp with pedals and basically create square waves for hours on end, I don't think the tubes will last too long. I've seen Xf2 Mullards turn red with every chord on a UA Ox box, they are not made for that
I don't use OD pedals so I think the tubes are fine in that regard. I have a pretty chill bias at about 50%.
 

Pete Farrington

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I don't use OD pedals so I think the tubes are fine in that regard. I have a pretty chill bias at about 50%.
Keeping the bias reasonable will certainly help things to cool off in those odd moments when it’s not screaming.
But when it is screaming, the particular bias setting is immaterial, the valves have effectively entered the digital realm, flipping between 0 anode current, and full conduction, as much anode current as they can draw via the load impedance (maybe 300 - 400mA for a typical EL34 amp).
Each push pull side is doing the opposite to the other, so average current measured at each cathode will then be about 170 - 240mA.
 

sdn25

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Keeping the bias reasonable will certainly help things to cool off in those odd moments when it’s not screaming.
But when it is screaming, the particular bias setting is immaterial, the valves have effectively entered the digital realm, flipping between 0 anode current, and full conduction, as much anode current as they can draw via the load impedance (maybe 300 - 400mA for a typical EL34 amp).
So average current measured at the cathode will then be about 170 - 240mA.
At the bias test points on the back of my amp, on v4, when cranked I see an average of 170mA draw, a big chord would be something like 205mA, and on v5 the averages are much lower. Is that reasonable?
 

william vogel

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I’ve had multiple failures of the current production Mullard EL34’s in 100 watt Marshall amps running 490 volts. I stopped buying them altogether. I have great success with the Tung Sol EL34B, no failures or wear outs yet. Strange that they are made in the same factory. Maybe I’m just lucky but I’ve heard the same results from multiple customers. The Tung Sol EL34B have a different sound (not going to explain) but their reliability is worth the cope.
 

sdn25

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I’ve had multiple failures of the current production Mullard EL34’s in 100 watt Marshall amps running 490 volts. I stopped buying them altogether. I have great success with the Tung Sol EL34B, no failures or wear outs yet. Strange that they are made in the same factory. Maybe I’m just lucky but I’ve heard the same results from multiple customers. The Tung Sol EL34B have a different sound (not going to explain) but their reliability is worth the cope.
Oh I see, are the high plate voltages a factor to consider in their failure ? Mine have about 440 on the plates.
 

Pete Farrington

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At the bias test points on the back of my amp, on v4, when cranked I see an average of 170mA draw, a big chord would be something like 205mA, and on v5 the averages are much lower. Is that reasonable?
Yes, that’s typical / reasonable. It would be beneficial to swap the valves in V4&5 over regularly, to keep the aging even.
What does your HT voltage sag down to on a big chord?
I suggest not to probe an overdriven output anode.
 

sdn25

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Yeah my pt does get quite hot. After a long session it becomes uncomfortably warm.
was wondering whether it is normal for pre amp tubes to get too hot to where if you hold them for more than 3 seconds you feel the need to pull you hand away? This happens after a long play session.
 

Pete Farrington

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Drops from 440 to about 375
Bear in mind that it may not drop so much with your new 50+50 cans.
was wondering whether it is normal for pre amp tubes to get too hot to where if you hold them for more than 3 seconds you feel the need to pull you hand away? This happens after a long play session.
I wouldn’t be concerned, their dissipation shouldn’t increase more than a fraction with signal / overdrive.
They may just feel warmer due to moist skin / more blood flow, following a session.
 

junk notes

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was wondering whether it is normal for pre amp tubes to get too hot to where if you hold them for more than 3 seconds you feel the need to pull you hand away? This happens after a long play session.
To remove doubt that you are not running hot, would not hurt to check your heaters that you are not getting more than 6.3v.
 

Matthews Guitars

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"It depends" is the only answer that's always right. Run them hot and hard and they won't last long. Run them at moderate voltage and current ratings and they'll last decades.

Some Marshalls still work fine with the exact same set of tubes that were installed in them at the factory in 1970. But I'll bet those weren't run at full output for hours at a time, night after night, for months or years.

Others may need new tubes every six months because they're beaten like a rented mule.
 

playloud

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It’s anode dissipation, rather than current per se, that sets the consumption rate. But with an inductive load, that’s pretty hard to assess accurately, as V and A aren’t locked in phase, W won’t equal VxA.

This thread goes into assessing anode dissipation under load; it gets quite convoluted! https://www.marshallforum.com/threads/hiwatt-dr504-build.121746/

Operational life will depend on what your end point is. GEC note output power dropping below 50% or gm dropping below 80% of initial value.
Using that, KT66 are at least 8000 hours when operated at their abs max of 30W. The abs max of EL34 would probably be somewhat higher than that.
You may notice some degree of degradation to the sound before the above points are reached.

This is great. 30W is a pretty generous power allowance too!

Would still be interesting to see a distribution of actual lifetimes "in the field". Perhaps someone could apply for a research grant to develop a "smart" bias probe that allows this data collection? ;) It sounds more interesting than a smart watch to me, and ultimately good for the environment to boot!
 

playloud

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If you slam the amp with pedals and basically create square waves for hours on end, I don't think the tubes will last too long. I've seen Xf2 Mullards turn red with every chord on a UA Ox box, they are not made for that

Like this?



I really hope I get my 8000 hours out of these guys! 🤞
 

StingRay85

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Even a lot worse, the plates were also glowing red. Meanwhile, my friend was listening to the sound on his headphones, and liked the tone. Luckily I detected the amp/tubes wouldn't survive long this way
 

playloud

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Even a lot worse, the plates were also glowing red. Meanwhile, my friend was listening to the sound on his headphones, and liked the tone. Luckily I detected the amp/tubes wouldn't survive long this way

Tell him to lay off the windmills! I'll bet he's right about the tone though
 


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