What is typical max 6V6 plate V?

[daz]

Specs seem to mean little compared to what people use in the real world, so what is a typical plate voltage for 6V6 and what is the highest you can run them w/o issues or low lifespan.

[Rob Mercure]

Daz,

As Enzo has pointed out about a dozen times in recent postings, it isn't the absolute voltage that zaps tubes but instead the the power, watts, the bottle is required to dissipate. Voltage is "push" - it ain't actual electrons.

As a sweeping generalization Fender ran their designs at higher output plate voltages than any other major manufacturer. So go to The Fender Feild Guide, review the designs, and you can come up with the "typical max" for at least on manufacturer.

If the vacuum's intact the heather to cathode voltage is more critical than the plate as the spacing's closer. While not a 6V6 I've run 50C5 types at over 300V on the plates which is far, far about any design curve you see - but I was careful on dissipation, screen voltage, and H/K difference.

Rob

[Enzo]

And something else I like to point out: The published specs are not limits on the tube or else desctruction, they are limits with a purpose. They are like speed limits on a highway. If the speed limit is 70, your car will not blow up if you drive 80.

The published tube specs like you find in the RCA manual were intended for design engineers. They were intended to be specs that will result in long life and reliable operation in consumer goods - TVs and table radios. Dad expected his TV to sit there in the corner and run for a long time on the tubes in it. and that table radio out in the garage also. He would not be happy if his radio needed a new EL84 every 5 months.

Dad's car was designed to provide safe and reliable family transportation. He COULD take it to the drag strip every Saturday afternoon and race the crap out of it. But it would no longer provide safe and reliable transportation. Just so our tubes. WE EXPECT to change our EL84s a couple times a year, we EXPECT to wear our tube out.

So we routinely use tubes at voltages and currents in excess of their design.

[daz]

Ok, so i assume the point is that at 424 V is ok as long as i bias them at say about 17ma ? Thats what ni have now and it sounds fine and all, but i just want to be sure i'm not going to be replacing my JJ's all the time.

[Wakculloch]

I would consider an amp that only lasted 5 months before it needed new valves to have issues with low lifespan !
If it is built for 6V6's and has a HT of 420V, so be it; but if one is doing a new build then i can see no advantage in exceeding the design max.
Being a dad myself, i expect 5 years from valves, and if they fail before that I want to know why.

[daz]

Are you saying thats what i can expect?

[Rob Mercure]

If you were to watch your plate current the type 6W6 should plug right in - this bottle has a higher plate voltage characteristic with slightly lower plate dissipation. That is, you can runn em with higher B+ but you've got to limit the current flow (higher value cathode resistor if cathode biased - more negative control grid bias voltage is fixed biased). The tonal qualities are quite similar. These is another plug in sub - something like a 6EY6 - that requires more heater current and of course you could use a 6L6 (no letters) with heater voltage penalty.

Rob

[Enzo]

When I see 5 year life from a set of EL84 in a guitar amp in professional use, I want to know who sells them. I expect 5 years from them about as much as I expect 5 years from a set of strings.

Tubes - like strings - lose their tone. They continue to function, but sound awful. Failure is not the only reason to replace tubes.

[daz]

So Enzo, in plain english (some things go over this noob's head !) can i expect reasonable tube life such as a year or 2 with 425 V on the 6V6's as long as i keep a conservative bias? If so, what would you suggest as a bias current for decent life and good tone?

[Enzo]

If you play an amp a couple hours a week in the bedroom, then your tubes probably WILL last 5 years. I mostly work with gigging musicians, and that long on a set of tubes is wishful thinking, in my opinion.

The voltage across the tube and its current determine the disssipation. Keep within reasonable bounds and you will get good life from your tubes.

[daz]

Ok, well to me "reasonable bounds" would just be a guess on my part. Or i can take one source's data and consider it coreect or gospel and go with that. So going by weber's bias info i would estimate 17ma to be a safe bet. In the average (60%) column they show 19ma at 375v and 18 at 400v. Thats as high as they show, but going by that scale 425 would then be 17. So i guess thats where i'll keep it.

[Paul P]

I don't understand your numbers, daz. You say you have 424v between
plate and cathode, and 17ma flowing between them. So (424*.017)
7.2 watts are being dissipated. If we take this as the 70% value for
push-pull, it would imply 10.2 watts at 100% of rating.

It is my understanding that you can use 14 watts as the maximum dissipation
for modern 6v6s without any trouble. Working backwards from this you get
9.8 watts for the 70% point. 9.8w / 424v = .023A. So you should be able
to bias to 23 ma with no trouble at all. Going to a higher current at idle will
also drop your B+ so you'll be able to go even higher with the current. It'll
probably sound better.

You could always check things while you're playing to see what sort of
maximum dissipation you reach when playing full out.

Paul P

[daz]

YOU don't understand it ?! You should take a quick stroll thru my mind and see whats going on in there ! I can only do what i'm told for the most part because i'm not a tech. So while i understand a bit, not enough to be sure what to do.

So i took your advice and the plates are now 404. But i'm afraid to leave it there because 1)-enzo said the determining factor for lifespan is current, and 2)-according to weber's page, 400 volts should be at a absolute max of 21 ma. If theres some reason i shouldn't use enzo and weber's info and go back lower, please explain it to me and i'll leave it as is. But otherwise i have to go back to 17 for fear of ruining a new set of tubes.

[Paul P]

2)-according to weber's page, 400 volts should be at a absolute max of 21 ma.

Using Weber's bias calculator :

http://www.webervst.com/tubes/calcbias.htm

and entering 6V6GTA and 404 volts into the top line of the calculator
produces 24.2ma of bias current which seems reasonable.

Oh, I just figured out that you must have entered 6v6s. You sure you
don't have 6V6GTs ?

Paul P

[Wakculloch]

Sorry to keep banging on about this, but i've just totted up my last years amp usage, ( two practices a week, two month long tours, couple of all day festivals), and I have it at 500 hours.
5 years gives 2500 hours.
With 6V6's run conservatively, this is totally unremarkable!
They have sofar been in for two years.