Safe Max. Plate dissipation??!!

[jimboyogi]

Hi All,

I have discussed this in another thread, but was keen to see if I could get some consensus.

I have built a SE amp with a 6CA7 output valve/tube. According to the data sheet these have a max plate disipation of 25W.

How close to this is a safe max. for long term running? 24W? 95%? Go eat sardines?

Cheers

[Steve Conner]

The Mullard 5-20 ran its EL34s at 100% dissipation, and I've heard some guys in the hi-fi forums say that it eats reissue EL34s for breakfast, they only last a few months.

But then again, our Bruce Collins says that it's fine to run a 6V6 at 100% dissipation in a Champ. YMMV.

[R.G.]

I have discussed this in another thread, but was keen to see if I could get some consensus.

Unfortunately, consensus isn't worth much on "will it live" issues. The opinion of a number of more-or-less informed people will not make a tube live longer.

The problem is details. The devil is always in the details.

I have built a SE amp with a 6CA7 output valve/tube. According to the data sheet these have a max plate disipation of 25W.
How close to this is a safe max. for long term running? 24W? 95%? Go eat sardines?

First you have to look at what datasheets are for. They are (1) advertising for engineers, with pornograhically (to engineers!) described power, voltage, current, and speed ratings. We (manufacturers!) want the engineer salivating over using our stuff; (2) they are an advertising promise, and as such, actionable in a suit over false and misleading advertising. The manufacturer may have to prove in court, somehow, that their products *could* be expected to work, on average, like the datasheet says.

There's an old quip that goes "Lies, damned lies, and statistics". It's more or less accurate. The manufacturer called to prove his datasheet numbers in court will present statistical evidence showing, for instance, that some high percentage of his production will meet the published numbers in the datasheet. This gets into confidence intervals and expert witnesses confidently saying what is "proof".

However what it does not mean is that every product carrying that type number it will perform to data sheet. Only mostly. This is why engineers, wanting to keep their jobs, will tear their eyes away from the powerfully attractive numbers and scale the actual stress on the parts back to something smaller.

With that as background, we're back to

How close to this is a safe max. for long term running? 24W? 95%?

The majority of 6CA7's will survive and work a "normal" lifetime at the specified max. There will be some dropout percentage from (1) infant mortality (2) the minority that can't take it. The specified operating conditions in the datasheet have to be taken into account. If the datasheet says "in 25C ambient air" and your sloppy designing lets the air temp in the box get to 26C, the maker is off the legal hook. If the datasheet says "with ambient air temperature of 50C and 100m/s forced air cooling" and you have 40C but 90m/s forced air cooling, the maker is off the legal hook. The list goes on.

There is an underlying communication in all datasheets: it's that the manufacturer is saying "I'll bet you the cost of your order that I can specify this in a way that makes it impossible for you to get your money back."

At least all this was the case back when tubes were made by companies that could be called to account in court for false advertising. NOS tubes are made by companies who are either currently existing but making counterfeits, or are long dead. In either case, they're beyond the reach of the law. Current production tubes are made by companies which are not reachable by US or European law by and large (JJ may be an exception) and therefore their datasheets have to be taken as laudable goals, but not provable. They may, in fact, be really, really good and every part that ships may meet the specs. But that's not the way to bet.

What I'm getting at is that if you're designing your own amp, go wild and do what you want. If you intend to sell these things and want to rely on the datasheet, you'd better do some sincere analysis of the variation of the operating conditions, voltages, dissipation, wall-voltage range, bias variability, and a few hundred other things, or YOU will be worried about whether you have to cover the warranty replacements.

The safety of running a tube or anything else at exactly the max rating is subject to a host of variables. You can make one of anything work, mostly. But if you're trying to make 100, or 1000, or a million work, that's another story.

[Wilder Amplification]

Hi All,

I have discussed this in another thread, but was keen to see if I could get some consensus.

I have built a SE amp with a 6CA7 output valve/tube. According to the data sheet these have a max plate disipation of 25W.

How close to this is a safe max. for long term running? 24W? 95%? Go eat sardines?

Cheers

One thing to note about Class A amps...the instantaneous dissipation will equal max dissipation both at idle and at the zero crossing points of the signal swing. When the signal is swinging above and below zero, instantaneous dissipation falls BELOW max dissipation. Instantaneous dissipation essentially falls below max dissipation on the rise of the positive swing, then rises back toward it on the "fall" of the positive swing, where it will be right at max when it crosses zero, then falls below it again on the negative swing, then rises back up to it when the signal rises back to zero. Assuming a Class A amp with 100% symmetry (i.e. biased right at max dissipation), this makes the AVERAGE dissipation (i.e. the power it dissipates for one full cycle of the sine wave) = max dissipation.

This basically means that in a Class A amp, the tubes are dissipating the same average power whether they're at idle or under signal.

As to whether or not 25W max is a liberal or a conservative rating, I'm sure that would depend on the manufacturer of the tube.

[jimboyogi]

Thanks Steve, R.G. and Jon, guess I can put those sardines away.

The 6CA7 I am using is a recent EH one, actually I have tried 2 of these at plate dissapation of ~ 24W, and neither has shown any signs of red-plating.

I have only 2 simple choices for cathode resistors to adjust bias with my power supply etc. setup -
220 ohm gives ~ 24W Pp
270 ohm gives ~ 21W Pp

If I was keeping this amp myself I would stick with the 220 ohm, but when finished I am sending it most of the way across the world to a luthier in Oregon USA in exchange for a custom guitar body!
So I do not want it to give him any trouble (hard for me to fix!), hence asking you all if ~24W is likely to be trouble

The flip side is that I am trying to squeeze as much clean headroom out of this as I can.

[Wilder Amplification]

Thanks Steve, R.G. and Jon, guess I can put those sardines away.

The 6CA7 I am using is a recent EH one, actually I have tried 2 of these at plate dissapation of ~ 24W, and neither has shown any signs of red-plating.

I have only 2 simple choices for cathode resistors to adjust bias with my power supply etc. setup -
220 ohm gives ~ 24W Pp
270 ohm gives ~ 21W Pp

If I was keeping this amp myself I would stick with the 220 ohm, but when finished I am sending it most of the way across the world to a luthier in Oregon USA in exchange for a custom guitar body!
So I do not want it to give him any trouble (hard for me to fix!), hence asking you all if ~24W is likely to be trouble

The flip side is that I am trying to squeeze as much clean headroom out of this as I can.

With pure Class A, maximum peak-peak headroom is achieved at a bias setting that yields 100% symmetry, which theoretically going by "textbook perfect world conditions" would be right at max dissipation. Colder than max and the negative swing will clip before the positive and vice versa (plus you'll overheat the tube going hotter than max).

[Steve Conner]

I am sending it most of the way across the world to a luthier in Oregon USA in exchange for a custom guitar body!
So I do not want it to give him any trouble

Well then, I'd go for the 270 ohms.

[jimboyogi]

I've just found that I might be able to get 250 ohm 10W resistors from one of the big internet component suppliers, if so this might be the best compromise, maybe ~ 22-22.5W Pp.

[pdf64]

'This basically means that in a Class A amp, the tubes are dissipating the same average power whether they're at idle or under signal.'

This is at odds with my understanding, which is that the tube/s in class A amps dissipate most power at idle, least power at full output. Because under signal, the speaker is dissipating the power that the tube would at idle.
Rather it's the power supply that has to supply the same current draw whatever the signal conditions.
Have I missed anything?

Sorry to pick on you Jon, but also you seem to be equating class A operation with 100% dissipation.
My understanding is that there's no direct linkage between dissipation and operating class.
It's just that in order to get the max class A output from a particular tube set, the designer will set the B+, load and idle current to get the most output from the tube/s, which inevitably results in close on 100% disspation.
But if the B+ was reduced say 50V, then then dissipation could be reduced also, whilst maintaining class A operating (ie tube conducting for whole cycle), though with necessarily reduced output power.
Conversely, increasing the bias on a class (A/)B amp until it reaches 100% dissipation won't necessarily move it's operation conditions into class A.
Again, please point me in the right direction if I'm not thinking this through correctly.

Also, in response to the OP, it would be beneficial to factor in increased B+ levels resulting from the line voltage tolerance, so set the bias a little conservatively. Pete.

[raiken]

This is at odds with my understanding, which is that the tube/s in class A amps dissipate most power at idle, least power at full output. Because under signal, the speaker is dissipating the power that the tube would at idle.
Rather it's the power supply that has to supply the same current draw whatever the signal conditions.
Have I missed anything?

This is correct. If, for example, you have an "ideal" class A EL34 tube set up to run at 250V/100mA at idle, for a dissipation of 25W, the plate dissipation will drop to 12.5W at full power with pure sine wave drive. The power dissipated in the load is subtracted from the power dissipated in the tube. The supply current will remain the same, as you pointed out.

It gets even more interesting with a square wave drive. If you had an "ideal" tube that could swing all the way down to zero volts, with infinitely fast risetimes, the plate dissipation would drop to zero at full power! This would be essentially class D, or switching operation, where the tube is only fully off or fully on, and it would not dissipate any power at either extreme, only during the transitions, and if they were fast enough, the dissipation would be inconsequential. All of the power in this case would be transferred to the load.

For a graphical view of what happens in class A and class AB tube dissipation, see this article:

Idle Current Biasing - Why 70 percent?

RA

[Wilder Amplification]

Whoops...got mixed up somewhere. Thanks for the correction. Was thinking of current staying the same at idle and under signal...not power (duh).

@pdf64, I know the dissipation doesn't set the class. That's just where the tube itself is able to amplify the full swing by itself, which is what makes it Class A.

[jimboyogi]

Thanks All,

Randall, in your article you state the following -

"In a true class A amplifier, you can bias at maximum plate dissipation because idle is the point of maximum active dissipation - the dissipation in the tube actually decreases throughout the entire range of operation, so you are in no danger of exceeding the tube specifications if you bias for maximum dissipation at idle."

So biasing my 6CA7 at 24W plate dissipation at idle/quiescent should be fine?

I'm particularly interested due to the amp failing today while playing I'll start a thread about it in the 'Troubleshooting' section.

Cheers

[Wilder Amplification]

Thanks All,

Randall, in your article you state the following -

"In a true class A amplifier, you can bias at maximum plate dissipation because idle is the point of maximum active dissipation - the dissipation in the tube actually decreases throughout the entire range of operation, so you are in no danger of exceeding the tube specifications if you bias for maximum dissipation at idle."

So biasing my 6CA7 at 24W plate dissipation at idle/quiescent should be fine?

I'm particularly interested due to the amp failing today while playing I'll start a thread about it in the 'Troubleshooting' section.

Cheers

If you have the proper plate load (OT primary spec) that doesn't allow enough current draw to exceed maximum dissipation at any point in the swing, Randall's recommendation holds true.