Are tube plate dissipation's peak or rms?

"Jeff Johnson"

** Nope.

** Correct.

** Yep - the speaker was an idiot.

BTW:

Dissipation cannot be "rms" - just peak or average watts.

.... Phil

Tubes are (or were), in fact, rated with much more margin than semiconductor devices. Semiconductor specs are "never, ever exceed" specs, where tube specs -- at least receiving tubes -- are "center design" specs. The idea was that the specifications given were what you designed to, assuming 117V (in NA) on the line, but if it went up to 125 for a while, no tubes would be damaged.

I never experienced tube equipment much -- I'm too young. But tales of putting 500V on the plates of 6L6's* and running the plates cherry red** abound.

Generally running a tube too hard might shorten it's life, but you had a lot more leeway before it'd go "phut" (eventually you could make something melt). Semiconductors die more readily.

• 6L6's are rated for something like 375V

** Which is OK for some tubes but _not_ 6L6's

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Transmitting tubes were commonly run with red or orange glowing plates.

John

"John Larkin"

** Yep - and in many cases those plates were made of solid graphite.

One reason that Tx tubes could stand such high internal temps is the DC resistance in series with the grid was normally near to zero ohms - making a joke of any plate to grid leakage currents.

..... Phil

"Tim Wescott"

** The only real limitation on max plate voltage for most power tubes is internal insulation breakdown. 6L6s are rated to stand continuous peaks of up to 2.5 kV on the plate, at zero current.

The max rated DC supply voltage for 6L6GCs ( the only kind made for decades) in a class B, push pull power stage is 800 volts - where each plate swings up to almost double that figure on signal peaks. One pair of tubes can deliver 100 watts of sine wave power this way.

** All such figures need to be looked at in context.

A pair of 6L6GCs is rated for a 450 volt DC supply in class AB1 to get 55 watts at low THD.

The final limit is internal temperature of the mica supports and the plastic base - which must not become so hot they begin to conduct. Making the series grid resistance low really helps keep the issue at bay.

.... Phil

Uh, they're not "tales," it's true. I've seen a "cherry-red" (actually, red-orange) plate on a 4-400A transmitting tube, but it had a "chimney" and forced-air cooling. Yes, you can run a 6L6 to cherry-red, but be DAMN sure you keep the glass envelope cool enough to stay firm.

Oh, OK, the voltage might have caused internal arcing, but as long as you hold the voltage low enough, it would be the plate current that heats it.

For this assertion, I'd like to see some documentation. Got the data sheet?

Thanks! Rich

This is because "rms" is only a number that calcultes "effective" power, right?

As far as "peak" watts, wouldn't the thermal inertia of the object figure into that somehow?

Thanks! Rich

"Rich Grise is a 100% FAKE "

** FFS - go to " basics" and ask the little kiddies here to help you.

Wanker

Phil, I've tried to be civilized, but I fear I must draw the line now.

"Rich Grise is a 100% FAKE "

** FFS - go to " basics" and ask the little kiddies here to help you.

Wanker

Most of the valve manuals i've seen (Brimar, Mullard etc) specify typical / recommended operating conditions and also absolute maximum ratings. The latter being continuous dc plate disapation, for example.

Had an amateur radio friend in the 60's who ran 4 x 807 tubes in the pa, where all the anodes would run cherry red if he held the key down for too long, but running them like this just shortens the tube life. They will take quite a bit more abuse than solid state devices...

Regards,

Chris

The real problem was often melting the glass.

John

That was the indicator I used to adjust the bias pot on my KT88 class-A amplifier... crank 'er up until you could see shadow of the screen grid, then back off slightly ;-)

...Jim Thompson

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      Remember: Once you go over the hill, you pick up speed

Tuibes generally have/had two max ratings: design maximum and absolute maximum. The latter is a never-ever-exceed rating while the former allows for reasonable departures from normal operating conditions.

I had an argument with my immediate superior at work some 40 years ago about tube ratings. He had a Ph.D. in electronics whereas I'd been teaching myself over the previous two years. He asked me to design and build a high-voltage constant current supply for electrophoretics work. I came up with a hybrid design using EL84 tubes and transistors. Where we disagreed was that my boss was aghast at the thought of using the tubes at their max ratings. I argued that it was OK to run them at their 'normal' max ratings, meaning design maximums. I won, and the product did useful work at the medical research center for quite some time.

The incident must have made an impression on my boss. We did a lot of work together on things like a digital patient monitoring system, an externally powered pacemaker, etc., but when we briefly renewed contact 30 years later, one of the first things he mentioned was that relatively minor project with the EL84s. He said "You taught me a good lesson that day......"

I've got an RCA handbook of transmitting tubes, from the early 50's, I think. Most of the glass tube types have a visible plate temperature guide, from "no glow" (unusual) to "white hot" (wowza!). So it goes by tube. I know that the plate material made a difference, but I'm not sure which ones were best.

--
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Which is the reason a lot of transmitter tubes were made of ceramics. EG: The 4X250 quickly became a 4CX250 because of thermal problems.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

I was going by me recollection of the plain ol' 6L6 data sheet -- which, when I pull it out and look, says 360V.

My point was more that you can take a tube -- like the 360V-rated 6L6 -- and put much more to it than it's rating (500V in transmitter usage) and get away with it. Which is basically what you say much more succinctly: all such figures must be looked at in context. 500V on a "360V" tube, in intermittent service, kinda works -- and if it doesn't, it's a socketed part...

I wouldn't expect to be able to waltz into a design review and present a design where's I'm putting 36V onto a 36V transistor, much less 50V onto a 36V part -- my colleagues would insist that I use a 50V part for 36V service, not the other way around.

--
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Well, yes, but it doesn't help much. I'm mostly going by collected stories of "how I melted my radio", for which I don't have cites. And, to be technical about it, you'd have to be SuperMan to see a plain ol'

6L6 plate glowing red, 'cause it was a metal tube. A 6L6G, maybe, but not a 6L6.

--
http://www.wescottdesign.com

I used to buy surplus 35T triodes when I was a kid, something like $1.50 each.

They ran orange-hot just fine, but the glass was mechanically fragile. Beautiful little tubes.

John