Running tubes at their maximum dissipation.

[G]

Hi all. I the issue with running tubes at their max dissipation a thermal issue or a insulation issue? Or both? I plan on running a output tube at it's max dissipation with good ventilation. Am I still going to run into problems? BTW the tube is the ever popular 12B4A.

[HollowState]

Truth be known, the real issue is short tube life for which temperature plays a big part.

[jackinnj]

I ran the 7868's on my Fisher 400 at max output and one of them just cracked at the base !!!

[G]

I will end up running them just below the max dissipation of 5.5 watts.

[Eli Duttman]

Gavin,

What is the max. allowed dissipation for any given type? Please take the fact that 3 different rating systems can be found on tube data sheets into account. Design center ratings are extremely conservative and you should never have problems at those limits. Design maximum ratings are pretty good for stating, this far and no more. Absolute maximum ratings are exactly that and should not be reached.

If that 5.5 W. figure you provided is design center, go for it and rest easy.

[G]

Originally posted by Eli Duttman
Gavin,

What is the max. allowed dissipation for any given type? Please take the fact that 3 different rating systems can be found on tube data sheets into account. Design center ratings are extremely conservative and you should never have problems at those limits. Design maximum ratings are pretty good for stating, this far and no more. Absolute maximum ratings are exactly that and should not be reached.

If that 5.5 W. figure you provided is design center, go for it and rest easy.

Hi Eli. The design center ratings, "characteristics and typical operation", have them running at about 5.1 watts. I will be running them at about 5.3 - 5.4 watts. They were designed to be used in TV sets, as I'm sure that you are aware, and as such I would imagine the ventilation and cooling conditions left a little to be desired in that application. I will be using them in the open air. I have about 6 sets so if I burn one of them up I will just change the operating points to something a little more humane.

[Miles Prower]

Originally posted by G
Hi all. I the issue with running tubes at their max dissipation a thermal issue or a insulation issue? Or both?

It's both. Hollow state isn't like solid state where you can horse up the current by slapping it onto a bigger heatsink. Solid state gets rid of its internal heat by means of conduction and/or convection (the large metal power transistors are usually filled with silicone oil). Since the bottle is filled with vacuum, the plate can get rid of heat by radiation only, so heatsinking does you no good at all.

Some types are restricted by insulation considerations. The max Vpp rating for an 807 is 750Vdc, whereas the rating for the virtaully identical 6L6 is limited to 400V. That's because the 807 has a plate connection to a button on top, and the 6L6 doesn't. Trying to run a 6L6 at 750V is sure to cause a flash-over across socket pins. Of course, this doesn't really matter since the best loadlines for the 807 occur at lower plate voltages anyway.

I plan on running a output tube at it's max dissipation with good ventilation. Am I still going to run into problems? BTW the tube is the ever popular 12B4A.

Again, hard to say. I'm using 6BQ6GTBs as audio finals. This type is rated like a 6V6 (Pd= 12W for both types). I'm using them at a dissipation of 16 --17W with no trouble at all. The 6BQ6 was designed as a TV horizontal deflection amp, so its ratings are very conservative in light of the rigours of the duty. Audio isn't so demanding, and these don't begin to show plate colour until you're pushing 20W of plate dissipation. That would quickly poof a 6V6.

Keep in mind that the 12B4 was also designed for deflection duty, in this case, a vertical deflection amp. Not so tough an assignment since the vertical deflection system doesn't also run other subsystems (CRT HV, B-boost). You might not have any problem running at the limit that you'd have with a pure audio VT, in which case, you'd want to stick with the usual 70 -- 80% derate.

[kstagger]

Yesterday with a friend of mine I was messing around with a mono 'breadboard' SE power amp and plugging in a variety of tubes to give them a listen on my UREI speakers. This thing runs on just regulators and I can determine the bias and B+ (0-500V) with a twist of the dial. We checked out some different power tubes and listened to some mono music (Ray Charles and Frank Sinatra btw).

My friend Chris brought over a well used and abused SED 6550 and EL34 over. By now these were both high hour tubes he bought a number of years ago.

Well in the spirit of tubelab we threw the EL34 on the amp and let it play for awhile (through a cheap Radio Shack speaker for this test!) and decided to see if we could get it to runaway. Cranked ~40Ws (450V plate) through the poor EL34 and the plates were only faintly glowing red. You could see the grid taking some abuse too, but it never went into runaway. There was a certain 'odor' coming from the tube...

The (already abused) 6550 went up to 50W-55Wss and I couldn't get the plates to glow red at all... though the screens were 'an interesting' color. I admittedly chickened out from putting any more current through them.

Obviously running tubes like this would shorten life! But I was surprised how much abuse a tube can take short term. SEDs are apparently very tough.

I also took the EL34 and used 150V for the B+. Dropped the bias to less than 1V and I was still getting (distorted) music through the tube. The current meter on the regulator was flopping around to the music signal...

My wife was looking at us like we were schoolkids, but hey it was fun!

[tubelab.com]

Well in the spirit of tubelab we threw the EL34 on the amp and let it play for awhile.... My wife was looking at us like we were schoolkids, but hey it was fun!

Now you know why I do it. The wife still doesn't understand.

There was a certain 'odor' coming from the tube...

Likely the glue that holds the glass to the base. Some times the glue loosens after extended "testing" sessions.

Some tubes, especially the older Chinese stuff will rapidly self destruct without the usual warning signs. I recently had two different Chinese KT88's (about 10 - 15 years old) fail spectacularly in a SimpleSE running at about 30 watts dissipation which should not bother a KT88. Sparks flew inside each tube, resulting in cracked glass on one of them.

I just returned from the Orlando hamfest where I purchased several hundred dollars worth of tubes transformers and chassis. This time though I only got a few cheap tubes for "testing" purposes.