Causes of vacuum tube over heating - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: February 26, 2016, 04:12:07 pm
content: Hi Guys,My basic question is what can cause a output tube/ power tube to over heat.I assume the only thing that can cause a tube to over heat is current flow. Assume for the moment all the voltages on the tubes are correct, plate voltage, plate current, heater voltage, correct bias voltage ect. Also assume the all the pre amp tubes are working correctly and the phase inversion is balanced.Take for example a 6l6 output tube. If the tube is disconnected from the load/speaker the impedance mismatch will cause to tube to overheat to the point that the plate will heat up until it melts or at least shorts out to other parts of the tube then causing current to flow to other parts of the circuit sometime causing a cascade effect taking out other components.Assuming the impedance of the speaker is correct and the speaker is working correctly and the tubes are working correctly only a output transformer failure of some sort could cause the output tubes to over heat.Am I missing something? Not understanding something??Thanks,Billy

author: Andy Watson
author_rank: Super Contributor
timestamp: February 26, 2016, 04:31:22 pm
content: Too much power will cause the tube to over heat - i.e. it's a combination of either too much current, too much voltage or both.Are we talking about the Fender amp again? You appear to have eliminated all the likely (cheap) causes in the other thread. Did you measure the voltages and plate current on the actual pins of the tube(s)?And don't ever, never ever run an amplifier with a transformer output without a load connected - not even for the briefest of moments! The unloaded inductance of the output transformer can generate very high voltages (and in tube equipment they are already high). The high voltages will break-down insulation and kill the amplifier - if not immediately, it will degrade over time - usually the valve sockets or the transformer windings become carbonised - i.e. conductive.Edit: I notice that the Fender has a couple of catch diodes on the plates - but I still wouldn't risk it!

author: woodchips
author_rank: Frequent Contributor
timestamp: February 26, 2016, 07:29:16 pm
content: Whilst by no stretch of the imagination a valve, or tube if you insist, expert I see that you are referring to the 6L6. In UK guise this is the EL34. These valves, in fact all power valves, are notorious for blowing up if they are plugged into something without being used for many years.Been lots of discussion about this on the vintage-radio.net forum but basically it is due to gassing. You must heat them up slowly with the grid nailed to a negative voltage to stop any gas induced runaway. The circuit grid resistance isn't sufficiently low impedance to stop the runaway. Run the valves for enough hours to make sure that the metalwork inside is really nice and hot and the getter can absorb the gassing.These dire warnings have stopped me trying any of my valves until I can build a real burn in rig, a 6L6 or EL34 is simply too valuable to risk. Note that a valve tester isn't the way to do this burning in, unless you want to destroy your valve tester, they are intended for short periods of use, not hours at maximum dissipation.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 26, 2016, 08:28:09 pm
content: Quote from: woodchips on February 26, 2016, 07:29:16 pmWould that not show-up as excess grid current on a tester?I knew there was a reason why I never got into valves - too many self-destruct mechanisms all wrapped in a fragile glass envelope!

author: Planobilly
author_rank: Regular Contributor
timestamp: February 27, 2016, 12:26:17 am
content: Hi Guys,Actually Andy this was another Fender that I own.Yes in fact I actually was asking these questions based on a Fender Hot Rod Deville that I just finished rebuilding.And in fact it did have a bad output transformer plus a bunch of other issues. All solved now and working better than new. That is the good news. The bad news is I own two other Deville , both not working...lolBut the question was also more of a general nature. I have never really understood output transformers very well to begin with. They seem like half science and half "Black Magic"...lolTube amps are really cool and some of them sound really great but.....they are kinda like having a lovely red headed girl friend...really sweet but a pain to deal with at times and always cost you a bunch of money...lolAnd for sure, crank up your tube amp with no load and you will pay dearly.I have a Variac and also high voltage regulated power supply to break in new tubes and fire up scarry old tube stuff that has not been turned on for years. I may have posted the video of the 1935 RCA Radio I rebuilt here on this site...don't remember.Face mask, Nomex fire suit, wife with the fire hose....lol....If one is going to do kinda dumb crazy stuff you need a good wife to bail you out.!!!!..lolWell..enough nonsense from me for a bit..See ya.BillyEdit: Yes Andy I always measure the voltages directly off the bottom to the tube sockets and have a "socket saver" rigged up to to measure the plate current directly.Anyone reading this needs to be aware that poking DMM probes into live high voltage amps can get you killed...no joke...so be safe

author: uncle_bob
author_rank: Unknown
timestamp: February 27, 2016, 12:50:23 am
content: HiSimple answer (as mentioned above) is power. It's not just voltage, or just current. It's power.Voltage alone can fry a tube, but not by heat. Current alone can fry one, again not by heat.The plate / anode has a power rating and you probably can find it on a data sheet. If it's a multi element tube (not a diode) each of the grids has a power rating as well. Finding that on a data sheet is not so easy for most tubes. It's quite easy to mess up the bias on a tube and overheat (fry) a grid. Their power ratings are minuscule relative to the plate. In normal operation they dissipate so little power that even those ratings are way more than what's needed.Bob

author: Planobilly
author_rank: Regular Contributor
timestamp: February 27, 2016, 07:33:41 am
content: Yes Bob I get what you are saying. I just normally think about it in volts and amps. The power P in watts (W) is equal to the current I in amps (A), times the voltage V in volts (V)The tube data sheets mostly only list max signal power output in Watts and plate and screen dissipation in watts. Most everything else is given in volts and milliamperes.I assume Joule's first law or Joule–Lenz law is at work here... heat produced is proportional to the square of the current multiplied by the electrical resistance of the conductors.Perhaps this is the less simple answer...lol..Billy

author: vk6zgo
author_rank: Super Contributor
timestamp: February 27, 2016, 08:22:33 am
content: Quote from: Andy Watson on February 26, 2016, 04:31:22 pmEven the little Mullard 3-3 would do this.I remember seeing nice "sparkplug like" zaps across the valve socket when the "bodged" speaker connection fell off.In great trepidation,I reconnected the speaker & fired it up---went like a dream!It continued so for years.Maybe it might be more of a problem with higher power units,but I never ran into it again,over many years around such stuff.Faulty output transformers weren't common among HI FI tube amps back in the '60s,either.The little ones on "mantel " radios were another story,& failure of these were not unusual.Of course,if you are fixing 1960s stuff,the transformers have had 50 years to deteriorate.

author: dmills
author_rank: Super Contributor
timestamp: February 27, 2016, 10:24:56 am
content: There is another lovely gotcha that you sometimes see.If you loose the anode connection, particularly in 'ultra linear' designs where the screen is tapped off the OPT, the screen grid can act as an anode, which it really is not designed to do, lots of light before it fails (Grid power handling is always a fraction of the plate rating), but it does sort of work.Suspect this if the valve glows significantly brighter when audio is applied.Regards, Dan.

author: Gyro
author_rank: Super Contributor
timestamp: February 27, 2016, 11:19:34 am
content: Agreed, In a power valve the screen is the most fragile part (unless you do something silly with the grid that is). That's why I like running them triode configured, even though less efficient.Quote from: Andy Watson on February 26, 2016, 08:28:09 pmOn the other hand they're the only semiconductor device where you can get the anode up to a nice cherry red and it will still work afterwards. I've even see pictures where the glass envelope has melted and collapsed in onto the anode.

author: uncle_bob
author_rank: Unknown
timestamp: February 27, 2016, 01:13:20 pm
content: Quote from: Gyro on February 27, 2016, 11:19:34 amHiThere are an enormous number of ways you can abuse any sort of amp to make it blow up. Add to that a low cost bill of materials and you get even more trouble.Try plugging the output of the amp into the wall and see what happens(Yes I *do* know people who like line cords as speaker extension cables ...)Bob

author: TimFox
author_rank: Super Contributor
timestamp: February 27, 2016, 08:45:33 pm
content: Quote from: woodchips on February 26, 2016, 07:29:16 pmActually, the American 6L6 is close to the UK KT66, not the EL34 which is closer to a 6CA7. The 6L6 is a beam-power tube, and the EL34 is a pentode. (The KT66 has a different heater current than the 6L6, but is otherwise similar.)

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 28, 2016, 01:20:10 am
content: I was recently working on a tube amps and I was biasing it. Long story short make sure your bias is set really good and its STABLE... The supply on the amp was all over the place and had no negative voltage adjustment. The cap was bad- and was causing the tubes to practically inscinerate themself. Luckily the owner didnt keep it on for long period of time before he noticed. I did recommend he replace the tubes.....With the power off AND the caps discharged I measure the OPT and write down the resistance- If its cathode biased then write it down the resistance of that.Simply check and calculate the current from the voltage diffrence across the device you measured. I estimate it for the high end of 70% the tubes capability.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 28, 2016, 04:41:16 am
content: Quote from: TimFox on February 27, 2016, 08:45:33 pmSoclose,in fact,that in Oz,they were commonly labelled 6CA7/EL34 on the packet.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 28, 2016, 02:13:14 pm
content: There is a trick used for years. Put a 100 watt light bult , maybe 200 watts for a fender amp in series with the AC . This way it is unlikely to over heat output tubes. Also leaving the amp in standby mode for a few hours will keep filaments only lit. This will activate getter magnesium to degas the pre-amp tubes. If memory servers a 6L6 has mercury in them to increase current flow so this does not help much. You have already tried the obvious but lets not be to sure of ourselves. Turn the amp from standby to on and observe the 200 watt light in series with AC. It should be just red hot. If it starts glowing yellow hot you have a problem. As there are high voltages with high gain cross coupling can be a concern causing RF osculations. Attention to wire dressing looking the same as manufacturer or was a repair done leaving the wire dressing out of place. I have had problems with lead dressing and it can be a nightmare to sort out. The heads up for this type of problem is the 200 watt light jumps from red to yellow hot. On the other hand a slow creeping from red to yellow hot could be a leaky condenser somewhere or bias current set too high. With a fender it is a pot in the corner of the chassis somewhere. I will attach a picture of it for a fender twin reverb. If a little freeze mist on condensers fixes high current then you found your problem. Fender amps and most other brands are well made and can take a lot of punishment. But with the older ones you can not trust coupling condensers. When in doubt change as .01 uf at 600 volts is cheap. That's my 2 cents on the subject. I am envious of you having not 1 but 3 fender amps to play with. Have fun with it.

author: TimFox
author_rank: Super Contributor
timestamp: February 28, 2016, 02:46:30 pm
content: There should not be mercury in a 6L6, but the getters can get exhausted over time.

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 05:06:11 pm
content: HiGetters in vacuum tubes are a bit of a mish mash of metals depending on what they were trying to "get". They all are metals that are expected to stay (mostly) solid at the max envelope temperatures of the tube (glass melts ...). The light metals are what you mostly see in the mixes. A pellet of metal normally was fired into the tube while it was still on the pump out system (really tricky to do). It then ran in the heat box for a while as the getter bonded to the glass and any crap in the pellet boiled off. In most cases (but not all) good getters are shiny / silver / metallic. Exhausted getters turn black or flake (yikes !!!) flake off the glass.Bob

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2016, 05:22:36 pm
content: Well, not quite...The getter assembly is part of the valve (tube). You can see it as a framework above the other elements of the valve. It is either a circular ring, or sometimes a flat trough in a square frame. In manufacture this is sealed to prevent it from reacting with the air. Once mounted it the valve, and at the end of vacuum pumping the getter is 'fired' by heating it with an external RF heating coil (the getter assembly forms a shorted turn). This causes the active material (normally Barium) to fire out and deposit on the inside of the glass, This mops up residual gas molecules and the layer on the glass provides lasting mopping-up of gas molecules either slowly leaking through the glass-metal seals or outgassing from the internal electrodes.A black getter is sometimes seen as a sign of a particularly good vacuum (the black is a result of deposition with higher surface area). The getter flash will slowly be consumed over the life of the tube. A white getter is a sign of a rapid loss of vacuum.Edit: The getter assembly is always at the top of the valve or sometimes on the outside of the Anode because it is important that the conductive getter flash doesn't deposit on the mica insulators (for obvious reasons).

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 05:28:02 pm
content: Quote from: Gyro on February 28, 2016, 05:22:36 pmHiIt depends a lot on the tube design and the plant it came out of. The ring getter eliminates the tricky fire the pellet thing, but it was more expensive.Bob

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2016, 05:35:23 pm
content: I've never seen or heard of a getter pellet actually being fired into the tube during pumping - it would be impossible to control where it is deposited. Maybe a misinterpretation of what 'getter firing' means.Edit: On some High power tube (eg transmitter or RF heading) an adsorbative coating (usually Zirconium) is coated onto the anode, but this needs to run at nearly red heat to be effective. Hence it can only be used on high power tubes.

author: uncle_bob
author_rank: Unknown
timestamp: February 28, 2016, 05:39:59 pm
content: Quote from: Gyro on February 28, 2016, 05:35:23 pmHiIt's pretty easy to see on the tubes. The shiny metal splash is the giveaway. How in the world they got it in there .. I've never seen the machine that did it. It could have been an item that melted at some magic temperature.Bob

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2016, 05:44:29 pm
content: Curious (the standard getter leaves the normal silver flash on the inside of the glass). Anyone else heard of this alternative process?

author: calexanian
author_rank: Super Contributor
timestamp: February 28, 2016, 05:49:17 pm
content: How has the bias voltage been set? Dynamically under load or just set to what somebody else or some datasheet days? Need to know that before going any further.

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2016, 05:52:32 pm
content: The bias voltage is normally set under static (quiescent) conditions.

author: calexanian
author_rank: Super Contributor
timestamp: February 29, 2016, 03:08:07 am
content: Is the overheating condition under load or at idle?