Plate voltage issues and it's effect in vacuum tubes - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: March 04, 2016, 04:20:25 pm
content: Hi guys,These questions relates to EL84/6bq5 pentode vacuum tubes. All the data sheets for EL84 tubes give a max plate voltage of 300V and a plate dissipation of 12 watts.Many amp designers run these tubes at 400V to 430V on the plate. That is 25% higher than the stated max values. These amps work without issue so the tubes can be pushed to these values.I assume that the higher the plate voltage the higher the plate dissipation in watts would be. Is this correct? If so how would one come up with the new plate dissipation value? Is it 25% higher? I actually don't think it is 25% and my guess it that the relationship in not linear.The reason for these questions is to come up with a reasonable plate current value and therefore a correct bias voltage.At the normal data sheet max values of 12 watts plate dissipation and max plate voltage of 300V the plate current would = 12/300 X .7 (assuming you want to run at 70%) which = 28 ma plate current. At 400V that would be 21 ma plate current based on 12 watts but it is obvious that is not correct because the actual plate current in at least one amp I own is around 35 ma. Also the tube is running at a temp around 265F which seems pretty hot but not damaging the tubes at least in the short term. I have found no max stated temp for a EL84 in data sheets. It is 450F for a 6l6 for example but that is a much larger tube.I guess one could say "if it works it works" but I want to know how and why it works.Cheers,Billy

author: TimFox
author_rank: Super Contributor
timestamp: March 04, 2016, 06:03:04 pm
content: You may be able to exceed the plate voltage specification, but the power dissipation specification is based on how hot the plate is allowed to get.If you have a higher voltage, you must reduce the plate current accordingly to keep the product (power = volts times amps) below the specification.Of course, there is no legal compulsion to do this, but the manufacturer cannot guarantee that the tube will last when run above any of the maximum ratings.

author: calexanian
author_rank: Super Contributor
timestamp: March 04, 2016, 10:50:39 pm
content: Very good question. I think I will go write a Hack a Day article about it.

author: Planobilly
author_rank: Regular Contributor
timestamp: March 05, 2016, 12:42:36 am
content: Thanks guys for your input.I am looking forward to the "Hack a Day" article.The more I learn about vacuum tubes the less I really understand at this point...lol. I think I have some really serious study to do to begin to truly understand this subject. Fun stuff but a ton of work.Cheers,Billy

author: vk6zgo
author_rank: Super Contributor
timestamp: March 05, 2016, 02:08:21 am
content: 6BQ5/EL84's can get hot enough to burn your fingers & still survive for years.Actually,the hottest operating receiving tube I ever ran into was the 6AQ5 (a miniaturised electrical equivalent to a 6V6).Trying to pull one of these out of an (operating) tube type car radio under the dash,is quite startling!Burnt fingers & dented head!Some of the amp circuits you have shown us,wouldn't have been considered good design back in the day,as it was usually better to run two big output tubes,rather than EL84s in parallel push-pull.Obviously,even when they were designed,the supply of audio power tubes was starting to dry up,hence the use of multiple (more available ) devices.I remember many years ago,a friend of mine referred to fixing a homebrew guitar amp which used 807 's in the output.In those days,there were still a lot of tubes which had the grid brought out on the top cap.A trick often used in troubleshooting was to touch the top caps,injecting a bit of hum in so doing.When it stopped causing a "blurt" at the speaker,you had found the faulty stage.807s bring the plate out of the top cap,& the"homebrewer" had neglected to use the proper insulated connector.My friend used the above technique,& received an unwelcome surprise!All of which brings me to ponder why Guitar amp manufacturers didn't use,not 807s,because they were already too old,but 6146s & the like,which have serious power capabilities,& were still readily available.Perhaps a lack of information about using them in audio circuits?

author: Planobilly
author_rank: Regular Contributor
timestamp: March 05, 2016, 04:20:50 am
content: Hi vk6zgo,One of the things I have started to do is actually measure the temp of tubes in amps that are working correctly and those that are not. As I get more experience doing this I begin to get a much better idea of what "hot" actually means. Also some tubes have max temps listed in the data sheets, the 6l6 for example. This provides another quick troubleshooting method.The cause of heat is current and the current has to be below a level that will damage the tube. The amount of current also changes the frequency response. To low of a current flow in a tube will cause a certain type of distortion.One of the issues that come into play with tube amps is that if one only looks at things from a electronics perspective there is a lot of things that would not be very obvious. Guitar amps are a part of a system, the guitar player/ musician, the guitar, and the amp. It is easy for me to understand why electronic engineers who are not musicians must think all guitar players are a bit crazy.There are many reasons why a certain tube would or would not be used . Better electronic design could be in some cases a function of larger tubes and larger tubes like 6l6 and 6550 tube are common in today's world. The main reason El84 tubes are used is not cost or easy access but the sound they produce. Even El34 tubes which are close to 6l6 in design sound different. It is hard to put into words but they also play different. In other words one does not play a guitar the same way with different designs of amps.There is a bit of an insane quest for "tone" among guitar players and companies making amps try to fill that quest.For myself I have an "insane" need to know the reason why thing works...lol Yes I play guitar, and yes I work on amps and yes I am always looking for "tone" I like better. I understand music pretty well...electronics, not so much.Electronics is a new adventure for me and I am in a very steep learning curve at the moment. A ten second look at this link will give you an idea of stuff I am studying and anyone who can read this stuff with ease my hat is certainly off too.http://www.tubebooks.org/books/pullen_conductance.pdfI can troubleshoot and repair guitar amps. That is actually not so hard to learn. Having the understanding to see why a designer did something a certain way is what I want to learn. That look like a pretty good size mountain to climb from where I am currently standing...lolCheers,Billy

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: March 08, 2016, 10:43:12 pm
content: Unlike transistors, tubes are forgiving when abused....to a point. The manufacturer's ratings are specified under certain conditions which guarantees that a given tube will meet the minimum number of operating hours before needing to be replaced (often at about the 50% point compared to a new tube). Tubes have various useful life span hours, usually starting at 1,000 hours and going to up about 10,000 hours for certain special tube types. In the case of power output tubes like the EL84/6BQ5, et al, the maximum specifications should be observed if you want the tube to last a normal life span, if you can afford to, you can run the tube 'hotter' than spec but there are two penalties that come with that, reduced life span and increasing risk of failure, often catastrophic as in arcing or melting of the grids. I have read of instances where the tubes were being run well above maximum ratings and they actually worked but invariably they will fail sooner. Higher voltages, higher currents, higher plate dissipation, higher grid dissipation all come with a price to pay.An EL34 running within spec may give you 2,500 hours or more life but the same tube running above spec can give maybe 250 to 1,000 hours before giving up, if the overload is high enough, you might not even see 100 hours before it fails. It all depends on the circumstances. Most power tubes like the EL34 will have a hot spot maximum temperature of about 185°C to 200°C and those need proper ventilation, keeping them in an enclosed box is going to shorten their life span even if run within spec.I have 6L6GBs that have run in one of my power amplifiers since the early seventies and are still reading good. Keep the tubes within spec and they'll be happy for a long time.