What causes vacuum tubes to red plate?

[@elsenordefiler1893]

Beautiful video!!! A customer just brought me a Orange rocker 30 and it's exactly a EL84 red plating. Swapped the tubes and it seems the trouble isn't anymore. BUT after watching this video, now I have to ensure something else is not failing. In this case, amp HT fuse was blown up, and that was the reason customer came for. I replaced with a 500mA standart fuse and it blew, then I tried with a 1A and it didn't blew. Then I replaced with exact type of fuse Bussman 500mA slow blow fuse and also it didn's blew up. And only then I observed one of the two EL84 red plating.

[@jkaley]

In my experience, dirty or loose pin sockets are the #1 cause of red plating. Also the easiest to fix! Thanks for the video!

[@w1nchester32]

Thank you so much. An excellent explanation of what has caused my Leak Stereo 20 hi-fidelity power amplifier to fail. I now feel sufficiently prepared to talk to a tech!

[@m.ericwatson968]

I've been trying to figure out why biasing of tubes is necessary when switching tubes in an amplifier, (6L6 to EL34) and I understood this explanation with basic electrical knowledge, thank you.

[@henniecronje3212]

Thank you sooooo very much. You saved me big. Having very little practical experience, even I was able to repair my amp. The least I could do was subscribe to your great channel.

[@budandbean1]

This was a really great explanation. I’ve always thought of it as the electric that it takes to make the tube operate, versus what is happening to the electric signal going into the tube. I thought I knew about red plating but not to this depth, this was very informative, thank you!

[@geomorphdog]

Thanks for the video! I had one red plating tube on a Sansui 1000. First time I'd seen it. I replaced the coupling caps on that channel and apparently fixed the problem.

[@curtisgriffin7924]

Excellent short and sweet explanation of how tubes get to this point. Thank you Mark

[@boonedockjourneyman7979]

Excellent. My only thought is the role of cold or otherwise poor solder joints. Increasingly over the years I am finding this to be a major issue. My practice now is to touch up everything on a given stage before I even reach for a DMM. Measuring bias when you have crappy solder work has been a waste of my time.

[@timka880057]

Thanks! Enjoyed the video. I certainly love vacuum tubes. its good to go over things to help us remember.

[@tonytaylor5087]

A leaky aged cap definitely did this in one of my old amps. Great video!

[@NotMarkKnopfler]

Thank you for this. I have a brand new (well, it's actually about 5 years old now, but has about 30 mins playing time on it) Carlsbro 30W amp that has this problem on one of the 6V6GTs. The amp was a total disaster (I got it un-opened from a music shop that went out of business). From the very first power up, it was wrong. The first fault I found was the heater pins on one of the power valve sockets was not actually soldered to the board. I fixed that, but now it red-plates. I managed to get a schematic, so this has given me the inspiration to dig it out and see if I can fix it. I'd send it to you but I'm in the UK and it's a 240V unit ;-) Cheers Mark

[@johnc8910]

It's red plating because it is drawing too much current. Check the grid bias.

[@GustavCI]

Hi Mark. Let's stay by the book. As John mentioned check the bias. Since the amp was running asymmetricaly, consider the output transformer could be fried. Just for reference check the resistance of the primary winding. (From the center tap to the anode of each tube). PLEASE if you do so set the amp to "dark and cold". This means UNPLUG the amp and DISCHARGE the filter caps. In "hot and working" check the voltage drop from center tap to anode. In case you encounter divergence in the readings, switch the tubes and figure out if the problem stays with the socket or the tube. It would be nice if you could provide the schematic. Once again lift yourself from the ground i.e. rubber sole shoes etc because, anode voltages are nasty somehow (some amps like Music Man go up to 700Volts!!!). Line voltage should not be a issue, step up/down transformers exist on both sides of the lake. Maybe your device has a voltage selector? For us in Europe only the power outlet plug changes. In Austria we use the "SCHUKO", it looks like a pignose....

[@JulianA-tr6pt]

In the case of EL84 guitar amps, I'd consider a bad tube a pretty common cause. Typically, these tubes are run flat out at 100%, if not a little more, often with 300+ volts. I've had several start to red-plate after a year or so of use. In the most recent case, one Sovtek EL84 red plated, had more blue glow (both in the bottle and the typical lines within the plate), and there was a noticable hum. Little orange specks were visible around the screen, too. Swapped the two tubes around, and after about 10 minutes it started to glow again, following the tube.

[@elsenordefiler1893]

Hi there!! In my case, I did the swap, and none of them went red plate. What could it be?

[@JulianA-tr6pt]

@@elsenordefiler1893 Oh so you had a red plate, swapped the 2 around, and the red plate went away? It could potentially be related to the bias on each specific tube socket, which may happen to not cause a red plate on the swapped tubes. It could still be a sign of a going-bad tube and/or very high 100+% bias. If they share a common cathode resistor, and voltages match up on screen and plate, that's unusual. I'd definitely check voltages to be sure both sockets are properly biasing tubes. If it's adjustable bias, similarly it'd be good to make sure the tubes are running at a reasonable dissipation. Coupling capacitors before the grid can also cause this when they begin to leak and allow positive voltages on the grid, but this failure is usually more noticeable.

[@elsenordefiler1893]

@@JulianA-tr6ptThank you for your reply! "Oh so you had a red plate, swapped the 2 around, and the red plate went away? " Yes it went away. This amplifier is self bias so there's no adjustment pot and yes, the two EL34 share a common cathode consisting on three paralleled devices: one 270ohm 5W resistor, one 330ohm 5w resistor and one 220uF electrolitic capacitor. For the coupling capacitors it has one 100n film cap for each EL34. Also there's an 12ax7 driving both el34. Worth mention, red plating occured not that fast.

[@eugenepohjola258]

Howdy. Nice. I have had my share of red plating in my designs. And You are correct. In some cases the root reason was parasitic oscillations. These were devilish to spot. When the oscilloscope probe was connected between the anode and B- the glow died. But eventually the scope was connected over the OPT secondary revealing violent oscillation at about 50 kHz. In some cases increasing the grid stoppers worked. In other a small cercap. from the anode to the grid worked. Like increasing the Miller capacitance. And yeah. A few coupling capacitors have proven to be leaky though brand new. My advice is to always check the capacitors with a multimeter before soldering. Also devilish to spot otherwise. Checking the DC potentials at various points eventually put me on the track. Best Regards.

[@qua7771]

Interesting. I only had it happen once when my bias was too hot. I want from one neighborhood to another with higher mains voltage, and the Chinese tubes I was using were known for thin plates, (I think). The sound dropped out completely, then I looked.

[@eugenepohjola258]

@@qua7771 Howdy. Thinking about it I had another root cause. I had followed the rule of using a separate wire as the B- bus connecting it to the chassis only at the input jack. Nevertheless parasitics were experienced. The cause was that the B- bus wire was too thin. I recommend using at least 1,5 mm2 as the B- bus. Also the bends in the B- were too tight causing inductance. Using as shallow bends as possible in the B- bus is recommended. Regards.

[@qua7771]

@@eugenepohjola258 Thanks for the tip. These are things I wouldn't ordinarily think of.

[@mobicus1]

Thanks for your clear, well presented explanation here. My brain feels just a little fuller now :) Cheers !

[@REDROCK123]

This video answered so many question for me.

[@michaelstreets7816]

Excellent video, thanks for posting this.

[@davekazoroski6548]

Good video! Personal red plating experience: Almost always a leaky interstage coupling capacitor. Once in a great while, a shorted cathode resistor bypass capacitor. I have never seen a voltage amplifier tube red plating. Have seen several output tubes red plating. Saw a few rectifiers red plating as the power transformer was smoking (shorted filter cap, one time a filter choke with an internal short to ground)

[@bart99gt]

Dave Kazoroski I would hazard to guess since most preamp tubes aren’t operated anywhere close to their limits (usually) that it would take a pretty catastrophic event to make one red plate. I looked around and couldn’t find a picture of a red plated 12AX7. I would assume that the voltage dropping resistors and plate load resistors would probably limit the current sufficiently to keep from damaging the tube if there were a loss of bias, ie., cathode shorted to ground. Unlike your power tubes whose DC supply is through an inductor with a low DC resistance.

[@davekazoroski6548]

Actually, some voltage amplifiers have the cathode connected directly to B-. They use something called "contact bias". A certain number of electrons coming off of the cathode "stick" to the control grid, thus causing a negative charge of a volt or so to build up on the grid. And as you say, the plate resistor of a voltage amp is high enough to limit plate current. With a power tube the series resistance is very low, enabling a very high plate current to flow. Plus, a power tube needs several volts of negative grid bias - contact bias won't work here - loss of bias results in a grossly overheated red plating tube. ( I've seen TV horizontal output tubes orange plating - hot enough to melt the glass)

[@markkett]

Thank you! Great videos!

[@williammiller5490]

The OB2 and OD3 are both a two electrode, ineinert-Gas-Filled cold cathode miniature tubes. Intended for use as a voltage regultor. Not a rectifier. If a voltage dropping resistor in series with this tube is not selected properly or changes its value, the tube will glow brighter with higher current flow. These tubes will fail and sort of "burn out" in an over current condition. Also, while an electron tube with filaments heating the cathode does cause electrons to boil off, they do not flow to to plate to create current or electron flow. A complete circuit with a voltage potential between the plate and cathode must exist to attract these electrons to create current or electron flow.

[@Blueglow]

That was a simple word mistake on my part. I am fully aware of what these are and how they work

[@williammiller5490]

@@Blueglow I thought that was the case but others ( newbies to tubes ) might take every word you say as The Gospel for tubes. You do a good job and I appreciate your videos.

[@Blueglow]

@@williammiller5490 thanks man

[@flintdavis2]

@@Blueglow thanks I appreciate it, I learned something about tubes so I just wont go swapping them out, but will try to test for proper voltage, current, amperage , 😢😅 Why can'tI just change the tubes? Put in ones that can handle the pressure, voltage, amperage, flow of current, ( excuse my frustrating rant) I just want an easy fix, that wont cause damage to this $1,100 fender bassbreaker 30 r amp.

[@oswaldwabbit8986]

Nice Video, Always something to be leanrt here, thanks for the explanation.

[@gavincurtis]

Amazing channel and have subscribed. What could cause the plate current to drop if B+ and B- is correct, cathode/anode resistors good? Cathode bypass capacitor good. Tube is confirmed good and it's compliment is suffering the same issue in this balanced push pull drive configuration. They directly drive high impedance headphones. No transformer. Swapping headphone channels (drivers) makes no difference. The amplifier is a little dot Mk6+. I am an electronics design engineer with 30 years experience, so no fears other than I am a tube noob. It is a balanced push/pull configuration for each driver...like an "H-bridge". So one set of 6080's for each channel, both affected by low plate current. It looks like this could be a grid bias voltage issue? Instead of overdriving the tube, it is instead "under-driving" the grids of the output 6080s???

[@abyssalsoul6216]

Hello. Thank you for this great video.So I have this vacuum tube amplifier with eight kt88 tubes as output. The amplifier plays well at low volumes but when I turn it up louder the B+ fuse (2A) blows. I did not have the chance to see if any of the tubes were red plating. Any ideas?

[@ranius7388]

If it happens in preamp tube , f.e when putting 12at7 instead of 12ax7 , it is also the same reasons ?

[@jeffammon3135]

Thanks for the information. Found it very helpful

[@edwardhannigan6324]

Great Topic and fab explanation..! I'm learning...Thanks for sharing your knowledge..Ed..U.K..😊

[@ke4est]

Great explanation Mark!

[@garybevis8691]

Current flow models confuse newbie EE's/Techs. To keep it simple and the way that all circuits make sense is electron flow model. As a vacuum tube is a voltage controlled device it makes no sense to envision a current flow model. Assigning the action of a valve to the control grid makes sense to most folks and is how I learned about tubes, FETs are the same thing essentially. Both are voltage controlled devices and need very little voltage to control a large voltage.

[@giulioluzzardi7632]

Learned a lot, thanks.

[@johnsenchakinternetsecurit8935]

Nice video , posting it on the EEVblog forum

[@joeblogs8204]

@ 13:30 leaky coupling cap..I am having problems with an el84 push pull..It is not red plating but I am reading about 1-2VOLTS DC after the coupling cap to the grid of the el84.Is this acceptable?

[@GustavCI]

Desolder the cap check DC-voltage cap to ground.

[@1974UTuber]

Ive noticed in movies when they want to show an old valve radio or any piece of valve technology they have sometimes deliberately wired it up to red plate for the glow effect of the tubes. Now days they would use LEDs or CGI for the effect but im sure they had old electro techs who would know how to red plate a valve for a quick and brilliant glow

[@ReneStover]

Electrical current flows from negative which is an electron source to positive. Engineers consider it the other way around, saying it is easy to think of current flow that way, but not for me.

[@glenmartin2437]

Thank you. This was a good review.

[@pietpaaltjes7419]

Great explanation. Thanx!

[@johnc8910]

Some tubes are designed to do that. The earlier 813 beam power tetrode had a metallic carbon graphite plate structure. Medium power tube, 125 watts plate dissipation. A pair could easily handle 750 watts in Class C. Plates would glow cherry red under sort of normal conditions.

[@davekazoroski6548]

Yes - many industrial thyratrons and ignitron tubes (high current controlled rectifiers) also ran red anode ( industrial electronic term for plate) in normal operation. They were designed that way. But, most receiving tubes were designed to run "dark plate". A glowing plate in audio or other home electronics is a definite sign of trouble.

[@greatPretender79]

Hi blue glow, I'm working on a 1931 Philco "baby Grand" model 90 cathedral radio. I have this exact problem after replacing my line filter caps. I'm thinking now that my problem might lie in my Bakelite blocks as a coupling cap for the #80 rectifier. I wondered for days what I did wrong, stared at the chassis and the schematic.. now, I think I know. Thank you!! Any input is appreciated as I haven't worked on anything this old much before! Mike

[@gillesswing1423]

Can someone explain to me the role of the 40uF capacitor in // with the 320ohm resistor? Thanks a lot.

[@silasfatchett7380]

It's a cathode bypass capacitor. If it were not present, a negative feedback signal would appear at the cathode, reducing the gain of the tube.

[@MemeReviewer]

The surprising thing is, My grandparents have a Grundig Majestic 2035 from 1954 and no red plate. It is amazing

[@larryshaver3568]

what about the100ohm or the grid stop ?no wattage is shown for them

[@larryshaver3568]

Wouldn't increasing the impedance on the grid resistor help to minimize chance of red-plating?

[@romelceschini6992]

Thanks for the post. Brazil

[@EngineeringVignettes]

A good explanation of red plating, thanks Mark. I will look a bit further into this. I'm curious about the long term effects of red plating a tube... the failure mode would be different from a silicon device as the silicon device would either short out or blow open the substrate.. in the case of a tube there is no substrate, just a vacuum. Perhaps the damage is due to plate warping or other physical movement of parts in the tube... I really appreciate these videos, thanks for taking the time to make them! Cheers, - Eddy

[@davekazoroski6548]

Red plating causes the plate to release trapped gasses in the metal, damaging the vacuum. In older TV's sometimes the horizontal output tube would red plate due to loss of drive. They got so hot the glass would melt. Red plating can destroy a tube in very short order.

[@davekazoroski6548]

Did a little more research (benefit of being semi-retired). When a tube is red plating, the current through it is much higher than normal. The massive flow of electrons to the plate litterally takes the special emissive coating on the cathode with it. As this coating erodes (normal over a tube's life) the tube gets weaker and weaker - low emission. Basically a few minutes of red plating is equal to several hours of normal use. So no matter how you look at it - red plating is bad for tubes.

[@turiddu9]

Question: Wouldn't an AC filament voltage being applied to a vacuum tube (as opposed to a DC filament voltage) cause the frequency of that alternating current to be also amplified and sent (and audible, measurable?) at the output of the tube?

[@michaelhelgeland4588]

Salvatore Rossignolo I’ve run filaments on 6.3 v dc instead of AC and it made no difference in hum level, so at least in the case of my amplifier, no.

[@teeeeeeeeeet]

Great explanation. Thanks

[@fastteddyb]

The red glow followed my valve around when I swapped it - with it's twin opposite side. I guess thats a valve for sure?

[@lousekoya1803]

Thank you !

[@ProperLogicalDebate]

I thought that current flowed from the Cathode, Emitter, to the Plate, Collector. From a place where there is excessive Electrons to dilute to where there are less. But while going there the Electrons must go past an obstacle of the Grid or be affected by the Base. Especially with a FET.

[@ProperLogicalDebate]

Maybe I am thinking of current flow of Electrons while you are looking at it as "Holes" as with a NPN or PNP transistor?

[@ncilmo]

Great video. Thank you so very much.

[@andrefiset3569]

Well done but a too high B+ can also be a cause. You need to adjust your cathode resistor accordingly if your plate voltage is in the specs.

[@rafaelatienza9389]

I need to know the pins data of 6k4 please help

[@larryshaver3568]

could a speaker with too high of impedance cause the red-plating issue?

[@JulianA-tr6pt]

Late reply, but for anyone wondering - probably not. While impedance mismatches can be a problem, if it's a 100% mismatch (16 ohm speaker running on an 8 ohm output), there probably won't be much of an issue. Another thing many may not realize is that the tubes will not be effected by speaker impedance unless there is sound being played through an amplifier. If the amp is idling, with no audio, you can run it without a speaker (or even a dead short circuit on the speaker output) forever with absolutely no issues. This is because the speaker is transformer coupled, and only the amplified AC signal will pass through it. The speaker impedance is only realized upon the tubes putting out AC signal, through the transformer. At idle, there is only DC on the primary side, for the tube plates.

[@ericsalinas1839]

@@JulianA-tr6pt Could someone run a tube amplifier without a load connected, without doing an harm, as long as nothing is plugged into the input jack?

[@JulianA-tr6pt]

@@ericsalinas1839 Yes, you can. The output transformer only "reacts" to AC signal, therefore none of the impedance issues exist when there is no signal. When there is no signal coming from the output tubes, the amp doesn't know what kind of speaker load it has. There's a good reason to not do this though - if the amp is faulty and has bad oscillation or noise, or a loud pop is created if you're working on the amp and taking measurements, it'll be running into no load. It's best to always use a dummy load.

[@hpelisr]

Very good info,Thanks

[@banjoperator]

awesome video, and as always informative..i do have one suggestion, in future vids when using diagrams. could you switch to a dark pointer, the reason i ask is that i resently lost alot of vision in one eye, this makes it difficult to follow a white pointer on a white diagram background..just a sugesstion for those who are in the same situation i have found myself in..cheers

[@michaelsingh7529]

I have a tube amp. athome and have decided to revamp it,its over 50 years o!d ,usinging EL34's love how it used to work but they started red plating,and brobably caused my electrolitic capacitor to leak,i am trying to sourcenew parts to service this amplifier.Can anyone help me source these parts? Thanks

[@TechTed1]

the section of wire of primary winding of transformer is more bigger than normal that is calculated for the right nominal voltage thats wy red plating

[@larryshaver3568]

I have a Magnavox single-ended stereo output amplifier that uses 6BQ5's

[@paulkielt9301]

Unfortunately, many guitar players bias their power tubes like crazy because they think that way their amp will sound better. Distortion and gain are too different things that are totally misunderstood! But sometimes it's just a component failure.

[@paulkielt9301]

Too much current on the grids! Sometimes one or more screen grid resistors has failed or the bias power supply has to be fixed. The first thing people think is they have a power tube going bad. Overwhelmingly, it's not the power tubes! That's what I experimented on my own.

[@bradt.3555]

Too much current flowing, quite simple, Or are we discussing figuring out why too much current is flowing?

[@russellhltn1396]

I disagree with you at 11:43 - it's not possible for the output load to reflect on the *DC* resistance of the primary. It will most certainly reflect on the AC impedance. In some cases, the lack of a load could allow the tube to oscillate which in turn could cause red plating, but that's different than saying the load affects the DC bias.

[@boonedockjourneyman7979]

Do some reading about transformers. The secondary side will effect the primary side in a number of ways. I can't possibly describe the topic in a KZbin comment. You need to read it for yourself.

[@russellhltn1396]

@@boonedockjourneyman7979 Yes, at AC, or during turn on/turn off. But at steady state DC? I don't think so. The coupling between the windings is from changing magnetic field. With DC, there is a field, but it doesn't change. Therefore, no interaction.

[@andreimarkulczynski2498]

Maravilhosa e didática aula. Parabéns! Muito obrigado! Wonderful and didactic class. Congratulations! Thank you!

[@nikoladivic2786]

Nice Mark, thank you.

[@daleburrell6273]

The way I learned it, the plate current flows from from the cathode to the plate-!!

[@Blueglow]

If you follow modern electronics "conventional current flow methodology" it goes from + to -, thus plate to ground through the cathode. If you follow "electron flow methodology", it goes cathode to plate.

[@silasfatchett7380]

@@Blueglow If 1mA is flowing in a 100k resistor, 100V are dropped across it whichever way it is flowing.

[@antoniograncino3506]

A cause of red-plating that you didn't cover: negative bias supply circuit faulty rectifier and/or capacitor. Great video otherwise.

[@johnsimms3957]

Wax and paper capacitors need to go the way of the dinosaurs.

[@flintdavis2]

Maybe just saved me from buy a set of tubes! The plot thickens, to bias or not to bias that is the question.

[@williamnichols2067]

"find a good tech to bias your tubes" Use Ohm's law ya dummies. Not at any one person, but its really not that difficult.

[@TechTed1]

the red plating is hapending by corona efect of high masses of discharging

[@TurdInternational]

I (stupidly) didn't turn a Fender amp off, before swapping the preamp tube, and ended up with a crackle and a redplating reverb tube. Swapped it with 3 different tubes; stock Groove tubes, new Mullard/Sovtek and the same model/year NOS Mullard that was redplating, and they all worked fine. Put the bad one in, and it was fine for a few minutes, and then starting glowing. I'm 90% sure I've cooked it, but now I'm wondering about what happened inside the tube to cause the red plate...