Tung Sol KT120: Why do some of them overheat and others don't?

In this video, I want to go over what I think is likely causing the overheating problems in some of these tubes. Clearly some perform to spec, while others don't. I know at this point I am dealing with a small sample size, but I am seeing a pattern and it makes sense to me, let me know if you agree or not!
This project is using output transformers from Thermionic Labs, custom made to use with KT120 output tubes, which you can buy here:
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[@kingarthur1953]

Your thought process along with your natural drive to resolve problems is what makes you who you are and a great resource. I am so glad the Upscale Audio tubes worked out. I am a fan of them too.

[@IrushaBitukova]

Hi Steph, it's director of the factory-manufacturer, Irusha Bitukova answering you: Yes, we moved the side getters purposely. These are reasons - 1.They used to be placed too close to the radiator edge what formed the plate-getter-radiator loop and, during getter spraying, caused high frequency breakdown between getter and the radiator rib. - 2. The getters were too close to both plate and radiators. This made the getter harder to spray and was leading to overheating of both plate and radiators and caused the plate material to spray onto the glass. - 3. In the previous design the getter was farther from the glass what gave a larger mirror which blocked the radiator rib and bigger part of the plate. THIS caused overheating of the plate and CATHODE of working tube as well. And THAT IS WHY you got higher current!!! - 4. In the current design the mirror surface is smaller and moved very slightly what opened shadowed before plate half and shadowed lesser part of another half. This made plate and cathode cooler and lowered plate current at same plate voltage but still the output power readings show 24W value when the lower limit is 18W. We'd love to get rid of the side getters at all but then we'd get the tube life time below 500 hrs. Anyway, thank you so much for being a fun of our KT120! You are great too! I'm your fun now and subscribed!

[@SkunkieDesignsElectronics]

I got much higher bias and power out of the amp before red plating with the old design. None of the newer version tubes will bias up past 80ma at 500V which is 70% of the rating. which is 40W not the rated 60W. I'm sure they work fine in a push pull amp, but this is not good for a SE amp.

[@IrushaBitukova]

@@SkunkieDesignsElectronics I understand, but we used to lo loose too many tubes in production… And I was talking about idle output power

[@ConorHanley]

Well that's a downer, as one depends on a hope and prayer you can find the 'good' KT120's.

[@NickP333]

Yeah! You rule, Steph! That’s great news! I’m always impressed by your detective work. Yup, the getter grabs up all the extra electrons, but great work figuring out why the tube was defective. I doubt I would’ve figured that out, and I work with tube gear pretty much every day. We get our tubes directly from New Sensor, but had to go to other people for a bit, and luckily haven’t run into too many issues. We do only run most tubes at 70%, as I mentioned to you. That little bit of extra power can make a big difference, especially when used with high sensitivity speakers. Thanks for letting us know about the tubes. Fantastic vid! You’re awesome, Stephe! 🔊😊🎶

[@nevillegoddard4966]

Great detective work Stephe! I'm sure you're right about why these stoopid 120's cook! As you say - why did they change the position of the getter? Crazy! You're definitely exhonerated now! You've done a great service! 👏👏👏! People will now just need to look at the getter & the position of the chrome patch on the side of a KT120 to know if it's a good or bad tube! You wait Stephe - I bet your discovery will go viral real quick! You should be proud of yourself! Someone needs to get on the phone to Vladimir to tell Dmitri to rotate those side getters 20°.

[@peterlundskow4061]

What a super detective job! It just shows again that you have a great natural understanding of how these things work! Thanks so much for this!

[@CP-qe2ym]

your sense of reasoning is impressive.

[@gregwilliams2746]

Hi Stephe - I just checked the four KT120 tubes (bought new in 2022) in my PP Luxman A3600 amplifier and they all have getter material centred over the seams of the plate! I'm lucky that I decided to bias mine at 75mA at 500V for long life and I guess I'll stick with that! I'm also blown away with how good my amp sounds - I'm reaching for more and more music now because it makes everything sound sooo good!

[@OpossumPiper]

Yay! I'm glad you got a working amp for all your effort!

[@anoxicfiltrationplenums]

Great video. I use older (2020) kt120 for my CJ- Premier 11A amp and they work great. Great base too like you noticed with yours.

[@NebukedNezzer]

your analysis makes perfect sense to me.

[@cobar5342]

That is a really intelligent theory. Well done

[@audiononsense1611]

Kudos!!! As a 40+ year EE tube guy I can say yes, most tubes made in the last 5-7 years are junk. Upscale does add loads of value in their knowledge and stock as I have personally recommend them and have always had a good experience (tubes / phono-stage and turntables... not on speakers however, they need serious help here)...

[@richbrown5852]

Thank you so much, I really struggle to understand and retain when I read, but your explanation here made things perfectly clear, I think I'll subscribe and embark in binge learning

[@krispineda5290]

Thank you so much for this video. I am really playing with the idea of building these monoblocks, as I have 8 KT120s laying around from when I built a pair of Bob Latino M125s (which are too noisy for the very efficient speakers I am currently using). All of my KT120s are from several years ago, and all have small getters that are off center of the plate...so even more reason to try your build!!

[@SkunkieDesignsElectronics]

You can do it!

[@krispineda5290]

Is there a list of materials somewhere? I can't find it. I'd like to see how much it would cost, total. Sorry if it's easy to find and I'm just blind... lol@@SkunkieDesignsElectronics

[@paulhovenga4138]

Awesome. Excellent deduction abilities you have, and I believe you are correct based on the trials, even down to how far the getter flash is over! This information needs to be shared with the manufacturer, because even though they should know, maybe they don't! The other part of me says, see if there is excessive current draw from the red plating tubes, and let them red plate to see if they do ok. That does however, keep one on toes as to when there may be an actual problem with the tubes. Output iron is not cheap! Ultimately, and IMO, it is time for another solution, possibly figuring out how to make a KT88 perform well in this circuit.

[@gadget73]

Probably welded where it is because the plates are being welded together anyway. Its just one more "thing" to clamp together in the assembly. It would make more sense over the beam plates though, there are a couple of extra radiator fins on either side of that weld joint in a KT120 for heat dissipation, and un-blocking those would make them even better able to handle it.

[@josepheccles9341]

I believe you are right about the getters. That coating is reflective on the inside as well as the outside. That means the heat would build instead of dissipate.

[@darrylbrueckner2812]

Love this content, thank you. Visual review of the tubes was the chef's kiss. If anyone had told me I'd be addicted to KT120 troubleshooting videos this month, I would've asked them what they've been smoking!

[@SkunkieDesignsElectronics]

LOL! Maybe smoking solder flux?

[@RobertKohut]

I assembled a pair of Bob Latino M125 mono blocks and ran 8 KT 120s in them and they sounded absolutely amazing!!!

[@tonelives1023]

Super smart my friend super smart

[@ahlbergmagnus]

Wonderful news! 🎉 Does this mean that you will run some tests with these new tubes? Can you check how much the tubes can be pushed to get more juice out of them but still be within a range where they will last quite long? Like perhaps test how much different settings of milliamps put out in wattage.

[@SkunkieDesignsElectronics]

Absolutely, but might be a couple of weeks before I get back on these.

[@frankgeeraerts6243]

Normaly , it's good practice to stay below the maximum dissipation ..typical was 75 %....................some sturdy build tubes withstand above their maximum announced performance and dissipation but that was in the PAST !

[@1mctous]

I'm cheering on the KZbin algorithm. Let's find 410 more subscribers to make 10K!

[@SkunkieDesignsElectronics]

So close!

[@NikitaNicholas]

Bought a brand new pair of EL84 from my Mid-century RCA console and almost immediately one tube started red platng! throw them in a box and continue to use the old RCA ones from the 60's ...lol

[@dtg474]

Totally great observation. Very good investigation

[@flyingmattyboy]

Just checked 3 pairs of tungsol 6550, 2 were from ebay and were low use but 5 years old, the other 4 were from a uk based supplier just after the war started. Thankfully they all have the silver film to the left or right of the join. Hopefully they are all good tubes. Guessing you like the TL output trans then.

[@SkunkieDesignsElectronics]

Yes, I like them a lot!

[@mr.george7687]

Can't wait for Western Electric to make EL34 tubes.

[@frankgeeraerts6243]

WE never made them ...........pentodes were patented by Philips.........US response was beam power ( tetrodes).....but guess what , if there's enough money to earn ...who knows ?

[@WoodandSteel]

I run mine at 80ma, and I'm happy with them. I think you might be on to something with the getter position. I had purchased 4 KT120's 3 years ago, and they have offset getters.

[@frankgeeraerts6243]

That's not the problem.....economics are the problem !

[@vespass225]

Not only does the mirror surface block heat that needs to leave the plate, the curved shape of the the mirror surface both bounces the heat back and focuses it to hit exactly to the middle of the plate

[@SkunkieDesignsElectronics]

Right, but of course there are people arguing that a shiny surface won't reflect heat...

[@vespass225]

@@SkunkieDesignsElectronics Really ?? It is the main principle of some effective sun-powered powerstations, so arguing seem kind of off...!

[@michaelcorlet2998]

Someone had a hangover attaching those getters on the production line.

[@Pootycat8359]

This might be informative. Try graphing the transfer characteristics of the good vs bad tubes, for various grid bias voltages, and plate & screen voltages & currents. That should tell you what's screwed up, though not why. But you might determine the values to make the "bad" tubes good.

[@Warclock07]

Thank you, I just compare 4 -KT120 that I bought 4 years ago on ebay they're the same as your good one but only running at 65mA. Will build a new one that can run up to 85mA.

[@SkunkieDesignsElectronics]

I bet you will hear the difference.

[@stiffwin]

Do you think it would be worth experimenting with cooling fans? A well-aimed, quiet cooling fan could help with heat dissipation, and could perhaps be powered from the 6.3 volt heater line...or an external power supply

[@SkunkieDesignsElectronics]

I did try a small fan when testing these KT120 tubes and it didn't change their behavior. The glass isn't overheating, the plate is.

[@davemillon6663]

Nice detective work Skunkie. Could you look and post what the KT150s look like?

[@SkunkieDesignsElectronics]

Already returned them but the getter was directly over the plate.

[@adamphillips1263]

I believe they had someone new on the assembly line when welding things together. They probably had a box of halos with the standoff wire/tab already attached. This would then be positioned and welded in place. It could have been that, based on the orientation of the base or holding fixture, it was more difficult to hold the halo flipped in one direction and easier to hold it in place in the other. It could be that the standoff wires/tabs should have been bent after welding - conversely, they were supplied bent to clear the plate joint but the assembler didn't know they should be bent so straightened them to look better (cussing the halo makers with every tube). Who knows? I just hope I can find a second tube that will bias to 90ma.

[@SkunkieDesignsElectronics]

All the above could be true!

[@frankgeeraerts6243]

The tube delivers enough current at a voltage and bias ...........but the plates redplate way below the rated wattage as announced in the datasheets . Older versions work correct as you experience . They have saved on the materials , quality and quantity of the plates ( and other elements ).........especialy reducing the amount of nickel deposit on the plate and the quality of the metals ! We have seen this before at the end of the tube era production ( example : redplating of the famous GEC KT88 due to the heavy concurence war with chinese shitty productions sold way less expensive ) Those low qualty badges that were refused by manufacturers ( hifi and guitar amps ) were returned to the factory but resurfaced many years later on the market sold as expensive nos tubes . Proof , the same tubes manufactured in the years earlier work fine ........most amplifier manufacturer switched to this family of tubes , where there is no alternative other manufacturer who builds the same type ..............all those customer will encounter the same problem with their expensive tube amplifiers ...........Mac, Audio Research, Jadis and many others , Good news that your amplifier works fine now ! Kind regards

[@petercalum9767]

One more comment... My measurements show that you get lower THD the harder you drive the tube. How does your THD look at 80mA vs 110mA ? I also completely agree with you that the KT120 sounds really really good. I had not tried it before at 500V and more than 90mA bias because i had not a high enough PSu then. I will definitely continue to work with the KT120 as an SE output stage. Thank you for your work.

[@SkunkieDesignsElectronics]

I haven't tested all this yet, but plan to in the near future.

[@stuartmitchell1908]

Hi skunkie. I have the kt88z that you suggested so I ordered more. They are 30mha so they will be ok biasing on a Muzishare x7? Thanks. Really enjoy your videos!

[@SkunkieDesignsElectronics]

The "matching" number rating has nothing to do with their power rating, so yes they are.

[@stuartmitchell1908]

@@SkunkieDesignsElectronics thanks! That’s a real help. Not much info about it and it’s good to hear from someone who knows what they are doing :)

[@whome8192]

You just created another parts shortage. 😊

[@SkunkieDesignsElectronics]

I try

[@tee-jaythestereo-bargainph2120]

I use KT 120 with my Doge 10 mk2b and with my Galion TS120 SE no problems at all ' Auto bias dials them in perfectly ' I have over 7 sets of kt88 and the kt120 bass ATTACk is better than all my Kt88 but the Golden lion kt88 resolution is hard to beat ' but If you have bright Crispy speakers Mullard kt88 is the way to go ' I actually use 2 golden lions kt88 and 2 TAD kt88 for power tubes ' and Wow !! Sound great with my dynes (Dynaudio towers)

[@SkunkieDesignsElectronics]

I'll say this one more time for the people in the back: push pull KT120 amps will never run into this biasing issue I ran into with a high power single ended amp. They aren't biased anywhere near these levels and don't need to be.

[@tee-jaythestereo-bargainph2120]

@@SkunkieDesignsElectronics appricate you love the vids '

[@mortenthue7304]

Would they redplate when running bias at 45ma? Great video btw🙂

[@SkunkieDesignsElectronics]

Oh not at all. All of them seem fine biased at PP levels.

[@MortenThueNielsen]

Thank you. I'm not sure what pp means?

[@SkunkieDesignsElectronics]

Push Pull

[@mortenthue7304]

How obvious;-) Thamk you.

[@ren67s2]

what the cost to build that amp for the parts

[@SkunkieDesignsElectronics]

Haven't added them up but with tubes and these chassis, guessing $1000-1400 each

[@frankgeeraerts6243]

First thing to verify when a tube redplates ( when normaly they don't or never did ) is to check the coupling capacitor ( leaky ) ...........in this case with the amplifier build by Stepf this is not the cause ! Second verification when it is with PP amplifiers biased with a shared cathode resistor , when aging one tube will do all the job and the other will be in starvation mode . It acts as a seesaw ........ Third reason when this happens is caused by grid current with high perveabce tubes , fixed bias that doesn't protect and compensates to prevent run away .......also to high a grid resistor value as seen to often ! ( not in your amplifier Steph ) Fourth reason ..............;;bad and low quality build tube ........ass seen with these KT 120/150/170 from recent production . These tubes that now one else builds are some kind of a bussiness prison .... You're lucky to find good ones or to have build up a decent stock in the past .

[@RobertKohut]

You might have been able to "getter" to the bottom of this problem!! 🙂

[@EskWIRED]

How is it that the tube engineers didnt realize this?

[@SkunkieDesignsElectronics]

They did I'm sure. I believe this was something that happened on the production line by workers who don't understand how tubes work.

[@EskWIRED]

@@SkunkieDesignsElectronics the problem clearly implicates a defective quality assurance regime. Back when these items were just a commodity, I could understand such a thing. But these days tubes are specialty items, much more expensive than they were back in the day, and they should be treated as such.

[@cobar5342]

That is a really intelligent theory. Well done

[@Pootycat8359]

FYI: The "getter" vaporizes calcium metal, which condenses to form that silvery deposit. Calcium is an Alkaline-Earth (Group II on the Periodic Chart) metal, which is highly electropositive, and the vapor reacts with the oxygen & nitrogen in any air present, to form calcium oxide & nitride, which are very high melting point solids.

[@mfr58]

Top observation. Just another sign of no quality control systems in the manufacturing process. I guess we need to see more examples to prove the point, but it looks highly likely you are correct. In the mean time something to look out for when buying tubes. Cheers.

[@davidkclayton]

We learn the most when things go wrong. Knowing this helps me deal with the disappointment of failure.🤔

[@SkunkieDesignsElectronics]

Very true!! It's only failure if you learn nothing from it.

[@tee-jaythestereo-bargainph2120]

2nd that

[@ramilrodriguez8340]

Maybe somebody made a mistake on the production line at the factory. Maybe a new employee

[@georgejobin1744]

I think your on to something makes sense to me good call steph.I knew you would figure it out

[@petercalum9767]

You are absolutely right. I have 6 KT120 tubes and 2 of them have getter position not as bad but almost like yours. I just tested one at 120 mA bias and it redplates slightly at 150 mA worse and not at 100 mA bias. So your theory is absolutely correct.

[@iainbowman8979]

That's a very intuitve observation but there must be a reason why Tung Sol put the saucers where they do. Either it's easier or they have to be in that posistion because of the internal construction. Hovever if this is what's affecting the power handling it would be an essential part of the manufacturing process to get right, which is terrible QC. One question I have is if this amp was built for use with KT88's would the sound quality (ignoring power) be as good because people like KR Audio etc make good KT88's.

[@SkunkieDesignsElectronics]

I honestly can't say they would be "as good", I've never heard those KR Audio tubes. There are a zillion KT88 SE designs out there.

[@frankgeeraerts6243]

The real TUNGSOL is turning in his grave.................

[@gregwilliams2746]

I think you underestimate the likelihood of snafu... new workers are shown how to make KT120s and one detail is left out (don't make the halo directly opposite the plate seam).

[@frankgeeraerts6243]

Please do have a look at old tubes before your era ....and look where the getters were put before ( you were born...)@@gregwilliams2746

[@Pinball-Audio]

Yet the same Tube Brand..... made in Russia.. I have found various tubes made in Russia 30-40 years ago, which I have many of... work far better than Russian tubes made today, especially tubes that never existed even 10 years ago...like KT120's...

[@frankgeeraerts6243]

A friend of mine runs his PP amplifier in triode mode, the tubes ( nos 6550A) are biased very hot in class A .............455 volt plate at 100 mA .............Six years later the tubes arestill fine and within specification ... WHY ,.................the plates are relatively and asthoningly small and fine .................those were made with three alloys ( tri-alloy clad structure )...........the different alloys were blasted together with DYNAMITE ! The tube .........6550A GE .........and some other US manufacturers...... Those were the days ...you bought the tubes, you used them without testing and in most cases all went well. Graphite plates as seen in big emmision triodes like the 211/VT4c and the 845 kive even longer when pushed a little into red plating ( seen in the dark ) , BECAUSE the anode was coated with zirkonium as extra getter..............when heated above a ceratin temperature they become very active as a getter ..........and the tube livesd longer............Seen in some amplifiers , more than 20;000 hours with more than 1000 volt on the plate........ So everything is relative besides badly build low quality tubes. To conclude ................as long as the anode gets it's electons out of the electron cloud without pulling them ( = stripping of the cathode coating ) there's no problem of shorter lifespan ....that's another myth in audio circles ............but the temperature of the glassbulb has to stay under the designed max temperature rated by the manufacturer.........

[@johnstuchlik5828]

Just curious if you can only run 950 volts on 845 tube se what range of plate current gives a long service life?(us made 845)

[@frankgeeraerts6243]

With US ( nos ) 845 tubes you can run the current to just have a faint red glow on the plates in the dark ! Sounds good also if the PS and OPT are of quality .@@johnstuchlik5828

[@frankgeeraerts6243]

You can also run only 700 volt on the plate ..@@johnstuchlik5828

[@lifetimesofamultiplemediam1003]

Well spotted!…

[@moodyga40]

ill stick with my 300 B shoebill amp

[@alexarif2835]

I would like to ask if you think it is possible to bring the "color" pre amp feature into this power amp by increasing the input signal into the 6EJ7 and reducing the output to maintain the same volume ?

[@SkunkieDesignsElectronics]

Someone can always add pots to an amp like this, but you would need to factor in the output drive of the 6EJ7 when sizing the power rating. + there is a LOT more voltage swing.

[@kb6dxn]

When are you posting the schematic and parts list for the 120 amp?

[@SkunkieDesignsElectronics]

Soon, I promise!

[@MortenThueNielsen]

Thank you. I'm not sure what pp means?

[@SkunkieDesignsElectronics]

Push pull

[@rogerweiland6384]

There is just too much fettling involved with tube amps. I don't want to buy a brand new amp and still have to tune it up like an old AMC Gremlin. Also too cost prohibitive. I was thinking about a hybrid amp maybe like the Dayton hta-200, but now I don't even know about that. Also, is a fully tube amp considered a class A amp?

[@SkunkieDesignsElectronics]

There are class A, sometimes called single ended. Then there is class AB, called by some as push/pull. The class A have a lot less power but sound cleaner IMHO.

[@rogerweiland6384]

@@SkunkieDesignsElectronics You don't have to be humble about your opinion, you obviously know your stuff. I've been in custom car audio since 1988 specializing in sound quality so I know my stuff but I'm not a bench tech. I have no problem designing and building speakers, and I know my way around ab car amps pretty well but tube amps are kinda like an ancient mystery to me. I'm familiar with tube sound and it interests me very much but I don't think I can afford it right now. All the design tinkering and tube swapping interests me very much, I love that shit, but I don't have the money right now. What do think about entry level hybrid amps like the Dayton hta-200? I might be able to swing something like that. I have no problem designing and building my speakers, but I have a lot to learn about bench work. Believe it or not after all these years I've never heard an understandable definition of amp classifications, so thank you. I have to explain why I hate D-class amps to people all the time, so I appreciate the info. How do you explain the difference between class ab and class d amps? I always compare class d to an arc welder. Lol

[@extracla55ic]

Fantastic video! I learned a lot about tube design from your video. Thank you for stepping through the tube parts. Fascinating! Your description makes it easier to understand how the tube operates. I’ve been using tubes for just a few years and really dig the elegance of the tube’s function. Good stuff.

[@JohnScheppler]

This is great information for anyone looking to buy tubes on the used market what to look for. Thanks

[@wpg_dude1930]

I'm using Tung-Sol 7581A from Tonal Tubes last 3 years and am happy with them.

[@gillesthibodeau7078]

That makes alot of sense! I would never have tought of it! Great work as usual. Thanks! I learn from you! :)

[@whome8192]

Interesting. My recent Mullard KT-88’s are just off to one side like the tube you show, sometimes the plate seem o just covered up, but the disk will be off to one side as the wire that holds it moves the disk away from the gap. No red platting, but as you said biased pretty low.

[@SkunkieDesignsElectronics]

And the heat load on a KT88 is much lower.

[@stephenkeever6029]

I never thought l would be learning new things about vacuum tubes so I’m enjoying being on this journey with you.

[@SkunkieDesignsElectronics]

Welcome aboard!

[@markkinney5030]

Wow that makes sense why they were red plating. I been using Tung Sol EL34's in my guitar amp never had that happen before. Getting tubes these days our getting harder.

[@SkunkieDesignsElectronics]

They don't have these type side getters most likely.

[@markkinney5030]

@@SkunkieDesignsElectronics You are right they don't! I'm happy you found out why they did what they did and the new KT120's working great! I wish i could here that amp i bet it sounds so sweet. A good tube amp is what it is about tone for days.Thank you have a great day!

[@paulgood2218]

Great explanation !!

[@SkunkieDesignsElectronics]

Glad you liked it!