Lots of great info Mark and thanks for the link to Rob's page, looks like a lot of great reading to do!

Mark thanks for the information. I have a Sonic Frontiers sfs 80 which was totally rebuilt. I currently am using EH KT90s. I was told that it could handle KT120s but was skeptical about the actual benefits of using a tube which draws much more current than what was intended. I am grateful for you taking the time to post this video.

Excellent. Perfectly presented and of high value.

Mark, outstanding video, in the EMI/EMC business, "it depends " is perfectly acceptable answer, same with tubes. Anything that has some form of non-linear function, it depends is appropriate. Thank you for your patience.

Great cautionary. Comes down to,"Careful what you ask for". Good lesson.

Really appreciate this one. Knew most of it; still got some good stuff. I built a a SE 6550 guitar amp. Have given some thought to a KT120 version. Since I like to go oversize on the output trans, that's some big iron.

Thank you for bringing up this trendy subject ....I had to repair several often expensive amplifiers after audiophiles swapped their tubes for these new generation power tubes ! There's another aspect to it and that's the necessary voltage swing to the grid off the output tubes which was optimized and calculated for the original design.... Even in recent ( mostly "sino made " but not always ) tube amplifiers , the transformers are sometimes underrated or limited in current handling to begin with ( it's about economics so they don't care about a lifespan od several decennies) ...so are the PS capacitors voltage ratings......I've seen 45O volts rated capacitors on 475 V HT supply ...worse if measured on the input stages just after start up, before the tubes warm up and draw their nominal current , cooking time for these capacitors ( example : 350 and 250 volts on a 500 V HT rail before current on the tubes will bring them in range of their working voltages ).... Also as the tubes age....and their current drops ....The HT voltages are climbing and come closer or can e trespass the voltage rating of these capacitors....so change your tubes before bad things happen to your amplifier....or replace such underrated components by better ones..... Even factory build , the filaments voltage are not always right !!!!....Wrong filament voltages have and will create problems , perform badly ore compromise the useful lifespan of the tubes. If all goes well they will hold the time of the warranty .............

GREAT explanation/discussion! Very clear!

Excellent summary. Thanks for making this video.

Great video, Mark! It's kind of like taking a Ford Fairmont with a four-banger, swapping in the optional V8, and fitting the 1bbl carburetor from the 4 cylinder to it. Sure, it'll start, idle, even run. But it will never be twice the engine. Sorry for the bad analogy.

Thanks for going to the trouble. Very educational and might save someone from frying their amp.

Very good presentation, however, WRT to GEC tubes I've used, the KT88s and 6550s are the other way around - the KT88 draws more Plate Current and has has higher Plate Dissipation than the 6550!

Holy shit, that KT170 makes me want to actually build a non guitar amp. Rob’s on the TDPRI “Shock Brothers” forum too, and he’s very active. There’s a lot of guys there that have been designing, building and using all kinds of tube related equipment for decades. Wealth of knowledge with probably an eon of experience between them. I’m not so big on audio reproduction, like home amplifiers, but I can tell you how to build a Fender 5F6A Bassman from memory.

This was an eye opener! How about substituting the EL-34s in a push pull design with KT-90s? Any suggestions? Are they too far apart?

Great video as usual. I think you got the labels for the KT88 and 6550 columns switched.

I have an amplifier made for EL34 with auto bias. It's a Swedish stage called Edison 30. I have changed the 4 tubes for Gold Lion KT66. The sound is better, the base is tighter, better control overall, but not better "push" overall, was hoping for more power. What tubes will you recommend instead of EL34? KT 88, 120, do they increase power? I understand it comes to what amplifier it is too

What if you go down, like putting NOS 5881s in a modern 6L6GC amp? It would run them at a higher operating point, correct? I’ve done it, and I like the sound. It breaks up at a much lower volume.

How does running them in triode mode effect the overall picture? I believe they'd draw less current that way so may be less taxing on the amp.? I'm guessing though I'm a noob😁! I've gotta kt88 set amp and I'm still trying to figure out if it's ok to swop the shuguang 88s for some old Russian 6650s I've had in the drawer for years?🤔

Fantastic explanations. Simply current can only be drawn so without the load no current. Then you need all the support devices to handle that load.

I've got a Mesa/Boogie Mark IV. Mesa amps have a set bias on them. They have this thing they call "Simulclass" amplification. Ya are able to run a quad of 6L6s, 6L6s and EL34s, or a quad of 6V6s...now my question is: since they are biased so cold is it possible for me to run a set of KT-88s on the outside sockets?...without harming anything in the amp? I know it will run them cuz I've tested this theory out but I didnt run them for long. I like how it sounds, but I don't want to burn my amp up. I'd like to run a set of KT-88s on the outside sockets and KT-66s on the inside sockets.

Great video. Always heard about tube rolling. Now I know what is really needed to do it right.

Analogy: Can I put bigger tires on my car? Will it go faster?

Great video. Thanks!

Excellent information on power supply/OT current handling. Thanks Mark!

I want to use EL34's in your circuit. Do I need to change any resistor values, or can I just plug it in?

I have been waiting on an Etracr till I knew more about load lines, thanks. One question, If you have a 50 ma choke. Now you need one rated for 100 ma. Can you get a 2nd 50 ma choke and run it in parallel ?

100% agree on this. Optimum performance mya not be achieved. Between EL34 up to KT120 its 56mA up to 135mA for 90% dissipation at fixed 400vdc B+. For 80% its between 50-120mA plate current.

Excellent explanation, thank you. In addition you may have to change the turns ratio of the output transformer primary too. One thing I don't understand is why anyone would use Ben Franklin's mistake called "conventional flow since, in a tube, electron flow is what happens.

A while ago I swapped the 6550 valves for KT120s in my Croft 4S amplifier. In my ignorance I thought I would get more power. I was nervous to switch on half expecting bangs and flames. The Croft has external bias adjustment and measuring points. I plugged in my meter. I used to run the 6550s at 80ma and so I expected the reading for the KT120s to be similar. To my surprise I watched the reading starting to sail past 95ma and so I quickly turned the bias to minimum. Another surprise was the lowest reading I could get was 93ma whereas I could get a lot lower with the 6550s? The amplifier did seem to get very hot but nothing untoward has ever happened. I hoped my transformers wouldn’t go into melt down but I assumed if the 6550s could run at 90ma it should be safe 🙏. A little knowledge is a dangerous thing 😁. It made me wonder if people without bias capability may have their amps running into excess current problems? As for the sound I seem to recall it being a tad leaner but with added clarity. The bass seemed to be lighter, but after adjusting my subwoofer I think it was lower and tighter 👍. The sound overall did seem a lot better although I suspect my ears readjusted to it a bit. I think for me it was well worth doing ending up with a very nice sounding system that could blow you away with adequate volume 👍.

Hi, Mark! Great info. I’d like to ask you about the 25 watt (output?) rating of the OPT versus the 40-something rating of the kt88. Can you explain the relatability, plz? Do you have any thoughts or comments? Thx again for the great content.

I'm aware of the KT88/6550 series of output tubes, as did my H-K Citation II almost 50 years ago. The KT120/150; never knew they were ever a thing

Beautiful.

Genius article

Excellent explanation. I never really got the attraction of "tube rolling". A design is tweaked to operate with specified components. I guess sometimes you can luck out and something else might work better, but have found, especially with things like cars, usually not...

What about going the other way and “down sizing” to KT88 (because of, say, availability, price) in a design made for KT120/150? Great videos, thanks.

Great video. It's not that simple! But I think we also need to emphasize another point here. How much more power are you going to get for your money? Most audiophiles have read the fact that a 100 watt amp is not ten times louder than a 10 watt amp. Realistically it might be twice as loud. Our hearing is not linear. So say you bump your 6550/KT88 from 60 watts a channel to 90 watts a channel, and that's after all the expensive mandatory component upgrades noted in this video. Listening wise what did you gain? Not much at all. And you have potentially compromised a well designed and balanced circuit increasing distortion and other sonic ills. The difference in power here is not typically significant in a home HiFi listening environment. Guitar amps, perhaps there may be some advantages.

Got a runt 50 watt, it has EL34.... So what kt types can I use 66,77,88,?

Swap a kt150 for a kt170? It would be possible ?

And just because an amplifier is powerful and new doesn't mean you should swap tubes either. For example the Rogue Audio Cronus/Atlas and the Audio Research Reference 75 were all designed for the KT120 tube. While Audio Research later updated the Ref75SE to use the KT150, they endorsed the KT150 for earlier models too. However Rogue Audio does not endorse using KT150s in their KT120 designed amps. This does not make the Rogue products inferior, it just shows that they made different design choices.

Mark, please, please take a 1 hours time and make a video of how to read and use those charts. How we choose the points, what is the best approach and everything. This is most essention about understanding vacuum tubes and parameters

Rob Robinette’s web site appears to be down...

Why does the list on the schematic include KT120?

Just found this. This is awesome 👍😎. Love the way you describe things ™️®️Get out your wallet. Merry Christmas 🎄🎄🎄🎄🎄

Please do iKT66 vs KT 88

I used Kt-170 for my Sonic Frontiers sfs 80 without any problem, the sound is much better than the KT99 or KT88, but I need to add one more filament transformer separately dedicated for all KT170s.

First comment that I would like to make before I even watch this video I have a pretty good idea what it’s going to be about but from my own experience from many many years working on tube amplifiers is trying to create more wattage with a bigger tube is really not going to get you very much because the amplifier itself is limited to the voltage and amperage that it is supplying to the tubes so you’re only going to get out of the amplifier what it is designed to do unless of course you Beef up the transformer and output supply transformers and a few other components that will allow the amp to push more current.

I holding a quad set of Russian made 6550 winged Cs.... I can confirm that they enter change well in my 1976 Musicmann HD130 it's an amp designed for EL34 x4. (Solid state preamps and 1 12at7 for a phase inverter l..... The y had at east 100 hours run over 2yrs maybe more.... i just pulled them for check up... they register almost exactly as the did when they were new out of box.... The difference in headroom volume and just pure ghetto thickness is worth all risk I know I'm taking. But my ears don't lie....

Not to mention that most hookup wire insulation is 600V to earth. Increase B+ and you have a rewire with 1000V+ insulation. For PCB components then you're looking at creep etc so the track spacing and materials have to be designed for it.

I run kt120s in my Doge 10 mk2 B tube valve integrated a 59 pound beast actually the desighner reccomends to atleast try kt120s normally i run kt88s I have pics up on me community page KT120s have hell of dynamics and sizzle on top !

KT88's are killer in an Ampeg SVT

a KT120 draws 120ma in class-a at 474V B+ ????? why was i figuring i needed almost twice that per tube at 455V B+ ??? help me to understand blueglow guy. EDIT: oh that was including the rest of the circuit too ....but still, that's a cold bias then ?

6550 vs KT 88 please

When calculating the the transformer current, you need to multiply the DC current by about 1.414 to get the correct AC current in the winding for +B. Even your original KT88 design with 2 X 81 mA, needs 230 mA not 200mA.

I have a Fender 400PS which uses three pairs of 6550s. They tend to be very hard on 6550s. I'm thinking that if I put in KT120s, they should run well and last a very long time and have no trouble delivering the full rated power of the 6550.

If you want high wattage,say above 80,it's better to go all transistor,imo.I had Quad 11 80 monos,and 99 monos going through Quad 63s and PMC OB1s,the transistors better in every area,and,no bottles to change.

Problem is in audio most things are based on myth more then engineering. I’m sure there are many amps damaged every year when someone sees something on the internet and just does it

What a about to use the KT170 ? I need to test some KT150 some curcuit for do it?

all comes down to anode current and plate dissipation and heater current

Th is so fun as a biologist , who can solder will but has done many experiments , wow

Terrific presentation, BUT: you didn't touch on the topic that brought me here. Like Bernard Nizynski below touches on.... KT88 and 6550 interoperability. Everyone says it fine, yet your charts suggest otherwise. To top it off, the numbers for these two look wrong in your most key chart. Would love to see you finish this by telling us your opinion.

I find it annoying that some guitar amp manufacturers are advertising all the different power tubes their amps can support........ Why not just build it ideally for one tube model.. and get the best out of that?

Kt77 is a good valve but people don't mention them match. Better solution is to get rid of the transformer worst thing for a valve amp put a switch mode power supply 120Khz much better results

15:35 conclusions .

It is always best to call the manufacturer of the amp.If you want a tube amp that uses kt120 or kt150 ,Buy one that has the transformers designed for those specific tubes.

Just rebuild the whole effing amp!

ehh the 6550 and KT88 are SWITCHED. PLEASE REUPLOAD THE VIDEO OR SOMETHING. It can lead to damaging the amp. Like putting 660V on 6550. Teaching about differences - making big mistake right at the beginnig.

So the answer is "NO"

the lecturer's diction is unclear. Please write what he says. This is a kind request from non-English people.