ARC VT100 with KT-120 tubes ?

I have seen posts about the KT-120 as a replacement for the 6550. Have any VT100 (Mk II) owners re-tubed their amps with the KT-120? What differences did you notice ? What bias setting do you use ? Is it an "improvement" ?
Well the bias setting is the same as with the 6550. I can tell you it's at the very least an improvement in power output. They sound very good. No down side. I've only heard them in a pair of Ref210's.
Power of the KT-120's abilities relative to the 6550's are theoretical differences. The KT-120 is capable of handling more power but only if the power supply voltages are increased. When plugged in as a 6550 replacement and no other changes are made, the amp will put out exactly the same power. No tube will allow ohm's law to be revoked. I'm afraid that the issue of replacement is misunderstood.

Thus, power should not be the reason to replace the 6550's. There may be other reasons such as added lifetime because the KT-120 is working under it's maximum power output that is an advantage. But, not power.

I do not know what Hifigeek1 was hearing. Imagination?

I wouldn't expect the tube change to make the amp put out more power. My interest is in longer tube life. And, I'm interested in knowing if there are sonic differences within the power limitations of the amplifier. In other words, will they last longer, and will they sound different (better) ?
thanks for your responses
Karma16. The KT-120 draws a bit more heater current per tube. There are possibly other variables with the KT-120 that I am unaware of... The Ref210's are mono blocks and are rated at 210w across 8 ohms with 6550's @ 120Vac line and that's what I measured. With KT-120's approx. 250w across 8 ohms with the same line voltage. I verified this on my service bench using a Dale 250W 8 ohm non-inductive wirewound resistor. Bias was the same as was plate and screen voltages. You can contact Audio Research and they will give you the before and after wattage outputs for 6550 vs. KT-120, just specify the amp in question. I fix their products in warranty.
HI geek,
OK, but can you explain how this increased power output happens? To get more power you need more power supply voltage and current no matter which tube is being discussed. We can't argue about this. First, did you use a good distortion analyzer to monitor the amps saturation state? Or, how did you monitor distortion? I use a Tektronics analyzer which goes down to .0001% THD and a Tektronix audio frequency sine wave generator which outputs at .00001 THD. Those are real specs. Great test equipment and ideal for this type of testing. They got a lot of use in my repair shop.

Typically, any power amp will use the power available from the supply to establish its limits. But, not always. The exception is if the power supply has extra head room and other circuit factors could then could determine when the amp goes into saturation where distortion increases rapidly. This is not the case with my D250. The power supply is the limiting factor as it is with most power amps including those made by ARC.

I did check with ARC about the KT-120's. They made the point strongy that the KT-120's will not, for example, increase the output power of my D250 Mk II unlesss the power supply voltages and current capacity are increased. I already knew this so it was confirmation. Of course, changing the power supply voltages and current is nearly impossible without a complete redesign. So, for most amps (all that I know of) the advantage of substituting the KT-120's for 6550's is possibly (yet to be proven) increased tube life, not increased power.

BTW, ARC will continue to use 6550's for power supply regulators. I'm sure you know this.

So, please explain. This is important because the 6550 owners of the world are going nuts over this new tube for, I think, the wrong reasons. There are no free Watt's in my world.

Thanks, Sparky
That is correct. You can not use a KT-120 for the regulator tube in ARC amplifiers. ARC is still in the process of determining power output of their amps with the 6550 vs. the KT-120. In some ARC products the KT-120 will not even fit. The KT-120 represents the latest design from the Russians. The tube is capable of running up to a maximum plate voltage of at least 700Vdc. The hope is that tube life will be extended.
HI hifigeek,
Well, you never addressed my questions. I don't know what to think. So, are we to assume that your tests were in some way flawed? That the KT120 was not resulting in more power but you won't admit it?

Since you made the claim of increased power but then won't back it up by defining your test methods, we should be very suspicious. Your good name alone won't validate the tests resultes for me.

If it were me, I would have tested the original 6550's to determine their freshness, run power tests with the 6550's using ARC's THD spec as the control, then, test the amp with the KT120's using the same ARC THD. If you wanted to be really precise the AC line voltage should be monitored. This is the only way to determine relative power outputs.

I think you know all these things. But knowing and doing are two different things. The folks who read this thread may not understand how power tests are run and, thus, have no way to evaluate your results.

Until you prove me wrong, I stand by Ohm's Law and state that simply installing KT120's will not increase an amps power output.

Karma16..Have a nice day.
I am considering purchasing a VT100MkII. Any preferences on/compairsons between the VT100MkII and the VT100NkIII?
Karma16: please back off Hifigeek. We should all be respectful to each other. Moreover, he has always provided good advice to those of us who are less informed about electronic theory, particularly ARC gear. Oh, my rig is all ARC: PH-7, Ref-3, VS-115 and CD-7. I can personally attest to the very helpfulp advice and information that Hifigeek has always provided.

BTW, the ARCDB website has some info on increased power output for various ARC amps that use the KT-120. See the News tab. I don't know, or frankly care for that matter, where ARCDB got their information.

If you do a broad "KT-120" search on the A'gon discussion forums, I think a number of ARC owners have posted comments about the superior sonic attributes of the KT-120 over the 6550. Is it true, I guess you'll have to spring for the new tube and try it yourself.

Will tube life be extended? We'll just have to wait and see. However, Hifigeek's comment about the KT-120's ability to handle plate current of 700V dc (versus actual bias being set at 65mV dc) is impressive. As a lay person, it would seem (?) that the new tube has a robust build quality that may possibly extend tube life over the 2000 maximum hours suggested for the 6550.

Thanks to everyone for their helpful advice.
Some slight technical corrections:

Bifwynne, you wrote:
"the KT-120's ability to handle plate current of 700V dc (versus actual bias being set at 65mV dc) is impressive."

700VDC is plate voltage (Volts DC), not current.

Karma16, you wrote:
"Until you prove me wrong, I stand by Ohm's Law and state that simply installing KT120's will not increase an amps power output."

Ohm's law does not apply to tubes, because tubes have plate current/voltage CURVES, not straight lines.
Ohm's law is a first order equation, therefore it represents a linear relationship between voltage, current and resistance. its graphical representation is always a straight line.

Also, a tube with a different set of voltage / current curves may interact differently with the same output transformer at the same bias point, because the OPT does not follow Ohm's law either. An OPT's main electrical spec is impedance, a non-linear measurement, it varies with current AND frequency.

So I guess we are all laymen and should wait for the manufacturer's findings.

In the meanwhile, do not pop KT-120s into your 6550 tube amps, KT-120s draw more heater current and may burn the power transformer beyond repair.
Enjoy the music!
Thank you Casouza for your helpful explanations. Just to be clear, I appreciate your clarification that current and voltage are different concepts.

However, I quote from Hifigeek above that "(t)he tube is capable of running up to a maximum plate (VOLTAGE) at least 700Vdc. The hope is that tube life will be extended." Casouza, I should have said that "the KT-120's ability to handle plate (VOLTAGE) of 700V dc (versus actual bias being set at 65mV dc) is impressive." Quite honestly, not being an electronics techie, I have no idea whether VOLTAGE handling capability will or will not extend tube life. Hopefully, it will.

Finally, I think your advice that one should not replace 6550 tubes with KT-120s because of concerns about increased CURRENT draw on power transformers is correct. FWIW, I spoke to an ARC tech rep about the issue. I was told that the newer models can handle the KT-120 without the need for a mod. FWIW, the ARCDB web site states that the output tubes on the VS115 amp were changed over to the KT120 tube in Nov 2010. By contrast, I didn't see the same notation for the VS115 on the ARC web site. I don't know where ARCDB obtained that info, although I believe it to be correct. In short, I too would advise anyone thinking about changing out 6550s for KT120s to call ARC or their respective amp mfg'er before changing out the tubes.

Thanks again Casouza for the explanations.
Bifwynne, I will be getting in a VS-115 with KT-120's for service. I will let you know what the power output is once the unit has been gone through.
I talked to ARC today 3/9/11, sorry I forgot the name, but ARC is now strongly recomending that even current products like my Ref 210s be sent back for some power supply adjustments before going with the K120 tubes. He said they have learned more about the K120.

BTW I did not go into any detail since I was calling about another 6550 tube issue and I have 650 hours on my set so I will not be making the jump soon. My issue is a 6550 driver tube that will not hold bias.

I am looking forward to the K120 for hopefully both sonic improvements and reliability improvements over the SED 6550 - but its not like they dont sound good IMO.
Thanks Tdaudio for the power supply info. That's good to know.

Hifigeek, do you know anything about the power supply adjustments that Tdaudio mentioned in the post above? Also, just curious about the VS115 you have for repair. What's the problem? I'm interested because I own a VS115.
Both the Ref210 and Ref610 require changes to one of the regulated power supplies in those two amps. I will probably be able to update the Ref210 at my office, the Ref610 on the other hand requires a more extensive update and therefore Ref610's will need to go back to the factory.
Tdaudio and Hifigeek: spoke with Chris at ARC today. Was told that REF210 and REF610 will require mods, as mentioned by Hifigeek above. However, the VS115 and many others are ok as is. Until something comes out in writing, I would still call to double check for yourselves.

Chris also said that ARC is not in a position to say that the KT120 will have extended life over the 6550C because there is not enough history behind the new tube and a lot of variables are in play that can affect tube life. However, ARC does believe that the new KT120 sounds better than the 6550C.

In my case, as I get closer to the 2000 hour mark for my current set of 6550C tubes, I will probably consider dropping in the KT120s. I'm very curious to hear how they sound. FWIW.
Hi All. Did talk to Chris O. this week as well about dropping those KT120 into a pair of Classic 120. His response was that ARC have not "tested" those in a pair of 120's and will not until they get a pair in for service. He told me that electronically speaking there should be no issues using KT120 in the 120's except that there may not be enough range on the pot to adjust the bias up or down. A resistor may have to be changed. Physically speaking, they may not fit within the cage enclosure as they are taller. However, i operate my 120's with the cages off so that is not an issue. He said to me just drop them in and report back to us. Otherwise, it may be a while before they are tested in my 120's and the many, many other amps ARC has designed in the last 40 years. I am not quite willing to send my amps down to the Factory from Canada just to see if they are a fit or not. However, Chris appears to be very confident that they should work just fine in those 120's.
Long story short, i have to make a decision soon as i have 2000 hrs on my SED 6550C now.
I think they will work great and given the amp runs the outputs in triode it should sound fantastic!!!
Thanks for being a good sport!

More plate voltage is not very important in the context of most tube amps, because the plate voltage is fixed by design. However, the ability to take more plate current is an advantage, for the following reasons:

The Svetlana 6550C has the following limiting values (only ONE limit can be exceeded at a time, and only momentarily, for example, during power up):
-680V plate voltage
-plate dissipation: 35 Watts as triode, 42 Watts as tetrode

For best sound / least distortion, ARC tends to run their tubes at about 450 Volts B+ and 65 ma current, which is aproximately 30 Watts, pretty close to the tube's limiting value.

The KT120 has the following limiting values:
-650 plate voltage connected as a triode, 850V as a tetrode
-Plate dissipation: 60 Watts

Running a 60W tube at 30W dissipation is much easier on the tube that running a 35-42W tube at 30W dissipation.

So, instead of plate voltage, power dissipation is the main KT-120 advantage.
A 60W tube running at 30 W dissipation will be loafing along and will probably last much longer than any 6550 tube, provided that the power transformer can take the extra heater current and that ARC makes the (depending on amplifier model) necessary bias supply changes.

I agree with you, this tube has great potential but is not a simple drop-in replacement.
Enjoy the music!
Casouza there's more to it. The construction of the KT-120 is far more robust than the 6550. Taller plate with heat sink fin structure to allow for more efficient heat dissipation, as well as a thicker glass envelope. The KT-120 also has getter flashing on the top and on the side. I do agree with you, it is NOT a drop in replacement.
Thansk Hifigeei and Casouza for the tech information. Hifigeek, care to share what's wrong with the VS115 you're working on??
Lyric Hi Fi in Manhattan indicted recently that Audio Research had issued a bulleting listing the ARC products that are compatible with the Tung Sol KT120 tubes. I was happy to learn that the VT100 MKII was included. I ordered a matched octet and installed them about a week ago. After initial break in of about 20 hours I am starting to get the full measure of the performance enhancement of the VT100/KT120 combo.

Admittedly, replaced 4+ year old power tubes with new ones will lead to improvements across the board. However there are specific areas of improvement that are due to the KT120s.

From the outset it was clear that the dynamic range was enhanced. Not so much in terms of sheer loudness: The gradations from "p" to "pp" and "ppp" were now more clearly defined. While quiet passages seem even quieter.

There was a greater sense of control, flow and ease to the music

There was greater refinement and smoothness. Highs were more extended and clear

The sound stage was deeper and wider

Bass had more control and power.

After about 20-25 hours the sound became noticeably more powerful and robust

Last night I had a surreal experience while playing Bizet Carmen Opera (EMI) with Maria Callas. It was breathtaking. The power, majesty and sound staging were incredible. Maria and orchestra were in the room with me. The climaxes and "ffff" were without strain or stress. The VT100 MKII had never sounded this powerful and suave before.

There have been no down sides so far. Surprisingly, the amp seems to be running cooler than before (which is contrary to what I would have expected). Presumably, the amp was coasting along for the ride.

I would highly recommend these tubes to VT100 owners
It is possible that the KT-120's are running cooler because the plate of the tube is much larger. Therefore, there is a greater area for heat dissipation and less chance of a hot spot. There are also heat sinks on the plate structure as well, making cooling more efficient. The KT-120 represents the latest state of the art in power tube design.
I bought 6922 input tubes and 6550C output tubes on the internet, but they did not meet the standard and it was impossible for my tech to get the settings right and the bias up to the correct value. He recommended that I got the “authorized” tubes from ARC and I ordered a complete set of new tubes and got the KT120 instead of the 6550C from ARC (or actually from, the Nordic distributer).

This time the tech had no problem getting the setting right and the bias correct and I got the VT100mkII back in no time.

The change in sound was dramatic. Before the sound was easy, soft and pleasant, the amplifier seemed to work fine and I was happy. But now it is much more open, revealing and tight. I listen mainly to acoustic jazz and the double bass is much better defined, the sound from the plucking, the string and body is more coherent as from one instrument (as it is in reality). Drums and especially cymbals sound natural and again the attack and ringing of the cymbal is coherent. The recording of the piano is clearly revealed, but still sound neutral and musical, not analytic.

I find that one of the greatest strengths of ARC gear, is the “recreation” of the human voice. And the voice of Ella Fitzgerald or Diana Krall shines through the VT100mkII with the new KT120 output tube.

Before I could read a book or work on my laptop while listening to music, but now it is difficult to concentrate and, as I am writing this, I have to stop and listen all the time, even to music I have heard a hundred times.

I can second what gmorris writes and I can recommend use of the KT120 in the VT100. And I will absolutely recommend buying the “authorized” tubes from ARC, it can save you and/or your tech a lot of time and trouble.
Farobari I try and tell my ARC customers that all the time. ARC tubes are more expensive but well worth it, especially if you want to get your amp up and running quickly. I also recommend factory trained technicians do this because you are dealing with high voltages in the amp and one wrong move and the results are not pretty. With ARC tubes, I don't have to sit there and let the tube bias settle in for a week. The bias is usually stable after an hour since they had been burned in for 48 hrs. I can run it for an a day, set the bias, do a frequency response and a power output test and return it back to the customer in short order. I had to recently re-tube a C/J amp and purchased tubes from a reliable vendor. Two minutes after I installed the tubes, wouldn't you know one of the EL-34 started running away. And these were supposedly tested tubes that I paid extra for. The point is you really can't trust many tube vendors out there. Granted, it's possible the tube got damaged in shipment. I find that with ARC products, installing non-ARC tubes in their products is like putting low octane gas in a Ferrari.