KT120 Amp Mk. 3

Discussion in 'Tube Audio' started by kward, Jul 1, 2018.

  1. kward

    kward AK Subscriber Subscriber

    I've been mulling over a few performance enhancements for my KT120 Mk. 2 amp. The major addition I wanted to add was an Enhanced Fixed Bias (EFB) output stage regulator. While I was in there, might as well throw in a few power supply upgrades I've been contemplating also.

    The basic design of all the iterations of my "KT120 amp" is:
    1. Two channels on one chassis with one power supply for both L and R channels.
    2. Mullard-like topology: triode voltage amp, 6SN7 cathode coupled inverter, UL connected output stage.
    3. KT120 power tubes.
    4. Triode Electronics clones of the Dynaco A431 output transformers (these have 33% UL taps)
    5. UL operation.
    6. B+ voltage of 475V supplied to center taps of output transformers.
    7. Fixed bias operation.
    The Mk. 3 is identical to the Mk. 2, except for the following enhancements:
    • The Mk. 3 utilizes an Enhanced Fixed Bias (or EFB) output stage regulator whereas the Mk. 2 does not.
    • The Mk. 3 utilizes a snubber on the high voltage secondary winding for some power supply noise suppression whereas the Mk. 2 does not.
    • The Mk. 3 swaps the positions of first and second banks of capacitors in the high voltage supply before and after the smoothing choke to reduce transient ringing within the L/C interaction of these components. In the Mk. 2, the first filter cap was an equivalent 275 uF and the second filter cap (after the choke) was an equivalent 110 uF. In the Mk. 3, these capacitor banks are reversed.
    The biasing approach of my original KT120 amp, which has carried forward through all iterations, is a fixed bias output stage circuit with individual bias adjust pots for each output tube. In these individual bias adjust circuits, there is no DC "balance adjust" pot per se. Rather, each pot acts as both the mechanism to adjust bias and to achieve DC balance. It has worked well if you know the bias target you want--just set all four tubes to the same bias target and all four tubes are both DC biased and simultaneously DC balanced. Simple--until you want to change bias without upsetting balance, which is exactly what I want to do with the EFB regulator so that I can find the best bias point that minimizes distortion.

    Sliding bias up or down without changing DC balance is doable with individual bias adjust pots, but each change in bias requires resetting DC balance--a tedious and cumbersome adjustment. So with the addition of EFB, I decided to add a bias adjust pot on the EFB board itself, where one single pot is used to set bias for all four output tubes. Then, each individual adjust pot that previously was used for both DC bias and balance is now only needed for DC balance. This is working well in that I can set DC balance once via individual adjustment of each of the four pots, and then slide bias up or down by adjusting the bias pot on the EFB regulator board.

    The test procedure then is basically the following:
    1. Set bias to 60 mA (that's what I used on the Mk. 2), set power output to 50 watts both channels driven, then measure distortion.
    2. Find the lowest distortion bias setting at 50 watts output, both channels driven.
    3. For giggles, substitute in new production Tung Sol 6550 tubes (because I have a quad laying around), and find its lowest distortion bias setting at 50 watts output, both channels driven.
    Here are the results:

    As stated in the Mk 2 thread, the Mk. 2 amp (with no EFB), measured with the following distortion numbers at 50 watts output, both channels driven, and at 60 mA quiescent current per tube:
    • 20 Hz: 1.35%
    • 1 KHz: 0.42%
    • 20 KHz: 2.0%
    The KT120 Mk. 3 measures with the following distortion numbers at 50 watts output, both channels driven and at 60 mA quiescent current per tube:
    • 20 Hz: 0.95%
    • 1 KHz: 0.25%
    • 20 KHz: 1.25%
    Comparing these two sets of distortion measurements clearly show that EFB is doing its handy work. Percent THD improvement between Mk. 2 and Mk. 3 under identical bias and output conditions is the following:
    • 20 Hz: 30% decrease
    • 1 KHz: 40% decrease
    • 20 KHz: 37% decrease
    But how much lower THD can the Mk. 3 produce if adjusted optimally? Lowest distortion readings were obtained at 100 mA quiescent current (per tube), at 50 watts output, both channels driven. Under these bias conditions, quiescent plate voltage clocked in at 455V, and this represents idling each KT120 tube at approximately 76% of its max plate dissipation. Here are the THD readings:
    • 20 Hz: 0.8%
    • 1 KHz: 0.15%
    • 20 KHz: 0.71%
    This represents really great THD performance. In fact, several years ago before I sold my Conrad Johnson Premier 11 amp, I measured its distortion at max power output. It was roughly similar to the Mk. 3 at 1 KHz, but my CJ amp could only deliver less than 1% THD up to 15 KHz.

    But, I don't like idling the KT120's at 100 mA each. That's a little too hot for my tastes. So I think a reasonable compromise here is to find the idle current that will deliver < 1% distortion at full power output from 20 Hz to 20 KHz. That idle current was determined to be 85 mA per tube. Plate voltage was measured at 465V, which idles each tube at 66% of max plate dissipation. Distortion at this bias setting is:
    • 20 Hz: 0.82%
    • 1 KHz: 0.17%
    • 20 KHz: 0.95%
    Which is still very respectable for a Push Pull amp.

    Now, since I have a quad of new production Tung Sol 6550's available, I thought I would measure distortion with them in the amp. Minimum distortion reading was found to be at 77 mA and 475V plate, at 50 watts output, both channels driven. Quiescent dissipation is 87% of max. Distortion readings are:
    • 20 Hz: 0.97%
    • 1 KHz: 0.165%
    • 20 KHz: 1.15%
    Still really quite excellent performance but again, this is a little too hot for me so I'd probably back that off to 60 - 65 mA at the expense of a little higher distortion.

    My gut says that if I were to convert the output stage to pentode mode (instead of UL) I would be able to achieve the same distortion numbers at lower static dissipation levels. But there is a tradeoff...usually more feedback is required with pentode output stages to obtain the same damping factor as with less feedback but in UL mode. And more feedback represents its own set of issues that need to be dealt with.

    So overall, the numerous iterations of this amp I've built over the last 6 years have lead me to what I believe is about the best this topology and configuration has to offer. For sure this is the best overall performing amp I am personally able to create. I keep saying that as I build each amp, as I learn a little something new with each build. But I think also because of the off-the-shelf parts used, this near world class performance within the stereo chassis configuration is within reach of the normal DIYer.

    My thanks again to Dave Gillespie for inventing the EFB regulator and making it public!

    I will leave you with the Mk. 3 schematic as well a few pics of the amp and the EFB board. This is a really pretty amp with a polished aluminum chassis that is clear anodized and with wood accents. The unique thing about this chassis is the transition from front face plate to top plate is a clean 90 degree angle made from one solid piece of aluminum. It makes for a very clean look.


    EFB and negative bias board:

    Schematic attached as PDF document.

    Attached Files:

    Last edited: Jul 1, 2018


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  2. primosounds

    primosounds Powered with pure tube sounds. Subscriber

    Terra, 21st century CE
    Very nice Kev. I know that we all want to hear what you new amp sounds like. But, since that is not possible, can you give us your impressions and if possible reference the sound to the music you are playing. And what are the differences between the different versions of the amp.
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    New Jersey
    Are there actually 3(?) amps or are you reworking the same hardware?
  4. IPADave

    IPADave Which one's Pink? Subscriber

    Wow! I love the look of your amp. The soft shine of the chassis, the black choke, caps and transformers, and wood side panels make for an elegant, but purposeful look.
    Well done!

  5. kward

    kward AK Subscriber Subscriber

    Jury still out on how it sounds compared to the Mk 2. I'll be going from aural memory in describing the sound differences (which is typically poor in humans), because there is only one current version of the amp that is built (the Mk. 3). It has had the output transformers and choke repurposed from the Mk. 1 to the Mk. 2, and now to the Mk. 3. I would have kept the same red chassis as used in the Mk. 2, but it started to fade and started to look really bad.

    Also I don't own this amp. I am building it for AK member yurka77. If I ever get a break, I will build one for myself.
    • The Mk. 1 used a 12AX7 voltage amp, tube regulator for frontend tubes, a toroid power transformer, and a very large chassis. It didn't sound that great (it was my first attempt) but provided the basis for the design.
    • The Mk. 2 got rid of the tube regulator, moved to a 12AU7 frontend tube, replace the toroid with an EI type power transformer, and reworked the HF and LF tuning. It delivered a very nice full sound, slightly laid back.
    • The Mk. 3 added the EFB output stage regulator and added some power supply improvements.
    mjw21a likes this.
  6. nerdorama

    nerdorama AK member Subscriber

    Seattle area
    That chassis seems familiar. I've seen some on the big auction that look similar. What size is it? Just curious as I'm trying to size a chassis for a similar project. Any chance of a picture underneath?

    Great looking project. Thanks for sharing it in such detail.


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  7. thorpej

    thorpej AK Subscriber Subscriber

    San Francisco, CA
    Excellent work, Kevin.
  8. crispycircuit

    crispycircuit AK Subscriber Subscriber

    near Buffalo NY
    Ya, the chassis looks to be the hard part to find. Good Lord , what a fine unit. REALLY NICE!
  9. kward

    kward AK Subscriber Subscriber

    Thank you for the replies, gentlemen.

    John, the chassis is 17" wide, 15" deep, and 3" high. The chassis was obtained from ebay from a retired machinist in Texas. I'm sure it's the same ones you saw. Gordon W used the same chassis manufacturer on his latest build also. One thing about this chassis is it's actually two chassis bolted together. I recall that the largest chassis he was able to make in a single chassis style was 12" or 13" deep. I needed at least 14" deep so we worked out a way to bolt two chassis together. You can see the seam right in front of the transformers on the previous pics posted. I had him drill holes in the internal chassis walls of the two connecting pieces so I could easily get wires between chassis sections.

    I will post a pic of the internal wiring later this week after I sure up all the solder joints.

    Primo, I've logged about 4 hours now listening to this amp. I'm starting to have some impressions of how it sounds. First of all I'm using premium frontend tubes in the power amp: 1959 brown base Sylvania 6SN7s, and an RCA clear top 12AU7. The right 12AU7 makes ALL the difference in this amp. I am listening through my vinyl frontend which consists of a VPI Aries turntable, JMW 10 tone arm, Grado Reference Sonata MM cart, and with a Jolida JD9 Mk. II phono preamp (that has been upgraded with Burr Brown op amps, early 60's Sylvania 12AX7s, and Auracap XO output coupling caps), as well as an Audio Electronics AE-3 line preamp with early 60's green label Sylvania 6SN7's (that has also been upgraded to utilize direct coupling between stages). What's spinning on my platter right now is Midnight Oil Diesel and Dust, Mannheim Steamroller Fresh Aire II, Talking Heads Stop Making Sense, and Jefferson Starship Freedom at Point Zero. The KT120 tubes are very coherent sounding top to bottom. The amp sounds very similar to the Mk. 2 version except on very loud passages that get it up there to max power, the bass is less woolly, more together. That's the biggest difference I've been able to notice so far. Talking Heads "Once in a lifetime" and Midnight Oil "The dead heat" have bass tracks that will thump your chest through this amp. Oh by the way, I should mention my speakers are Cambridge Audio S30's. These are smallish bookshelf speakers but they REALLY come alive with a few watts behind them. Even though the woofer is 4.5" I really don't miss a subwoofer with this amp and with these speakers. There are some shortcomings of this amp certainly. Sound stage width is less wide than say my former Conrad Johnson amp. The CJ amps of that era (90's) were colored in my opinion, whereas the KT120 mk 3 sounds more like a decent transistor amp on the low end, but with that glorious midrange of tubes with harmonic texture that transistor amps can only dream of (all my opinion of course).
    mjw21a likes this.
  10. Selmerdave

    Selmerdave Well-Known Member

    Kevin can I ask about the choice of 12au7? It seems to get a pretty bad rap for distortion and sound compared to 6dj8, 12at7 and 12ay7, but I'm not a subscriber and my preamp sounds pretty good with some clear tops. But just curious if you have thoughts on the 12au7 reputation.
  11. kward

    kward AK Subscriber Subscriber

    I picked the 12AU7 because they are plentiful and decent old stock or NOS can be had for not much cash outlay relatively speaking. If the 12AU7 is used in the right application I think it can be a wonderful performing and sounding tube. The clear tops especially sound wonderful to me.

    When using the 12AU7 in the frontend of a power amp that employs feedback that includes the AU7 stage in the feedback loop, I'm not that worried about the extra distortion it produces over say a 6FQ7, because the feedback loop negates a bunch of that. In this amp, the output voltage swing of the AU7 stage isn't that huge--it's about 7V peak to drive the output to full power--and most listening is done down under 10 watts, where the swing is much less than that.

    It wouldn't be my first choice to use a 12AU7 in the inverter or driver position of a power amp because of the larger demands placed on the tube in that position. But for the first amplifying element in a power amp with global feedback around the 12AU7 stage, I think is a very good use for this tube. The distortion numbers I quoted above support this. In a power amp, distortion is largely due to output stage non linearities, not the 12AU7 frontend.

    Just my thoughts and opinions...
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  12. mjw21a

    mjw21a Super Member

    Brisbane, Australia
    Wow. Just stunning. I love your work :)
  13. kward

    kward AK Subscriber Subscriber

    I finally forced myself to rip the amp out of my system to shore up the soldering and take some shots of the underside. And here it is:


    As I mentioned previously, this is two separate chassis that have been bolted together. I requested the fabricator drill some holes between chassis so I could run wires back and forth between the the two sections.


    Also mentioned previously, the full chassis size (both pieces taken together including wood accents) is 17" x 15" x 3".

    Now that I've had another week to listen to this amp, I can say it is sounding very very good. I wasn't satisfied with the 12AU7-WA I had in the amp so I rolled in a Sylvania 12BH7 and a GE 12BH7A. The winner by a tube length was the GE 12BH7A. Wow what a difference that 12BH7 made in the high end and mid range!

    The same impression of sonics I reported last week after a few hours of listening still hold: the amp has amazing bottom end growl and chest thump-ability. The mid range is neutral, more so than my previous DIY effort I dubbed the "modded out Moto" (a motorola console redesign/refurb), and the highs are smooth and slightly rolled off. The image thrown by this amp is slightly behind the plane of the speakers. I had a friend come over on 4th of July to listen to it. He said he only had about an hour...but guess what...he ended up staying 7 hours jamming to his own crate of vinyl he brought with him! Pink Floyd's The Final Cut was just jaw dropping good on this amp!

    He also brought with him his Anthem 7 channel transistor amp with room correction to compare to the tube amp. After a few hours we rotated in the Anthem. It took him somewhere between 20 and 30 seconds and he was done with it, and we went back to the tube amp. His comment was, "the speakers disappeared as soon as I put the tube amp back in."

    Well, it's time now to send this amp off to its new owner. I am definitely going to build a clone of this amp for myself. I don't think I can go back to my "modded out moto" now..this amp is sooooo much better than that.
    Last edited: Jul 13, 2018
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  14. Brice

    Brice AK Subscriber Subscriber

    Hoboken NJ
    Beautiful Kevin.

    I see you have coupling caps in parallel, so could you fit Teflon caps under the hood ?
    I am telling you, I just tried 0.22uF FT-3 on 4 KT120 mono blocs and the impact was unbelievable.
    Maybe when you make your clone you can provision enough space for it?

    I also made THD measurement and I get similar results. I can get up to 70W before I clip. PS limitation.
    And as you, the hotter you bias the KT120 the lower the distorsion figure are obtained.
    I was getting the best results around 90 mA, but above this the PS choke was complaining (choke input), and indeed the tubes generated quite a bit of heat or course. I now run them around 60+ mA @ 485V. Still class AB, but sounds damn good. I can run them 8 hours straight and the PT gets barely warm :)
    I really like the KT120.

    Once again, fantastic build.
    Thank you for sharing.
  15. kward

    kward AK Subscriber Subscriber

    Those Russian caps are huge! It's good to see other measurements that roughly match my own with this tube. I thought the bias point for lowest distortion was awfully high, but, measurements don't lie, so "it is what it is," so to speak.

    Thanks for your comments, Brice.
  16. Brice

    Brice AK Subscriber Subscriber

    Hoboken NJ
    You are welcome.

    Big? You said big? Ha ha.
    Really worth every square inch in your amp, really.


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