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EL34 vs. KT120

Okay guitbanger,

Tonight I tube rolled SED Winged C EL34s vs. Tung Sol KT120s. I started by biasing the EL34s at 50 mA (100% of plate dissipation), and the KT120's at 50 mA (40% of plate dissipation) and then later at 80 mA and then 100 mA (70% of plate dissipation).

For my test track I used Janis Ian Breaking Silence, Analog Productions 200g LP, track 4 on side 1: "What about love." My front end is a VPI Aries with JMW 10 tonearm and Grado Reference Sonata Moving Magnet cartridge into a Jolida JD9 II phono preamp. All I've done to the JD9 is upgrade the output caps to Auracaps 1.0 uF. Everything else is stock. Out of the phono preamp the signal goes directly to my power amp. I have a 100K Alps Blue Velvet logarithmic pot for volume control right on the chassis of the power amp. My speakers are Vandersteen Quatros.

First I gotta say the Winged C's sound awfully good. I do think they sound better than the Mullard EL34 new issues. The Winged C's have a really nice midrange, are slightly more forward in the midrange than the KT120s and are just a bit warmer. Highs are very nice, and top to bottom, the sound is better than the KT120s at 50 mA bias.

The KT120s biased at 50 mA just do not sound as good! At 50 mA, the KT120s are brighter, harsher, cooler in the midrange, and bass is more sloppy. So overall at 50 mA, the clear winner are the Winged C EL34s.

But..the story's not over there. At 80 to 100 mA bias on the KT120s (I couldn't tell a difference), the KT120s just sing. They are more controlled top to bottom. They are just a bit cooler in the midrange still, and are not as forward as the EL34s. Bass is very tight (beating the Winged Cs hands down). I can hear just a bit more detail with the KT120s, and deeper into the recording.

The EL34s are not bad at all, and I could live with them if I had to, but the KT120's just edge them out by half of a stride when biased up higher at 80 mA or so. That's my analysis anyway.

Compared to the Winged C EL34's, are the KT120's good enough to drop everything and go out and purchase a quad? No, not if you already have a good pair. But if you're looking for a slight upgrade and have the juice to run them hotter, I think the KT120's outshine the Winged C EL34s, but just barely.

Your mileage will probably vary, or in other words, all in my opinion.
 
Thanks a bunch kward. Interesting to know (at least to me). I'm surprised that raising the current 10ma. on the EL34 resulted in 100% dissipation. I would have figured it would have been at about 85 to 90%. The extra feedback you predicted would have been right on as to what a MKIII used (20db). And (unless I'm reading it wrong) your observations as to the sound of the EL34's at 40ma. and the KT120's at 50ma. is about the same would you say? And the current bump up on both types yielded better sonics with the KT120's having a slight edge? Nice build on your amps btw (followed that thread and learned alot just by observing!) Like I stated earlier I have a pair of the potted A-430's that I'm going to build into something someday. Hopefully something different and a little better than my MKIII's and threads like your amp build are certainly inspirational and food for thought. Again, thanks for taking the time to do this.
Danny/guitbanger
 
guitbanger said:
And (unless I'm reading it wrong) your observations as to the sound of the EL34's at 40ma. and the KT120's at 50ma. is about the same would you say?
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Last night I started both tubes at 35 mA, then bumped up by 5 mA per trial until I got up to the limit of what the EL34 supports (50 mA). At every trial, the EL34 beat the KT120 in the sonics department. It wasn't until I bumped the KT120s up to 70 mA that they outperformed the EL34s, and the KT120's got even better at 80 mA, and that's about where they maxed out sonically. (Plate voltage at 80 mA was 450 volts, or 60% of max dissipation.)

guitbanger said:
Nice build on your amps btw (followed that thread and learned alot just by observing!) Like I stated earlier I have a pair of the potted A-430's that I'm going to build into something someday. Hopefully something different and a little better than my MKIII's and threads like your amp build are certainly inspirational and food for thought. Again, thanks for taking the time to do this.
Danny/guitbanger
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Thank you! These Dynaco output transformers are really excellent performers. I think you'll be very happy with them once you decide what to build around them.
 
Resurrecting a 9 yr old thread that is still relevant. Fast forward from 2013 to 2022.

Reading through this and @kward updates from years ago here, I too was a huge EL34 fan myself for many years (still am), recently jumped to KT150s for a while in 2021, nice, yet too pure some times, and now reverted back to KT120s wondering if I could recover some of that older vintage tone. CJ owners were still commenting how many preferred the KT120s over KT150s even today, so I picked up a quad of KT120s. Have been testing at 40/50/60ma with newer Tung Sol KT120s.

Seems to make a difference in the sound stage too, brining it a tad more forward with more midrange presence moving to 50ma and above. A little more recessed at 40ma with KT120s. Running them with big transformer mono amps. Starting to like the throwback KT120s more than I thought I would. KT150s are back in their boxes for a while.
 
I have not changed my overall opinion of the KT120 since that last post in 2013. Still a great sounding tube! Back then I don't think the big boys were using them routinely in their amp designs yet, or perhaps they were just beginning to notice this wonderful tube also as I did. Since 2013 I've also built an SE-UL amp using KT120s, and it too does not disappoint! However, I have discovered one Achilles Heel of the KT120 since those "early days" of experimentation with this tube. There are some conditions under which the Tung Sol KT120 (as well as the new production Tung Sol 6550) will exhibit parasitic oscillation. Whereas (for example) the Winged-C 6550 under those same test conditions does not. I discussed this in a thread a few years ago describing my SE-UL build.
 
kward, these are very interesting findings indeed. A thread I previously missed. Could you post the schematic of the amp you did these tests on?
 
I don't know if I have the specific schematic any longer, but it was the first instantiation of the "KT120" amp I built. It was a Mullard 5-20 like frontend with a 12AX7 gain stage direct coupled to a 6FQ7 differential pair inverter/driver, feeding a fixed bias output stage and using a Triode Electronics A431S Dyna clone output transformer. Stereo amp on one chassis with a common power supply. Power transformer was a 360-0-360 toroid. It also used DC filaments throughout (why I thought that was a good idea I don't know...) and a regulated power supply for the 12AX7 and 6FQ7 stages. Global feedback was 14 dB. I'll see if I can hunt down the original build thread. It's been a while (10 years??).
 
@kward gotta question for ya. Was that amp you designed and built in a regular push-pull -or- parallel single ended type configuration? If not, have you ever experimented with KT120s in a PSE/Triode type of configuration and how did it sound? I ran this by my own amp designer and he was not in favor of anything but pentode push-pull configuration, if that makes sense. Your thoughts?
 
I have never used the KT120 in a PSE configuration. I’ve only used this tube in PP UL and SE UL. I know there are some polarized opinions on UL vs Pentode. I have built one PP triode amp (pentode strapped as triode) but that was with sweep tubes so I can’t really compare it to the KT120.
 
Finding out KT120s (that I overlooked in the past the first try) DO settle in and become a bit more musical and a tad more laid back on the top end after you get past a few hundred hours. Not as forward midrangey as EL34s can be, yet maybe having a deeper soundstage and a bit more clarity in my rig. Not sure I could go back to ultra-clear and ultra linear KT150s again, or not for a while. Kinda liking what's happening with KT120s now, and much more so than the first time I tried them and prematurely pulled them. Giving them some time to settle in does seem to help. Piano seems to sound about how it should right now :)

This is probably old (been-there-done-that) news for those with CJ, ARC, Aric amps which came stock with KT120s. Still interested in hearing from others who are still enjoying their KT120s over EL34s.
 
opnly bafld said:
Don't leave out Rogue. :)
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Doing a quick re-check on current amp sellers, appears various are still using KT120s as standard or optional interestingly enough.
The tube has not been completely overtaken by KT150 or KT170 it seems. I do prefer the 120 over my 150s in some ways, and my amps were specifically designed to run KT150s natively.

Other +amp makers still utilizing KT120s today.

Conrad Johnson
Aric Audio
+ Rogue Audio
+ Audion
+ Audio Research
+ Icon Audio
+ Promethius Audio
+ VTA ST 120
+ Jadis JA 120
+ Cary SLI-80HS (option)
+ Carver Crimson, Raven
+ Quicksilver older V4, Mono 120
+ Quicksilver new KT Mono (option)
+ Line Magnetic 501
+ Mc Gary Audio
+ Octave RE 290
+ Raven Audio (option)

...feel free to add to this list anyone for those I missed....
 
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Learned a good lesson this year with the KT120s. Be patient, let them play and settle in for a while, up to ~200hrs. It makes a difference. They do change and sound nice once they've played for a while. Etch disappears, midrange comes forward, sound stage opens up. Nice.
 
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