29 June 2010

Tung-Sol KT120 Tubes - SE Impressions

Up to a few weeks ago I didn't even know the KT120 tube existed.  Thanks to Bruce, of Oddwatt Audio fame, I not only heard of the tube but now am using them in my simple two stage single-ended (SE), ultra-linear (UL) tube power amp.  The tube is the new Tung-Sol KT120

A very simple amp
The amp started life with a 6N1P driver tube and a KT88 in each channel. Not just any KT88 but in my case a JJ KT88 Blue Glass matched pair. This all exists on a bread board layout and I label the amp “Be Bamp”. The 6N1P driver tubes remains but the KT88 has given up its ceramic socket to a goliath of a power pentode.

The amplifier is based on Mikael Abdellah's single-ended KT88 tube amplifier circuit.  I have built a few of these amps and find them an excellent and inexpensive amp.

Simple Improvements
With some assistance of Bruce I increased the current flow through the KT88 from 63mA to 93mA. This allowed the KT88s to really deliver and lifted the amp from good to excellent. This modification does restrict tube rolling with the cathode current now being high and HT+ at 400V.  But nothing had prepared me for what was going to happen to the over-all sound and power delivery of this amp just by plugging in a couple of inexpensive (if not over-sized) tubes. How easy is it to remove two tubes and replace them with others and make no other changes? Can it be that easy? Yes it can because I did it.

The higher current through the KT88s had lifted the bass substantially. Not deeper just a delivery with greater weight. The power of the amp did not increase and remained at 7.5W RMS. What would the KT120s do to bass and would the power remain the same?

Under test with KT120s in place
My speakers are 6ohms impedance so a 6.8ohm resistor was used to load the amp for all the tests.  With Edcor XSE15-8-5k the -3dB points at 1W are 26Hz and 36kHz. The power increased to 9.7W RMS.  The amp had a reasonable power lift by replacing the power tubes. The cathode resistor is 300ohms and cathode current increased to 112mA. This occurred due to the tubes characteristics. 

I had listened to the amp for some time before measuring max power. I had felt the power had increased based on how the amp sounded.  Bass changed too. It sounded far more controlled. Mid range sounded clearer, crisper more depth. Treble about on par with KT88s in place. 

A few side effects of greater current drain are now the power tranni gets very hot. After hours of play you can only put your hand on it for a short time. With increased heat, listening pleasure has increased. I have now spent many hours listening to a variety of music both vinyl and CD through this amp. I can say with authority the amp is a much improved animal with the benefit of the KT120s driving the speakers.

Authority and passion
I'm a firm believe that the measure any system is its ability to move you musically.  Not by producing hundreds of watts and physically shaking you but by moving you emotionally.  If a system can convey the passion of the music I think it is bringing the music or performance to you.  To do this the main components, which make a system, all have to work in a synergy which "touches" you.

The power amp has a main role to play in that synergy. On a number of occasions now this map has played its part in bring powerful emotion to the music. It has powered my speakers with ease and authority and even at good volume shows no sign of clipping, strain or harshness. Music just flows from it and engulfs you.

Support equipment
Listed below is my equipment of the main listening area of my home and which the KT120 Be Bamp has been a part of for the last few weeks.

Analogue: Rega Planar 3 with RB300 arm and Grado wood body Reference Sonata cartridge
Phono Preamp: opamp based two stage battery powered
CDP: Oppo 980H upgraded by Vacuum Sate – Terra Firma clock
Preamp: Two stage Golden Dragon 12AX7s, Jensen copper, paper and oil caps with tube rectification
Power Amp: Be Bamp with KT120s
Interconnects: Pure silver and Transparent 200s
Speakers: Osborn Eclipse, three way large floor standing, 91dB sensitivity

Most of the gear is home made or modified commercial.

Listening notes
I audition the amp to a fellow DIYer who has both SS and tube amps. The night was based on a variety of pop, classical, opera and jazz vinyl. I will base my listening notes on that night but re-listen to each track for a refresher.

Clannad – Macalla [RCA/Victor LP]. On track one, "Caislean Oir", chorused voice present in an eerie manner with Maire taking some solo parts. Voices massed, appear natural, reverb was heavy but faded quickly. Individual voices could be heard in the mix with ease. The next two tracks sounded balanced with good bass but not dominating. Pace of the three tacks is easy going and sounded about right. Drums sound clean and snare naturally defined. On track three the bass drum had drive and bite and appeared forward.

Scheherazade - Rimsky Korsakov - The Chicago Symphony Orchestra [RCA/Victor LP].  This LP would have to be one of the best system test classical or demonstration LPs available. From the very first track which opens with a big brass "blast" to a solo violin then onto massed strings and builds then drops to flute and violin. The Be Bamp presented this whole LP with all the power passion imbedded in its tracks. You never felt you were missing anything. Big, bold, loud and delicate and sweet where it had to be.  Huge sound stage with good depth with the double basses throwing weight into the whole performance. Unforgettable! Articulate and defining.

Kate Bush - Aerial [EMI -180gm double LP]. Sea of Honey side B: "How to be Invisible". Driving bass and bass drum with clear electric guitar forward  of other instruments. Kate's voice, now mellower than her earlier days, sounded clean with all the intonations clear and discernable. She was really in charge on this track over quite a heavy back ground of a few instruments.  In "Joanni"  Kate's voice is back further in the mix with synths and heavy bass tracks. But once more you had no trouble discerning every word. The KT120s have a clean, crisp mid-range. To me this bought Kate her own space in a very complex and heavy, dark tones.

Donizetti - L'elisir d'amore / Sutherland · Pavarotti [DECCA LP].  This Opera opens with the prelude and introductory chorus and then Mr. Pavarotti takes over. From Pavarotti's very first phrases you are totally hooked. The clear and detailed presentation of the KT120s is very evident.  Pavarotti's voice floats into the room and engulfs you. You can "feel" where Pavarotti is standing on stage (left of stage) and you sense the size of the theatre this Opera is being performed in. Sutherland follows Pavarotti and sounds on pitch and well placed against stage and orchestra. This amp would be wasted if it never saw classical and particularly Opera.

Diana Krall – When I look into your eyes [Universal Music 180gm double LP].  The first side of this LP has excessive reverb. It sounds like false digital reverb which never sounds right. With a result sibilance is distracting. Dianna's voice is detailed and forward. The backing band sound excellent. Dianna's piano sounds a little bloomy and not quite natural.  Highs are crisp and airy which is what I come to expect from the KT120s. Rhythm and pace for the whole double LP set sounds right, even "jump'n".  On the second track, first side, the double bass is exquisitely articulate. You can follow every note and only the lower register gets a little boomy. Very enjoyable.

The downside and the upside
If you are a heavy rock, pop or R&B enthusiast stay with the KT88s in the same amp. The bass from the 88s is bloomier, a little more bloated and heavier sounding. This may appeal to some. With the KT88s in place sibilance is more controlled. The KT120s really open up the mids but with that comes accentuation of sibilance where it is badly recorded. The 88s control sibilance well, not masking it but not let it dominate.

Both tubes have excellent high frequency extension. So nothing to note at the top end. Except there maybe just a hint more air from the KT120s. But mid-range:  voice, acoustic guitar, strings, brass  and piano is where the KT120s really shine. Not only defining those sounds in the total mix but giving them weight, space and clarity. And with that comes pace and rhythm. Not that I ever felt the KT88s could not swing with the best.

If you enjoy a wide variety of music and want a little extra power from your SE UL or triode amp then purchase some KT120s. Drop them in place of your current tubes (assuming over all compatibility) and get use to better vocals, a tighter bass and airy highs. Who could ask for more?
Mark Houston – retro-thermionic.
To email Mark, type out the email address.



  1. Hi Mark, I am interested in knowing what component values and ratings you used for the cathode resistor and capacitor for the single ended KT120 circuit.
    Thank you in advance,
    Rob in the UK

  2. Check the forum post for the final schematic for the KT120 SE amp.


  3. pls let me more...output trnasformer current handimh current for KT88 is around 90mA. how do you use KT120? it's around 120mA.
    output transforner will be satulated that caused un recovered demage.
    of course in case KT88 output trnasformer current capacity up to 120mA, it's ok but it's unusual.