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Comparing Eight Tubes For Low Power Amplifiers
ArticleSubmitted By Mike Zivkovic OfTeresonic
Thearticle compares eight different output tubes of low power in Single Ended (SE)mode, both triode and pentode, for compatibility with classical and jazz musiclistening using Lowther high-efficiency drivers in Teresonic speakers. My goldstandard is still the 2A3, but this project showed me that the EL3N is veryclose. – Damon Coffman
Even though I never made a career in music (choosing physicsinstead) my love of music never ceased. At the arrival of our first child, Isold my stereo to pay hospital costs, but quickly hand-build a replacement basedon a Popular Electronics article. Thus it was strange that by the early 90's Iwas not really listening to classical music any more. I‘d converted my vinylto CD's and could afford a decent system by then, but something was missing. Ona whim I bought a pair of used Manley Labs mono blocks, picked up a platter andsome yard sale vinyl, and found that I enjoyed listening to music again. Thatsent me back down the valve amplifier road. Over time I've come to recognizeSingle Ended Triode (SET) amps can closely reproduce a live acousticalperformance. SET amps need efficient speakers and that led me to Lowtherdrivers. Then about eight years ago a few estate sales set me on a new path. Atone I purchased 50,000+ vintage tubes and next landed some 3500 records from aserious classical collector. As a kid I'd read the RCA handbook and knew thattube amplification wasn't hard. What I didn't know is how much fun it would beto make amp that sounded so good for so little money.
Music Is For Listening
Icare not a whet for specifications — if my ears object,who cares that measured distortion is almost nonexistent? Our ears, instruments,concert halls, and subjective senses are all extremely non-linear. We experienceand love music as a combination of all these elements together. SET amplifierscombined with full-range drivers come closest to reproducing the live andintimate concert hall experience. Why full range drivers? Because there is nomechanical phase distortion between complementary harmonics due to separation oflow, mid, and high drivers. To my violin-trained ear this phase distortion isobvious and unacceptable. We expect to listen to music as though it were comingfrom a point source.A PM2A Lowtherdriver in the Teresonic enclosure sounds like live music in your living room.
About a year ago my new Teresonic speakers using Lowtherdrivers arrived. During the early listening phase I discovered my volume settingwas always very low. I do tend to listen to music louder (according to my wife),but this is primarily to hear the subtlety in the background — if the clarityof the speaker does not mask the details then the volume can be lower. Lowervolume and clarity greatly improves listening as dynamics are expandedsubjectively (per the Fletcher-Munson equal loudness curves.) In addition, lowvolume equals low distortion. This got me to thinking about options of comparingdifferent low power triodes for sound compatibility. If one or two watts areenough, the options for a power amp are huge — such possibilities led to thisproject. To put this in perspective, consider that many top end speakers havesensitivity of 85 dB, compared to 103 dB for Teresonic. This means that drivinga Teresonic with one and one-half watts equals well over 100 watts driving theother speaker.
The motivation for this project is to make a design that fitsalmost any DIY'ers budget. Most of current tube amplification world is in theprice stratosphere. Surprisingly, one can build an amp for less than $1000 (oreven $300 with yard sale help) that equals an $8,000 commercial amp — if youare willing to make some compromises, primarily in power output. Then you canspend your money where it counts most – at the beginning and end of the audiochain, or on speakers and phono cartridge. I've spent hundreds of hourslistening to various revisions and freely distribute my results in the hopesthat more people will do what I did – pick up a soldering iron and head to thebasement (or attic in my case) workshop.
This project includes output tubes that are (mostly) stillavailable, ranging from the esoteric to the nearly free. I chose eight tubes asrepresentative of what the novice DIY'er could do. The design is broken downinto the drive and output sections for simplicity.
Drive Section
A choke loaded 5687 provides a gain of 16+ and can swing over 100VP-P without distortion. A drive tube should be able to swing the full biasvoltage X 2 without distorting. For example, the EL3N has -8 VDC of bias andthus 500mv P-P input drives to full power. Since a CD player or phono pre-ampwill put out 750mv at 0 db (max signal), no extra amplification is necessary.Thus, a passive line stage could work for the EL3N. A normal line stage willallow another 12db or more of amplification. I was able to run all tubesdirectly from my line stage without any extra amplification in the drive stage.
Output Section
Single Ended Triodes (SET) power amps are the very essence ofsimplicity. It is almost impossible go wrong if you follow the manufacturerrecommendations. See the table in the output schematic for proper values.
1 Pentode Mode
2 Using 5K transformer
* Pwr – measured 5% 2ndharmonic distortion in Triode mode – double in Pentode mode (may vary based ontubes)
** Vk – Grid bias sets themaximum driving input voltage by formula
Vin=2 x Vk
*** 3rd harmonic is lowest of all in the list at rated power for EL3Ntube
Deviate from these values carefully; more power is possible byincreasing the current draw, but tube life suffers. All these tubes are pentodesor beam power amplifier tubes. To run a beam power or pentode tube in triodemode, strap the screen grid to the plate (e.g. for 6V6 this would be pins 4& 3.) The suppressor grid is internally tied to the cathode, except in thecase of the E80L where it must be connected externally (pins 9 & 3).
In pentode mode the power output is usually doubled, but thedistortion in the odd harmonics increases. This may not be a problem when usinga high efficiency speaker as you will be at very low output levels anddistortion is only discernable as the power output approaches maximum. To run inpentode mode (except for 6BQ6), connect the screen grid to B+ supply with 1000ohm resistor (see schematic). Alternatively, you can use a transformer withscreen grid taps (ultra-linear mode). The E80L is designed to run as a pentode,but it makes a beautiful triode. If using the 6BQ6 as a pentode, follow thebreakout schematic and adjust the screen grid voltage to give ~75ma of currentdraw. This should yield between 150-170 Volts at the screen pin, depending onsupply voltage.
Measurements And Notes
Equipment: Tektronix TPS 2024 scope, Tektronix AGF 3022function generator, Hagerman Vacu-Trace analyzer for matching drive tubesections.
Power: Measured at the 5 percent harmonic distortionpoint, or -26 dBV for 2nd harmonic. This would have been the old ‘RMS' hi-firating. Later ratings allowed 10%, which could double the rated output power.This sounds like a lot but the ear finds the 2nd harmonic pleasant and enhancingin the high volumes. On the other hand, very small amounts of odd harmonicdistortion are objectionable. I like to see the 3rd harmonic below -48 dbV atfull power.
Drive section: Choke loaded circuit gives full gain of5687. Same result could be obtained using 12B4 but with half gain. Mu = 17 for a5687 thus, 0dB input will drive over 30volts P-P, plenty for many power triodes.Most active pre-amps will easily output +12 dB. This could then drive 300B tofull output. Signal is ultra clean up to well over 150 volts P-P. If more gainis absolutely needed use half of 5687 as a Plate Follower with 6 to 12 dB offeedback. This will reduce RF response slightly.
5687 should draw about 10 mA per tube. This leaves voltageregulator drawing 15 mA – right in the sweet spot.
Results
Ratings are obviously subjective to my ear. I think that most CDand solid state aficionados would initially prefer the 6973, and then over timebe wooed to the EL3N or E80L. SET lovers are going to want the EL3N.
Initially I noticed very significant differences betweendifferent tubes. This was surprising, but I went with the flow and spent about aweek listening to the permutations. I became convinced that I was not hearingtube differences; rather I was hearing the effect of my bench regulated powersupplies. The large difference of current between tubes could have a very markedeffect based on differences in transient response, etc. Once I connected up thenew power supply, 'The Beast', many differences disappeared and a whole new setof subtleties became audible. This exercise underscores the importance of thepower supply in the equation — many times a mediocre amp sounds that waysimply because of an inferior power supply.
Most of the time when you read a review there is a descriptionof how such and such a musician on such and such a track sounded so and so. WhenI finish reading those articles I have no idea what it really sounded like —so I haven't learned anything. I do know that when I listen to music I want tobe excited by what I hear. If the recording is a dud, no amount of tube greasewill fix it, but if it is great music a great SET amp can surprise you over andover again. My favorite recording of all time is a scratchy 1953 record ofSviatoslav Richter playing the Mussorgsky 'Pictures at an Exhibition' in
A good 2A3 amp with vintage tubes does wonderful things togreat music and has been my favorite amp for a long time. My goal in thisproject was to find a (vintage) triode strapped pentode that could equal themagic of a vintage 2A3. New tubes from
The tubes fell into three categories: 'yawn','verynice',and 'great'. In the 'yawn'category is the 807 tube. I can't recommend this tube in triode mode at all. TheRCA manual doesn't recommend it either, and now I know why. It doesn't even workfor background listening.
Inthe 'very nice' category are allthe 6V6 style beam power tubes. In fact, with the upgraded Beast power supply itwas hard to find a distinct difference between the 6AQ5, 6V6, and 6L6. This isnot surprising since they all have very similar construction. The 6AQ5 is alower power 6V6 in a 7 pin package used originally in car radios, and the 6L6 isan upgraded 6V6 with higher voltage and higher current. You could make a choicebased on cost or availability and loose nothing to quality in this group. (Allof these tubes are actually beam power tetrodes, not true pentodes, designed assuch to get around the 1928 Philips pentode patent.)
I would also put the 6BQ6B / 6CU6 in the 'verynice' category, but it takes a lot more drive voltage so an extragain stage may be required. The 6BQ6 is a TV sweep tube and can be had foralmost free. It is a great tube for guitar amps. I actually found that Ipreferred the 6BQ6 to the 6L6; the detail and articulation seemed slightlybetter. If you build a 6BQ6 amp, consider adding a switch to toggle betweentriode and pentode mode. Also, make sure to put a cage on the amp — you don'twant a high voltage accident from a child sticking their finger on the platecap.
The 'great'category included a bronze, silver, and a gold medal winner. [Note that all ofthe tubes in this category are true pentodes, not beam power tubes. RCA claimedthat beam power technology sounded just as good as true pentodes, but I beg todiffer.
Equipment And Parts
Line Stage:
Phono Stage: EAR 324,
Turntable: Immedia Platter, RPM II tonearm, Koetsu Urushi Cartridge
Digitizing and Playback: Echo Audio Layala 24, 32 bit X 96KHz tracks = 4GB per record.
Output Transformers: Audio Note (152?) circa 1975. I used 4 ohm tap for5K ratio.
Power Supply a.k.a. ‘The Beast': Re-made from an organ power supply.High voltage via solid state bridge feeding a double PI filter with a 4uF oilfilled input capacitor; good to 350ma with < 1mv ripple.
Listening Room: Lab: 18' by 14' with 10' cathedral ceiling. Very intimatesound with Teresonic Intregrums with Lowther PM2A silver voice coil, 16 ohms.
Schematics and tips for the various amps will be also posted at www.galaxySETlabs.com
The bronze winner,6973, is an ultra clean tube — detail and articulation are truly fantastic.Internally the 6973 is almost identical to the 6BQ5 and 6GK5, but it sounds muchbetter. The 6GK5 makes a really cheap good amp, and the 6BQ5 improves on itsomewhat, but the 6973 stands alone in this group. The attacks are all crisp andcorrect. A cymbal sounds like a live cymbal, which is rarely the case inamplifiers (and never the case from a CD recording). My only objection was thatthe sound could tend to be a little clinical in some passages. You don't want tobe beat over the head with subtlety; you want to be surprised by it. It is kindof like the difference between listening to a violin three feet away and fifteenfeet away. At three feet a violin can hurt your ears from the abruptness of thedown stroke attacks. At fifteen feet it mellows into an acceptable sharpness.Even so, I could listen to this tube all day.
The silver medalgoes to E80L as it adds beauty and warmth to the articulation found in the 6973,softening to just the right amount for the Teresonic speakers. The 6973 is agood match for the Medallion II's, but the E80L still gets the nod formusicality. We are talking very minute differences here, ones that may not bedetectable in a less sensitive speaker.
The gold goes tothe EL3N. It is closest to the 2A3 sound of any of the triode strapped pentodestested. The EL3N makes music exciting – I plan to build a vintage EL3N amp aspart of my collection this winter. Historically, the EL3 was created as a roundcathode version of the AL3 by Philips in 1936. The EL3N was an improved versionof the EL3 with lower filament current and better cathode. The 6V6 was createdas a competition to the EL3, while the 6L6 was created to compete with the EL5– the high power successor to the EL3. The final successor to the EL5 (andEL6) was the EL-34, still loved for its beautiful tone, especially byguitarists. Even in pentode mode, the EL3N is a beautiful and exciting tube. Iwould like to test this tube using ultra-linear design in the future. SET loverswill want an EL3N powered amp if they can find a source for the tubes; try www.radiomuseum.org,(I bought mine in
The first part of my Multi-Amp project is complete, although Idid not specifically test a few tubes that might be favorites like the 6CA7, orthe EL34, or even the 6080. However, there is a lot of information out there onthose tubes and I wanted to go a little off the beaten path. The next part ofthe project will look at comparing direct heated triodes: 45, 2A3, 6A3, 6B4G,6S4S (or 6C4C, the Russian version of 6B4G). Mygold standard is still the 2A3, but this project showed me that the EL3N is veryclose.
Damon Coffman,
About the author
I am a violinist (a hackernow, but a concert master decades ago) and want the music I listen to, primarilyclassical and jazz, to sound like live performances. I remember clearly the dayI began really listening to music. It was winter 1955. My father had left hiscollege music scholarship to support a new family. We wintered in
A mighty 807parallell CFB push-pull amp
Here we have something encouraging, namely young bright guy that wants a tube amp and takes the challenge with his friends that he'll build one from scratch. His work helps him, since they just have declared that all tubes are redundant and there seems to be quite a stock of 807 and 6SN7 and 6SJ7 and OD3 tubes available for servicing. ( They serviced the amps used for the trainstation broadcast service ). He also had a stock PIO 47µF/630 volt caps.
So he ask me for a schematic with these tubes and he wants power for possible PA application but he lacks cash, so El Cheapo please.
First schematic had 6SL7 input tube and 6SN7 long tail phase splitter and 807PPP connected in triode or penthode mode through LL1620CFB.
It was unstable, normal, he had zero experience and it showed, long cables going back and fort without screens and the whole thing was motorboating and oscillating full power above 100Mhz ( couldn't lock on it with my 100Mhz scope ). Back to scratch new gnd and more screened cables and a week later but problems where still there. The funny thing is with only 2 807 the amp played nice but not very loud. so i advised a LL1664PP for more power and cheaper, but that didn't solve the problem either. Screened cables to the output transformer improved the situation and a 6SN7 driver also helped but still not 100%. He then tried something i completely overlooked ( shame on me ) he used the OD3 tubes and put the G2 of each 807 on a fixed voltage of 300volt and suddenly all was ok. Georgeous sound, loud undistorted and we could run a comparison between LL1664PP and LL1620CFB as each channel had 1 wired to them. The LL1620CFB was clearly the way to go, at the cost of a few watts outputpower loss far better, especially bass was much more controlled. Midrange was also better, wider and deeper soundstaging amazing how much difference this makes.
The Schematic :
A few pictures of the inside of this amp!
A beautifull view in the dark
807 Amplifier Schematics
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