A Single Ended Vacuum Tube Amplifier With A Modern Twist

Despite the oldest solid state audio circuitry now qualifying for a pension and a bus pass where this is being written, the thermionic tube retains a foothold in the world of audio — cherished by enthusiasts for the warm sound it is claimed to impart. For  the electronics enthusiast a tube audio amplifier makes for an interesting and unusual project, and for that reason it’s one tackled by many. [Keri Szafir] is no exception, and she’s produced a stereo tube amp with a few modern features.

Electrically it’s a relatively conventional single-ended design using a double triode and a power pentode for each channel. It follows a so-called ultra-linear circuit, with a tap on the output transformer feeding one of the pentode’s grids. The modern features come via a switched Bluetooth input and a motorized volume control, something that would have never been found on such an amp when they were the cutting edge.

We have to admit a soft spot for this type of amp, and we particularly like this one for its very period construction style using cable lacing to keep the wiring under control. We more often see these amps using the cheaper integrated triode-pentode tubes which makes them especially easy to build, so the separate preamp is a little different. We’re not sure we’d have spent extra on the fancy E88cc tubes though.

40 thoughts on “A Single Ended Vacuum Tube Amplifier With A Modern Twist

    1. Don’t get me wrong, I do like tubes (in my RF projects tbh), but given the way people are drawn to them, and other historical oddities like vinyl records and nixies and 8bit micros and what-not, wondering when (or if) people will someday get all misty about stonkin’ big TO-3 cased PNP transistors. I know they’re not very flashy, they were usually relegated to the Back of the Unit, back there with the cooling fins and the fuse holder, but maybe they have their own, underappreciated sound? What better to amplify your 8-track tape than a big ol’ 2N2955 or some such? I know germanium transistors have a bit of a following these days, why not plain old silicon but PNP. Think about it, they’re quirky things, the voltage rails are upside down, the majority carrier is *holes* for Ffffeynman’s sake. What’s quirkier than electrons flying through vacuum? Places where electrons *aren’t*!

      Just trying to stir the pot ..

      1. you got me worried that someday you won’t be able to buy a bag of more 2n2222s than you can shake a stick at for pocket change! had to go looking just to make myself calm down, and that 100ct bag is apparently up to $8! pfew at least it’s still available

        not that i’m a huge fan of them over anything else, i just like a couple drawers full of “generic TO-92 BJT” so i never have to think about it, at least for signalling applications

  1. I got my “tube sound junkie” fix a little easier. https://josepheoff.github.io/posts/blaupunkt20300-4

    We have an old Blaupunkt tube radio from the 1960s with a DIN line-in connector. I whipped up a little adapter to plug a Bluetooth receiver in to the DIN line-in. We can play anything from our smart phones (music, video, phone calls) through the old Blaupunkt. It sounds fantastic.

    Tube amplifiers are not objectively better than other amplifiers. In truth, a tube amplifier is objectively worse than other types of amplifiers. What they do is sound pleasant. The way they “mess up” the sound is subjectively more pleasant than the sound from amplifiers that are objectively better.

    In other words, tube amplifiers aren’t better. There’s just a lot of people who like the way they sound.

      1. The SK-2 is not for me. A bit too much “cheap plastic” look.


        The old Blaupunkt Granada has several short wave bands besides the typical FM and AM (Mittelwelle) bands. It has a built in directional antenna (with control dial on the front panel) for tuning in far off short wave signals. I can pick up a BBC station (England) from where I live in Germany.

        “Geschmacksache,” sagte der Affe in biss in die Seife. Das gilt für uns beide.

    1. “The way they “mess up” the sound is subjectively more pleasant than the sound from amplifiers that are objectively better.”

      I have no skin in the game with tube amplifiers. Their mainstream days were just a little before my time and though I have had some old tube based equipment I’ve never really had anything meant for HiFi music so I don’t really have any preference that I know of for a tube amp.

      But.. I kind of feel the need to point out that some might say the whole point of the amplifier is to make a sound that the owner finds pleasant. Thus as long as the owner is happy it is at least subjectively better and since that’s the point and owners vary “objective betterness” isn’t really a thing.

      I get what you are saying, equating faithful reproduction with “objective betterness” but it’s a music device meant for enjoyment, not a piece of lab equipment meant for measuring the original signal. Whatever the user likes is best.

      1. If you listen to the hifi folks, they’ll tell you that they want true, faithful reproduction of the original sound – then they’ll tell you all the things they do that are NOT faithful reproduction of the original sound, including equalizers and tube amplifiers.

        I LIKE the sound of a tube amplifier. I LIKE to turn down the bass and turn up the treble. I know that these are my personal preferences. They are just the things that I like.

        There’s a lot of hifi “golden ears” who will tell you that their preferences are objectively better and the only correct way. They are full of (brown) stuff. They’ll tell you that you must have a tube amplifier because it is objectively better. They’ll tell you that you must use a certain speaker because it is objectively better. In both cases, it is just the listener’s preference. Tube amplifiers suck (objectively) and their preferred speaker just happens to match their expectations for frequency response.

        If you say you like the sound of tubes better, there’s no argument. If you say that a tube amplifier objectively reproduces the original sound better, then you are wrong.

        1. Is there really such a thing as “faithful reproduction?” Where modern music is concerned, the question must be asked: “faithful” to what?

          Consider: The awesome sound coming from a Marshall stack is far divorced from the “faithful” sound of the actual vibrating metal guitar string. In truth, the “sound” of an electric guitar is synthesized, albeit through analog means.

          Most people who like that fat punch of a snare drum would be very disappointed to find out that a real one, without close-miking, gating, compression, and short gated reverb sounds very puny.

          And don’t get me started on modern vocals… some of the best pop signers of 2023 can barely complete with the C-level singers from…say, the 1940’s, because back then, a good vocalist knew how to project, to articulate, modulate, and to sing in tune. Nowadays, all you have to do to be a “singer” is look good half-naked, and everything else can be fixed “in the mix.”

          I agree with your assessment of most self-proclaimed audiophiles. They’ll spend a king’s ransom on flat-response this, and zero phase distortion-that… and then crank up the base and treble controls (in a living room populated with standing waves, heavy curtains, and an overstuffed couch,) because it “sounds better.” In fact, If you want to hear an album the way the producer intended, you’ll need to listen to it with HIS speakers, HIS amp, in a mixing booth with anechoic tiles pasted to the walls and ceiling.

          One other thing… There is something to be said for circuitry (tubes, FETs) that produce even vs odd-harmonic distortion. Even-harmonic distortion products are related to the fundamental by octaves… so, while distortion non the less, they are musically related to the fundamental. This results in that “warm” tone that many people like.

          Odd harmonics, not musically related to the fundamental, lead to a harsh, grating, dissonant quality to reproduced sound.

          In the end, music is in the ear of the beholder, I guess.

        2. Equalizers are frequently used to correct for deficiencies in the owner’s equipment or listening environment. There’s an advantage to using an equalizer that costs $50 to provide 12 dB boost at 20 Hz rather than spend $2000 for speakers flat to 20 Hz.

          Occasionally I come across a recording that’s been obviously badly mixed. An equalizer (or just ordinary tone controls) can make a recording with overpowering shrill treble into something listenable.

          For people familiar with orchestras in large concert halls or rock concerts in their appropriate venues, etc., accurate reproduction helps the illusion that they’re listening to a live performance. Accurate reproduction lets the listener experience something closer to the composer’s or performer’s intention. That seems to me to be a valid concern for someone doing serious listening (as opposed to background music.)

        3. For flat freq. response, tube amps don’t “suck” any more than any other amp that isn’t reference quality. Besides the shape, size, and surfaces of the room matters at least as much as the amp+speakers. Odd that very few “audiophiles” do more than hang up some pricey acoustic panels in their 10x15x8 “listening” room and call it done.

      2. This debate reminds me of the definition of “good vodka:” one that adds the least additional flavor of its own, much like a very “flat” microphone (e.g. Rode NT1). Whereas scotch drinkers strive for “just the right aroma,” like a vintage Neumann U87 that makes female vocalists sound better. One is a scientific argument, the other is aesthetic. Both are right.

    2. tube amps handle clipping much better than solid state amps. push a solid state amp into clipping and compare it to a tube amp that’s clipping and tell me again how the solid state amp sounds better. by the way, it’s not unheard of for a short trnsient like a cymbol or drum to push an amp int clipping.

      1. P=I • E , is it not? Rumor has it that a tube amps power goes a long way compared to a SS amp. How is 89o possible with, say a 12 wpc tube power amp compared with 150 wpc SS amp? You mentioned tube amps handle clipping better and my example with the tube amp having a 176 total watt deficit, if playing music at a moderate volume using on average 5 watts, it appears to me there is not a whole lot of headroom compared to the available peak power of the SS amp.
        So wouldn’t the tube amp be clipping a considerable amount of time compared to the other amp, and how does it deal with clipping better than the solid state?
        I would like to eventually build one myself once I take the mystery out of it.

        1. so you want to compare an overspecified 150 watt solid state amp against a 5 watt tube amp? why not compare amps with similar output power? are there any other advantages you want to give to the solid state amp? how about using more efficient speakers but only for the solid state amp? seems consistent with your argument.

          1. High power high quality tube amps are prohibitively expensive and heavy. For the same price, solid state equipment can produce more power and less distortion.

    3. Your absolutely right but what a wonderful sound that distorts in a totally different way. Having worked with tube amplifiers, old altec gear that age had something special. Those that are that old know what I am saying. Those amplifiers were made to be serviced for ever. Meant to be repaired not replaced. Just people really don’t understand what specifications really say but their ear tells them what they like. Cheers from Nova Scotia

    4. Objective by what standards? THD+N? THD? Form factor? Not even all harmonic distortion sounds the same, like at all. Tubes naturally have more euphonic low order distortion without having to employ nearly as much negative feedback, thereby decreasing phase incoherence at the output– Can I definitely say that tubes are objectively better? Nope, because we still haven’t discussed the application or need of the user. Remember amplifying circuits are designed for human ears, so if one sounds better than the other, it more accurately meets at least that particular design goal.

      1. “Tubes naturally have more euphonic low order distortion without having to employ nearly as much negative feedback, thereby decreasing phase incoherence at the output”

        That is nonsense approaching gibberish. By the way, if phase performance is important to you, you don’t want your wideband audio signal going through a transformer, as almost all audio tube amplifiers do.

        1. Speaking of gibberish, it appears you understand neither negative feedback nor how transformers perform in audio amplifiers. There are drawbacks even to properly designed transformers in said circuits, but phase coherence is not one of them. They can flip phase at their outputs in some cases but that doesn’t matter since many other parts of circuits do the same thing and if the output is not mixed with the input, then there is no chance that the phase coherence between the two signals can occur, which, by the way is exactly the effect I was referring to that results from certain applications of negative feedback. You can read more about negative feedback here (since it sounds like that might be helpful): https://www.tutorialspoint.com/amplifiers/amplifiers_negative_feedback.htm

  2. “In other words, tube amplifiers aren’t better.”

    And I don’t miss them one bit.
    As someone who grew up at the end of the hollow-state
    era, give me 12VDC(or less) any day. I still have holes in my
    fingers from HV.

    1. There’s one notable property thermionic valves have, though.
      They go softly into saturation. Transistors don’t do that – they get stuffed.

      That’s one property that’s being forgotten these days.
      As receiver frontends, valves can still be useful.
      They can handle strong signals without failing or taking damage.

      Personally, I think that valves could be well integrated into modern technology.

  3. I have several tube amplifiers but all for guitar. I did however get to demo a set of infinity reference standard speakers pushed by all tube amps ( don’t remember the manufacturer). It was literally like sitting in a room with an orchestra. Something about the response of the various frequencies. I’m sure the speakers played a big part, they were like 50k back in 85

  4. These simple designs look good, but I’ve got a pair of large Magnepans, so 10 or 20 wpc isn’t gonna cut it! I still play with tubes, got a large supply of ’em and 4 or 5 tube testers. Tubes, to me, are primarily an intellectual exercise (and a way to keep 2 Hammond organs going). Although I do want to convert a pair of Heathkit Williamson-type amps to directly drive my Stax ‘phones (no output transformers).

  5. Going all the way for tube amplification and using bluetooth, what a joke. Far worse than tubes vs. transistors argument. Completely out of the ballpark. Unless this is done with 5.0 up-to-date it’s a waste of hearing.

  6. Dear all,
    I am an R&D engineer in electronics. I redesign an building tube amplifires to best performance. I am using those tubes who fit best for my need. Not because they look nice and implessive. I can tell you, if you need some power, like 2 x 50 W RMS and low Harmonic distortion like <1% at 25W and a – 3dB point from 10Hz to KHz. You need a lot of tubes 🙂
    But the whaw's and hoho's I hear from visitors are not bacause I play my Harmon Karton high tech chain but my Full tube lne with a Lenco L78 using a Pickering XV15 MD. For me this is convincing.

  7. Yeah. Tubes look cooler! And sound great in guitar or bass amps. All the best ones use tubes, period. They sound and distort more musically and are more versatile when it comes to Making music.
    That said, transistors do have their uses.
    My 7591 tube stereos are just fine with me.
    I LIKE the ‘good’ distortion they give.
    And I AM hsppily set in my ways!

  8. I never understand the hate that engineers give to tubes. The human ear interpretes 10 octaves of information straight to the brain. You cannot just look at THD and efficiency and say solid state is better if it isn’t pleasing to the ears.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.