GU50 8w Monoblock Amplifier

(2 customer reviews)

$600.00$999.00 US

SKU: N/A Category:


Get ready for the amazing sound of a Single Ended Pure Class A amplifier.  Clarity defines Class A and with clarity we get a great stereo image and with those two key things we get a wonderful soundstage!

We’ve carefully selected every part and component. From the Solen coupling capacitors to the solid Cherry Wood Plinth and Hand Brushed Aluminum Top Plate, to the Universal Hammond power transformer.

The GU50 comes as a Full Kit that includes all parts and an unfinished Solid Cherry Wood Plinth, Finished 3/16″/ 5mm Aluminum Top Plate and 2mm Aluminum Bottom Plate.

Build Information will include a Color Schematic of the Power Supply and Amplifier, also a Build Series of Videos is available on YouTube (Mellow Tone Kits).  The videos are the Build Manual.


SE (Singled Ended) Pure Class A (no Negative Feedback)
Rated Output Power: 8 Wrms
Universal Power Transformer
Household Voltage 100-240vac
Input Sensitivity: 5.45 Vrms
Signal To Noise Ratio: 64 dB
Input Impedance: 470 Kohm
Output Impedance: 4 Ohm / 8 Ohm
Power Consumption: 73W

Tubes – Driver: 1 x 6N6P or 6N6P-I
Power: 1 x GU-50

Size:  10″w x 13″d x 6.5″h (25.5cm x 33cm x 16.5cm), 16lbs/7.3kg

Sound Signature:  Musicality, Drive, Natural Bass, Rich Midrange and Treble with Amazing Detail, Clarity, Stereo Image & Soundstage.

System Compatability:  Feed the GU-50 with any of our Quality Preamp Kits and Drive any medium to higher efficiency speakers of  about 89dB or more.   Fun Fact:  We run custom open baffles (dipoles) that are about 93dB efficient and for the full volume “Live” test, I needed to wear hearing protection and 1/2 volume was already too much!

Build Complexity:   Medium – if you already make stuff, and are a thoughtful careful person, who can focus for an hour – You Can Build this Kit Amp.

Shipping Extra (to check on rate, make an account, place item in cart and go to check out page)

Discount Codes – don’t work on Kit Amps – the margin is too tight to offer discounts.

Not included:  Tubes, IEC power cord, solder


6N6P Preamp/Driver Tubes

GU50 Power Tubes

IEC Power Cords


Full Specification Sheet

Frequency Response

Frequency Sweep

Power Supply Schematic

Amplifier Schematic

Build Videos – Follow along with Jim on YouTube as he guides you through the build process step by step.

First Test Builder – First Listen!

Recommended Tools:  Link to Tools in the Store*

Temperature controlled solder station (I use: Lonove 926LED III available at Amazon)

Electrical Solder  63/37 tin/lead rosin core 0.8mm/.031” (Kester “44” 24-6337-0027)

Rosin Soldering Flux (I use: Caig “no clean” paste RSF-R39-2)

Volt/Ohm Meter (CAT III 600v – minimum) c/w probe & clip-on leads

Variac  0-130vac/20A or 0-260vac/10A  (helpful but not required)

Hot Glue Gun – a mini craft type will work just fine

1/4″ Socket Wrench & Deep Sockets 3/16″ &  7/32″ c/w  3″ (75mm) extension

*Wire Stripper

*Solder Sucker

*Needle Nose Pliers

*Side Cutters

*Ratcheting Crimper (Note: this is NOT your standard red/blue/yellow crimper)

*Mini Hot Air Gun (very very useful)

*Mini Utility Knife

*Mini 1/8″ Slot Screwdriver

*Torx Driver T15

2 reviews for GU50 8w Monoblock Amplifier

  1. Stanton Bailey (verified owner)

    The GU-50 monoblock amplifiers are a fantastic way to improve your audiophile experience while also learning a ton about tubes and the actual electronics you’re using.

    First the sound. I substituted these amplifiers for my outstanding Primalunda Evo 300 power amplifier and used the Evo as my comparison. I ran the GU-50s using a SOTA Comet turntable with an Ortofon 2M bronze and streamed from an Ultrarendu into a Schiit Gungnir/Freya+ combo. I used three sets of speakers with dual Rel subs to listen with the following efficiencies. Zu Audio Omen Def Supremes – 101 dB, Sonus Faber Sonetto IIIs – 89 dB, Von Schweikert VR-1s – 89 dB. All of the pairings were loud enough for my comfort and listening style with plenty to spare. The Omen Defs could shake the room but I felt like the Von Schweikrts paired the best with the speakers disappearing into the music in an amazing way. The first thing I did was just sit and smile amazed at how much better the music seemed to be. I noticed a much stronger, tighter bass. I had to dial down my subs. The result was a nicer timbre from string instruments. Not just the upright bass but violins, trumpets, and electric guitars sounded more quick and alive. Acoustic guitars had a richer tone. Kick drums and cajons were more palpable and punchy. Midrange was full and engaging as if the singer and sax were there in my living room. The treble didn’t seem to change significantly for better or worse. The next huge difference was in the black deep silence between notes and pauses. Turned all the way up without input there was just silence from my 101 dB speakers. No amp I have used was this quiet. The Primaluna has a notable hum with an ear next to the speaker despite all my efforts to eliminate it. I was just plain gobsmacked. Probably the next very notable change was in instrument separation and the spacing around singers and instruments. Singers in duets and groups seemed to move apart and I noted a greater depth to the sound stage with a bit more holographic imaging. Remember with taxes we’re talking $5,000 for the Primaluna vs. $2,000 for the GU-50’s. Also putting new “Budget” tubes in the Primaluna would be around $400 vs. $200 for “Premium” tubes in the GU-50s. I racked my brain to come up with a criticism regarding the performance and I couldn’t think of one.

    The build was just plain fun and so straightforward forward I think anyone can do this. I come from a ham radio background and have built numerous kits in the past. This would be a project I would recommend for anyone as a starter kit. The soldering is dead simple but the build can get complex so one has to be attentive and exacting at times. Jim and Charles literally hold your hand through the whole build and explain everything in such detail I think a high school student would easily be able to complete this without a problem. I recently built a Bottlehead Crack and found it more difficult than this kit. However the hand-holding would be my only criticism regarding the build. I would like to see a written guide so you don’t have to go through all the videos again for the second amp. I literally watched the videos and wrote my own instructions so I could just build without having to start and stop the video on my iPad while I worked.

    Bottom line – fantastic sound, fun easy build. For me it’s a joy to listen to and having built it brings me a great sense of pride and ownership. It will be something I hand down to my kids when Jim and Charles come up with the next best thing.
    Thanks guys

  2. Stefan Auer (verified owner)

    Finally, I could accompany my recent but first ever own built MTK 6or12SN7 Universal Preamp with an appropriate power stage!
    This time, I even dared to apply as a test builder what I do not regret! More than that, I feel honored very much that I got the chance!
    Like for the preamp, the build videos and schematics guided me well through the build and I enjoyed it very much and I also improved my skills in this regard.
    In a way, I found it almost easier than the preamp kit but I’d suggest to emphasize handling the top plate with the heavy transformers on with extra caution and best to lay it upside down into the plinth when working on the wiring.

    My background in Hifi electronics and loudspeakers goes way back to my teenage years but I must admit, I’m fairly new to building tube amp kits. In fact, I started my journey on this with the MTK preamp mentioned above only 6 months ago. (And got addicted right away).

    My current gear in use:
    Preamp: MTK 6or12SN7 Universal Preamp (my favourite tubes at the moment: Sylvania 7N7 – with loctal adapter)
    Loudspeaker: Tannoy Cheviot 1978 (original from first hand owner)
    CD: Exposure 2010
    DAC: Chord Mojo (1) & Cambridge DacMagic 200M
    Turntable: Micro Seiki DDX1000, Cartridge: Grado Statement Reference 2 & Denon DL-103R
    Phono preamp: Audiolab

    Now regarding the sound:
    I could use a lot of words to describe the sound but still it would not be enough – If I had to choose some: dynamic, clear, tight, crisp, detail – detail – detail but still all warmth, utmost lively!
    And in addition, that’s what I call synergy – A perfect match to the 6or12SN7 Universal Pre!!

    Well, I must say that I have absolutely liked the Willsenton R8 and I still do, but the GU50 monoblocks clearly play in another league!
    I always thought that you only can get precise and detailed sound by sacrificing warmth… Obviously, that is not necessarily true as these amplifiers prove. That must be due to the SE Class A design, I had only heard about before until now…

    The first record I played was the new album by Brad Mehldau “Your mother should know” (the Beatles covers) – “I am the walrus” among other tracks is simply outstanding! It is indeed a really good live recording though but hearing it through these amps makes you feel it is right happening in the room!
    The same applies to Nick Caves solo (alone) piano concert “Idiot Prayers”, Keith Jarretts “Sun Bear Concerts”, etc.
    The dynamics on “Oslo” by Michael Wollny is in fact astonishing! So is Cathal Coughlans “Black River Falls” and Kham Meslens album “Fantômes… Futures”! Jan Harbecks saxophone on album “Balanced” sounds absolutely incredible!
    Only to mention a view – I could go on for ever…

    Another funny thing is, as that I hear the music differently than before, I started liking albums which I didn’t particularly before.
    Bass is tight and precise – Moreover, it really sounds so natural and the bass drum for instance actually has a ‘tone’ and is not only heard as a “Bump”! I do like that! In fact, that clarity undeniably applies to the whole frequency range – And all that while still carrying the warmth! And not only that, it really carries emotions!

    Also the stereo separation, the soundstage and first and foremost the detailing and layering capability is phenomenal! Very, very, very natural!
    What is really pleasing me is the details to be heard apart from the music itself: the slightest “pffft” sounds and even the operation of the flaps from a saxophone or pedal noises from a piano, not to mention the attack and the timbre of string instruments!
    All in all, I’d like to use an expression Jim uses to say on Mullard EL34 xf2’s: IT SOUNDS LIKE LIQUID GOLD! I would say that nails it perfectly also here!

    When listening in the evening when it’s already darkened outside, by looking at the amps, it is like looking into the open fire of a chimney on a winter evening…

    While I’m currently simply enjoying listening on my current gear, I look forward to building another Mellow Tone Kit – I can’t wait for the Headphone Amplifier Kit…!

    As always, thank you very much, Jim & Charles, for keeping us up on quality vintage tubes, the weekly very informative Tube Lab videos and the development of the fantastic Mellow Tone kits!

    Sincere regards from Switzerland!

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