Here’s my first tube photo in a while: a Russian 5U3C (Cyrillic: 5Ц3C) rectifier tube from 1976. It’s so-called New Old Stock (NOS), meaning that the tube has never been used.
The tube was bought for a Kondo Audio Note KSL-M7 pre-amplifier clone produced and sold by the Chinese brand Sunbuck on Aliexpress; this is not to be confused with an official Kondo Audio Note product – nor a product from similarly-sounding Audio Note UK.
When I bought the amplifier, it was listed as an Audio Note product – now the name of the product has been changed to ‘Sunbuck KSL-M7’.
The same preamplifier is sold by AIYIMA SMSL Amplifier Audio Store, also on Aliexpress, under the name ‘AIYIMA SMSL KSL M7 Line preamplifier’.
Preamplifier from Aliexpress
I’ll save the ethical considerations of the project for another post. The Kondo clone itself was ordered from the Chinese site AliExpress for $238, shipping included. When it arrived via UPS, VAT added roughly another $71 to the total. The listing mentions that the components are from Nichicon, Philips and Siemens.
Original KSL-M7 versus clone
As described in a previous post, I’m very satisfied with my Leben CS-600 integrated amplifier, but there should always be room for experimentation. What drove me the the KSL-M7 clone was 1) wanting to use a minimalistic preamplifier with only one function – amplification & a volume knob. No bass or treble boosts, no reverse function, only one output and one input and no tape in our out functions – combined with a pathway as short and clean as possible.
Tube-based preamplification + solid state power amplifier
I also wanted to explore the combination of tube-based preamplification with solid-state power amplification—to give my Harbeth M30s a bit more wattage to work with. The CS-600 delivers a respectable 32 watts when configured with 6L6GC/5881 output tubes, or 28 watts with EL34s. (It supports multiple tube types via two selector switches.)
ECC81 driver tubes & 5U3C/5U4G rectifier tube
The Audio Note Kondo KSL-M7 preamplifier is based on two ECC81/6201/CV4024 driver tubes fed by a single 5U3C-family tube (5U4G, VT-244, 5Z3P)
I got two matched rectifier tubes on eBay, which is a new principle – when needing one, I get two for future use, as supplies are drying up for a lot of these classic tubes (produced before, let’s say, the 80s) that find use in a lot of different designs / configurations (other types of tubes see so little use that no new ones will ever be produced, presumably, such as the 6CS7 variety.
1976s Svetlana Winged-C 5U3C
This particular 5U3C was manufactured in 1976 and bears the mark “OTK-1” (Отдел Технического Контроля), indicating it passed quality control before leaving the Svetlana factory. The OTK stamp was a standard quality assurance mark on Soviet electronic components—especially those intended for military or industrial applications.
About the Kondo Audio Note KSL-M7
The current Kondo Audio Note KSL-M7 — where KSL presumably stands for Kondo Sound Labs, and the M for either Mains or Meister — is based on a single 6X4 rectifier tube (which is a lot smaller than the 5U3C) and eight 12AY7 driver tubes.
Judging by weight
Weight can be a good parameter by which to evaluate an amplifier’s design. The original weighs in at 17 kilos, whereas the clone weighs around 4 kilos — eliminating any doubt as to whether the components and design are in any way comparable.
Differences between the original and the clone
However, it differs from the original in that it lacks multiple inputs or a built-in phono stage for turntables, in addition to differences in the number and type of tubes.
The Kondo Audio Note is based on a 6X4 rectifier tube and eight 12AY7 tubes.
How did the clone sound?
It’s always an interesting experience plugging in new equipment. Normally, I use Harbeth M30.1s as speakers, but being somewhat skeptical of the product, I hooked it up with a set of Snell K IIs and a vintage Kenwood M1, fearing the unit might be defective in some way and potentially could harm a power amplifier and speakers in the process.
Visual inspection of the preamplifier
Before hooking it up, I took a look under the hood to inspect and see if everything looked right. I’m not that technically skilled, but I checked the soldering, the wiring, the inputs, and the transformer for any oddities. Everything, to the untrained eye, looked alright.
Testing the preamplifier with old and cheap equipment turned out to be a very good idea. I installed a set of Electro-Harmonix ECC81 tubes (which I had been using in a Leben RS30-EQ RIAA before upgrading to old General Electric and Telefunken tubes) and the 5U3C tube in the photos.
Heavy hum from Aliexpress ECC81 & 5UC3 clone
I turned on the preamplifier and waited a couple of minutes for it to warm up. I then turned on the power amplifier, which takes about 10 seconds to activate. Then — heavy, heavy hum. I turned everything off as quickly as possible. How could a unit like this pass quality assurance? Presumably by not undergoing it in the first place.
Not a good start. I thought it might be an issue with one or several of the tubes — all had come pre-tested with good ratings — but really wishing the project would turn out well, I gave it another shot using different input and rectifier tubes.
The result was the same — but the hum got worse. What to do?
Asking for help on the Hifi4all Forum
On Hifi4all, which is one of Denmark’s oldest fora for audio equipment and music listening. I shared a description (in Danish) of what I had experienced along with photos of the internals of the amplifier.
In spite of the photos not being the sharpest, one user, Tonni, spotted a problem:
“One of the electrolytic capacitors definitely looks defective. It’s bulging at the top. Comparing it to the reference images you shared, it looks like it’s installed the wrong way around!
Other than that, the pictures don’t really offer much help. They’re not sharp enough and don’t really give a clear overview.”
Another user, Boyd, elaborated:
The electrolytic capacitor (yellow one bulging at the top) is damaged and needs to be replaced. The reason is that the Chinese installed it the wrong way around.
Look at the stripe that marks the negative side (green arrow). It’s facing the opposite direction compared to the reference images (the first two pictures in your post).
This causes the capacitor to behave almost like a short circuit and places a load on the voltage it’s supposed to smooth out… and yes… it will hum like crazy. Don’t turn it on.
The next thing that happens is that the capacitor explodes, spewing a bunch of foul-smelling paper fluff.
Install a new capacitor with the same rating as the one that’s there, but make sure it’s oriented correctly—then you’ll see… it’ll probably work as intended.
Boyd then, very kindly, offered to see if the pre-amplifier was in a condition that would warrant a repair and/or upgrade of relevant components.
The least impressive components in electronics seen
The first analysis didn’t sound promising:
“Got it unpacked and taken apart today… Let’s just say it’s probably the LEAST impressive components in terms of electronics I’ve ever seen.
And it needs some thinking, because it’s FAR from following the schematic that was shown earlier. They’ve also used cheap fake China-Wima capacitors, and not even the values from the schematic or what’s printed on the PCB.
At first glance, it feels more like: “What leftover parts do we have lying around that we can use?”
Fixing the Aliexpress pre-amplifier
The reversed electrolytic capacitor had, of course, been acting almost like a short circuit, which ended up “frying” a 2.7K resistor.
But the other one, connected to the second electrolytic cap, was already starting to turn quite brown—so bottom line: they’re undersized.
They’ve now been replaced with 2.7K 6-watt resistors (I didn’t have any 5-watt ones on hand).
Then there are the three large white capacitors. Two of them are located at the output of each tube.
No matter how thoroughly I search the markings on them—whether through Google, eBay, or AliExpress—nothing remotely similar turns up.
However, when I search the exact text printed on them, I get one result after another for motor start capacitors or X2 line filters—same category.
This just confirms my suspicion: they’ve thrown in whatever they could find lying around in the scrap bin.
The only components that actually seem decent are the two choke coils and the mains transformer.
Again—the choke coils are nothing like what’s shown in the schematic. These are something else entirely… moving further and further away from anything that seriously resembles a “Kondo” (but hey, maybe it’ll still sound good).
So far, I’ve done the following:
Replaced the 3 X2 capacitors with proper polypropylene types
Swapped the fake Chinese “Wima” capacitors for genuine Wima units
Replaced the two electrolytic capacitors with high-quality equivalents (and oriented them correctly)
Replaced the two undersized resistors with more robust ones.
Summing up: Bad audio karma?
I’m grateful to Boyd for his analysis and work on the AliExpress ECC81 & 5U3C/5ZP4 preamplifier.
Overall, it saSudly confirms all fears of what one can expect from a product like this — one that looks good on the AliExpress site, has good reviews, and is sold by a seller — Sunbuck Official Store, owned by Guangzhou Shanyu Electronic Technology Co. — with many transactions and good ratings too.
I haven’t dared to ask how many hours went into the analysis, measurements, and repairs of the unit, but it’s quite certainly been a time-consuming project.
One thing is receiving a product with components that are different from what’s shown in the product photos; another is receiving a product that could potentially hurt you and your equipment — and perhaps even cause a fire if it had been left on unattended.
As mentioned in the opening paragraphs, there’s also an ethical issue at play — piggybacking on a very renowned audio equipment manufacturer like Kondo Audio Note Japan.
I’m listening to Person I Used to Know by Bill Evans (from Bill Evans: The Paris Concert: Edition Two album).
The preamplifier sounds good, equipped with a vintage 5U4GB rectifier tube and two General Electric ECC81s — thanks to Boyd.
Contacting the seller and Aliexpress
I sent the most important parts of the analysis along with five photos documenting the assembly mistake and stressed the severity of the issue — that the capacitor would have exploded had the pre-amplifier been left on longer — to the seller, Sunbuck Official Store, owned by Guangzhou Shanyu Electronic Technology Co., on AliExpress, curious as to how they would tackle the situation. They didn’t reply. I tried two more times, then gave up.
Then I contacted AliExpress with the same information, including photos. The support agent at AliExpress said they’d look into it as fast as possible — then radio silence. Which, in a sense, is the opposite of music.
I wrote this review in the hope that others do not receive similarly faulty equipment that could potentially harm loved — and perhaps also quite expensive — audio equipment.
More audio posts
The Leben CS-600 ~ An Introduction to Leben Tube Amplifiers
The Leben RS30-EQ RIAA, 6X5GT & ECC81 Tubes
6CJ3, 6DN3, 6CM3, 6CH3 and 6DW4B damper tubes
6CS7 Tubes: Data, Specifications & Usage
