• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

FunKenya Chinese 300b Monoblocks

Hi everyone!
I just got a pair of Chinese 300b Monoblocks from eBay and have some questions and comments, and wonder if anyone who knows what they are doing has looked at them.
The amps that I got are sold by a bunch of different people but they all look the same. There is very little actual identifying labeling, the vendor that I chose has a funKenya banner on the pictures.
I got the version that came with PSVane 300bs and Russian rectifier and driver tubes. It only took two weeks to get to Pennsylvania from Hong Kong! It was very well packaged and the packaging and contents were undamaged.
There was no manual of any sort included, but I don't read Chinese so that isn't a great loss. The case has no identifying markings at all but there is a sticker on the bottom.
The first thing that I did was peal off the warranty stickers over two of the screws holding the side panels so that I could verify which socket is for the driver and which one gets the rectifier. The case is labeled 110 volts in several places so I checked the heater voltage for the 300b and it was almost 6 volts. I built power cords with 10 volt bucking transformers and reduced my line voltage from 123v to 110v and the heater voltage to 5 volts.
I tested them out on the workbench with small speakers and they appeared to be operating properly, so I brought them upstairs and hooked them up to the big system.
Right off the bat they sounded pretty darn good. Not really competitive with my big VTL monoblocks, but they had something to offer over the Skunkie Designs modified A12 that I had hooked up yesterday. After a couple of hours, they really started to sound pretty darn good!
I brought them downstairs and put them back on the bench with the small speakers out of phase and pushed up against each other, and the Sheffield Labs burn-in track running on repeat. I'm going to hook them back up shortly, after 4 hours or so.
My questions:
Has anyone who knows what they are doing looked at these things? They seem more "engineered" than the A12 but I still wonder if the circuit is actually properly designed and implemented.
I would love to get rid of the Russian tubes and use more common types. It comes with a 5U4M rectifier and obviously a 5AR4 would give me more choice and availability, similarly with the 6H8C driver tube, a 6SN7 is a lot easier to come by. I'm pretty sure that I can't just use the "equivalent" tube types, lol.
There is a multi-turn potentiometer on the back but no exposed test point that I can find. Does it seem possible that this amp is configured with adjustable bias? That raises several other questions.
I'm about ready to bite the bullet and order some better 300b's for it but if I can do something about the others while I'm at it that would be great.

Hum balance! That makes sense, this is my first 300b amplifier. So, I just diddle the knob to balance out the 60hz hum? The amps are very quiet, even with my Klipsch LaScalas.
I was pretty sure about the driver tube being a direct replacement, less so for the rectifiers. Now I have to decide how much to spend on tubes! As well built as they appear, I can't imagine that the OTs are worth the big-buck stuff.
Hey guys, I just want to report that I retubed my amps with all JJ tubes and swapped the 5U4M for an 5AR4 and the 6H8C for a 6SN7.
I have to say that the amps are reborn! I burned the new tubes in for a few hours and kept to material that demonstrated their strengths rather than any weaknesses but they sound pretty marvelous.
I've used Russian 6H8C's in lots of my amps and can't fault them at all. Why shell out more money for a 6SN7 when you will probably not hear a difference?

Yon amps look well built, have you done any tests like OP power THD, frequency response etc? Did the amps have any spec sheet?

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I figured that as long as I was retubing, lol.
I don't have any test equipment other than my ears, which is why I am hoping that someone with more technical expertise than I has looked these things over. They came with no documentation whatsoever.
BTW, it appears that the actual manufacturer is BRZHiFi.
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Careful! The 5AR4 is NOT the same as Russian 5U4M (I assume you have the Chinese equivalent), which is not the same as an American 5U4GB. The Russian 5U4M is more like a 5Y3GB that has higher voltage drop than the 5AR4 you replaced it with, so you likely have a higher plate voltage than the amp was designed for. It may be OK, depending on the circuit and robustness of the 300B cathode resistors, but it deserves verification. Good luck with your amps!

Got the following from http://www.r-type.org/exhib/aaa1820.htm

The Russian 5C4M (5ц4М) is an example of a typical high current rectifier for small equipment use. It is indirectly heated but the cathode is tied to one side of the heater. The OTK mark indicates that it was made for military use. The data-sheet indicates that the 5Y3GB is equivalent.​

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I was concerned about this and would like to check. Is the correct procedure to pull the 300b and then check the voltage between anode and cathode, pins 1 and 2? Checking the voltage while the tube is installed would mean poking around inside and seems much less safe than going in from the top.
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I apologize in advance if I talk at too basic a level, but not knowing how experienced you are I wanted to be understandable. Your concern about poking around in a live HV tube amp is VERY important. Unfortunately you will have to measure with the tubes in place to get the voltages under load. In this chassis layout I suggest the use of grabber test leads and make connections before power is applied. I believe you don’t have schematics or voltage charts, so, the goal of these measurements is determining the operation conditions for your 300b output tubes, to ensure they are not running too hot due to the increased supply voltage as a result of the rectifier swap:

1. Measure B+ supply voltage, and plate supply voltage to ground; really needs to checked under load (i.e. when the 300vs are conducting.) The question is where. You will have to work it out by following the wiring and PCB through the power supply to the point where the B+ connections are made to the primary winding of the output transformer. You may want to this B+ measurement for both the original 5U4M and new 5AR4 to determine the difference. The plate voltage you will measure (under load) at the other end of the primary winding where it is connected to the 300b output tube plate. You need the plate voltage to calculate the 300b dissipation.
2. Measure the voltage drop in the 300b cathode resistors (of known or measured resistance) and calculate the current through each 300b and make a note each cathode voltage.

Dissipation in each 300b will be the current through each tube times the voltage across the tube (difference between the plate voltage and cathode voltage). Then compare your operating conditions with those listed in the WE data sheets for 300b, which list a long list of acceptable conditions. Hopefully you are within those.

Thank you very much for your detailed response, I know just enough about tube amps to understand how dangerous they can be.

I just got some alligator clips to build test leads and found my old analog VOM that can measure the voltages involved, so with your help I now have everything that I need. I like the gradual power-up of the indirectly heated rectifier but don't want to kill my 300bs.

As I mentioned above, I'm using bucking transformers to reduce my input voltage from 122 or 123v from my wall to 110v so my heater voltages are right on. Unless I misunderstood how transformers work, I've also reduced my B+ by 10% compared to plugging directly into the wall. It sure sounds like doing the rectifier swap without the bucking transformers would be an especially bad idea!

I'll report my results back shortly, probably tomorrow.

Take care,
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Nothing more .
Breeze audio(Weiliang,Hifi Exquis) is a brand that produces a whole bunch of stuff dedicated to audio, finished products and kits of all kinds.
I will not be surprised if you find a diagram on the net, either of the product itself or of someone who would have drawn it himself.
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As I mentioned above, I'm using bucking transformers to reduce my input voltage from 122 or 123v from my wall to 110v so my heater voltages are right on. Unless I misunderstood how transformers work, I've also reduced my B+ by 10% compared to plugging directly into the wall. It sure sounds like doing the rectifier swap without the bucking transformers would be an especially bad idea!

It is good that you used the bucking transformer to supply the line voltage it was designed for.

The 300b can function over a relatively wide range of conditions as the WE data sheet shows. Without a schematic of the amp and primary impedance of the output transformers I think the measurements you are taking and calculation of dissipation, compared to the published operating conditions, will suffice for considerations of safety and longevity of the output tubes.
Okay, I'm not sure that these numbers entirely make sense to me but they certainly seem conservative.
5U4M - 5AR4
383.7 -- 385.4 B1
369 -- 369.2 B2
29.6v -- 30v 470ohms
63.6 mA -- 63.8 mA
0.93 Watt -- 1.04 Watt

I was using a CD player as input and had a small speaker hooked up and the amp was playing at a fairly low volume.
I followed the wires from the output transformer back to the board and took my measurements from each end to ground. They are the red and blue wires to the right labeled P and B.
I'm less sure that I found the cathode resistors, lol. In any event, the voltages certainly appear well within limits.



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I have been listening to these amps quite a bit and I'm very impressed with them. So, I decided to replace the WIMA MKS coupling caps with something better. I decided to go with Audio Note Copper Foil caps from Parts Connection.

The removal and installation went well enough, although I think that I installed one of them backwards. My understanding is that it is "preferred" to have the outer foil facing the lower impedance end of the circuit, which is presumably the tube side. I'm not going to switch the orientation at this point, lol.

The operation was a success in the sense that they are back in the system and playing music, and it sounds like music, so I didn't break anything! At first listen, with the system ice cold it sounds good. It's currently warming up and I'll spend some more time with it this evening and report back when they have a chance to break in.

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My new Audio Note Copper Foil capacitors now have 35 hours or so on them, and I'm very happy with the results. The improvements in soundstage depth and focus are substantial, as are the improvements to the system's naturalness and detail recovery.

Audio Note says that this line of caps are specifically targeted to the single ended triode market. They say that the only downside of their new mylar in oil construction is that they take longer to break in than the older paper in oil version and sound worse while doing it, but they will sound better eventually.

I looked up the WIMA cap that I removed, $1.72 each or 70 cents per for 1000. Replacing them with 70 dollar parts took the amps to another level, lol.