A fellow on this site was offering a little 4 tube amplifier kit for a very reasonable price. I love building kits and decided to buy it. It is offered by S-5 Electronics and is the K-12G , 8w/ch amplifier. Fun kit to build. Took about an hour and a half. No issues except I had a polarized cap in backwards.....Two of the three face one way and the third is opposite....It got me!.....Flipped it around and all is good! Here is a quick start up video: https://public.fotki.com/Rbertalotto/hifi-stereo-stuff/4-tube-power-amp/p1050125.html
Attachments
Here we go again with the "No Ground" issue. This amp is not AC grounded and doesn't even have a polarized plug. I'm going to have to do some research on how to add a three prong plug and where to ground. Any advise is appreciated.
Without a complete and accurate schematic of your amplifier, giving any advice on how to ground the amplifier is not a good idea.
It might be a good idea to use some sort of cover / case, to prevent curious fingers, cats, and dogs from contacting dangerous voltages. Yes, I admit most points are only accessible where the wires go through the PCB.
Safety First!
Prevent the "Surviving Spouse Syndrome".
It is hard to sue a company that is another country.
But if there is a "Surviving Spouse", and the company is rich, there is a lawyer who will take care of that.
It might be a good idea to use some sort of cover / case, to prevent curious fingers, cats, and dogs from contacting dangerous voltages. Yes, I admit most points are only accessible where the wires go through the PCB.
Safety First!
Prevent the "Surviving Spouse Syndrome".
It is hard to sue a company that is another country.
But if there is a "Surviving Spouse", and the company is rich, there is a lawyer who will take care of that.
Last edited:
If you have power mains that have 3 wire with ground, it is a good idea to use a 3 wire IEC power cord, and IEC socket on the amplifier. The IEC ground needs to connect to the aluminum chassis.
The power transformer, and output transformers need to be grounded to the the chassis (transformer end bells paint needs to be scraped and use lock washers to ensure ground of the end bells and laminations. And the output transformer secondary needs to be grounded, both for safety and for the negative feedback to work.
I would guess that most of your signal sources have 2-wire power cords, and no ground connection to mains power outlet.
That is OK for consumer products such as a CD player.
You have to pay attention to the multiple ground loops that can cause hum.
Mainly: the B+ ground loop; and the signal input ground loop.
The negative of the B+ bridge needs to connect directly with a short wire to the negative of C8, and a wire from negative end of C8 to negative end of C9. Then connect from the negative end of C9 to the central ground point, which also connects to the chassis there.
Connect the ground ends of these parts this way:
R1 to R3, then to central ground
R7 and R6 to R8, then to central ground
C5 to R1 and R9
The other channel is done the same way
The RCA input connectors need to be insulated from the chassis, but then connect the RCA input connector ground returns with a very low length wire to the bottom (ground end) of the volume controls and the ground ends of R1 and R9 (each channel wired individually).
The power transformer, and output transformers need to be grounded to the the chassis (transformer end bells paint needs to be scraped and use lock washers to ensure ground of the end bells and laminations. And the output transformer secondary needs to be grounded, both for safety and for the negative feedback to work.
I would guess that most of your signal sources have 2-wire power cords, and no ground connection to mains power outlet.
That is OK for consumer products such as a CD player.
You have to pay attention to the multiple ground loops that can cause hum.
Mainly: the B+ ground loop; and the signal input ground loop.
The negative of the B+ bridge needs to connect directly with a short wire to the negative of C8, and a wire from negative end of C8 to negative end of C9. Then connect from the negative end of C9 to the central ground point, which also connects to the chassis there.
Connect the ground ends of these parts this way:
R1 to R3, then to central ground
R7 and R6 to R8, then to central ground
C5 to R1 and R9
The other channel is done the same way
The RCA input connectors need to be insulated from the chassis, but then connect the RCA input connector ground returns with a very low length wire to the bottom (ground end) of the volume controls and the ground ends of R1 and R9 (each channel wired individually).
Don't buy hobbyist kits with bare parts on a wood board. (Did the kit supply that board?)
Aside from stray 120VAC, there is 220V?DC on parts pins, and the power transformer apparently runs so hot they put a warning on it. Getting a wedding ring across the heater circuit might heat the ring and burn your finger to the bone.
Yes it did. A VERY nice board at that. These amps have been sold for a couple decades, thousands have been sold. (Along with a few hundred thousand Dynacos, ELAC, Heathkit, KnightKit, electronics with two prong plugs).....Not one person has died or lost the ability to count to ten. Folks that are DIYing tube amplifiers know where not to put their fingers.........
I agree with PRR.
It was very late here when I typed Post # 5.
I was not thinking properly, your kit uses PCB construction, not point to point wiring.
Safety is the thing to take care of.
The amplifier circuitry is Floating, the parts are exposed, and that is a danger.
A 3 wire power cord, and a cover is needed.
Cats, Dogs, and young kids do not read.
The output transformer secondary needs to be grounded, as in the schematic.
The power and output transformers mounting screws do not connect to the PCB ground, they can have leakage currents from the U-frame, or windings might short to the frame (laminations and U-frame). Needs to be grounded, wood is a very bad conductor.
If the amplifier is mounted in/on/to a chassis, the amplifier ground needs a connection to the chassis.
And the 3 wire IEC connector needs to connect to the chassis.
If there is any hum caused by wiring or ground loops, there is nothing that can be done to the PCB.
The only ground loops that can be modified are external wiring to the PCB.
The RCA input connectors need to be insulated from the chassis, but returned to the amplifier ground to reduce that ground loop.
It was very late here when I typed Post # 5.
I was not thinking properly, your kit uses PCB construction, not point to point wiring.
Safety is the thing to take care of.
The amplifier circuitry is Floating, the parts are exposed, and that is a danger.
A 3 wire power cord, and a cover is needed.
Cats, Dogs, and young kids do not read.
The output transformer secondary needs to be grounded, as in the schematic.
The power and output transformers mounting screws do not connect to the PCB ground, they can have leakage currents from the U-frame, or windings might short to the frame (laminations and U-frame). Needs to be grounded, wood is a very bad conductor.
If the amplifier is mounted in/on/to a chassis, the amplifier ground needs a connection to the chassis.
And the 3 wire IEC connector needs to connect to the chassis.
If there is any hum caused by wiring or ground loops, there is nothing that can be done to the PCB.
The only ground loops that can be modified are external wiring to the PCB.
The RCA input connectors need to be insulated from the chassis, but returned to the amplifier ground to reduce that ground loop.
Last edited:
The amplifier has very low hum. Speaker on ear with dead quiet room hum. One channel has a buzz. But switching the tubes, left to right moved the buzz to the other channel. New tubes are on the way...Thanks
The sticker on the power transformer informs readers that tubes get hot and that there is high voltage involved. It doesn't say that the power transformer itself runs hot.
Since I restarted this hobby some 15 years ago, I have built a lot on wooden boards. For me it was more or less a natural continuation of the breadboarding I did in the beginning, but getting neater and better (like for instance grounding all metal parts on the board) with time. We don't have children or pets (and I don't wear rings). My diy equipment is all in my attic so as far as I was conceirned there were no irresponsible saftey issues (I know and trust myself enough for that; from about 8 years of age I was fiddling around in working TV's with live chassis and survived all that too).
Only when using the high-sloped ECC88 (in a 85 Hz crossover) and E130L I encountered some oscillation problems which could have to do with my way of building. I also (ofcourse...) experienced that with low level sources like phono you need shielding, so you need to use some kind of metal casing. But except for an OTL amplifier with E130L's I didn't encounter any hum/noise/oscillation problems with the amplifiers I made for line level input, including the ones with GNFB.
Since a couple of years I only build with metal chassis, repurposing chassis from tube radio's and other tube equipment. My old style of building began to take up too much of my space.
Built a cabinet for the little amp today. Will be making some kind of cover with metal rods to keep curious hands away. Need to figure out where to get 12V AC to run the meter driver board.
You have the 12 Vac filament supply. But because that supply is grounded (at the centre-tap of the filament winding) you most likely can't use it also for the meter driver board since the meter driver must have a connection to ground too (which has to be made somewhere after the rectifier circuit on the meter driver board).
A possible solution would be to undo the grounding at the centre-tap of the filament winding. The filaments than more or less 'piggyback ride' on the grounding of the meter driver board. But this is far from ideal and might lead to hum/noise (the first stage of your amplifier has an unbypassed cathode resistor which makes it a bit more prone for filament hum issues).
So if there's room enough in the cabinet for a small 12 Vac transformer (use a toroid for low radiation), than probably that is the way to go.
A possible solution would be to undo the grounding at the centre-tap of the filament winding. The filaments than more or less 'piggyback ride' on the grounding of the meter driver board. But this is far from ideal and might lead to hum/noise (the first stage of your amplifier has an unbypassed cathode resistor which makes it a bit more prone for filament hum issues).
So if there's room enough in the cabinet for a small 12 Vac transformer (use a toroid for low radiation), than probably that is the way to go.
Thank you...I saw that 12V on the schematic, but realized that it was more complicated that just tapping into it. A small Xformer will be ordered.
