• 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.

(Re)Building a Chinese 6P3 SE kit

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A few months ago there was a thread about cheap Chinese 6P3 SE amps:

Another Newbie Who Bought a Cheap Chinese Tube Amp Kit

What I said in that thread:

So I ordered one last week. For about $115 shipped, it's not a bad deal. I'll be building it as a triode output with no global feedback and re-working most everything else as well. I'll document it in another thread, but for now, my initial impressions:
1. It's small. Real small. The length/width is about that of a sheet of paper.
2. The iron is close together. Like sharing the same mounting screws close together. I'm a bit worried about magnetic coupling, but we'll see.
3. The power transformer seems to be big enough, from initial mass estimates.
4. The sheetmetal is SHARP. I sliced open my thumb within the first minute of unboxing. Five minutes with a bastard file and some sandpaper and now it's only mildly dangerous.
5. The packaging was excellent for the price point. No bespoke foam liners (naturally), but lots of foam and thick cardboard around everything.
6. The components look to be quite value oriented. Most everything but the iron, chassis, hardware, sockets, tubes, and maybe the power switch will go into the "use, but not on something critical" spare parts bin. Most of the components I use will be from my massive parts stash.
7. The output transformers are small for SE. Maybe half the size of my Edcor 25w P-P transformers. So maybe 5w max in SE?


I’m planning on changing it from single ended pentode with gNFB to a single ended triode with no global feedback, among other changes.

Attached is a picture of the parts after unboxing:
parts.jpg

Schematic I'm going to use. I'm also planning to use a CRCLC input filter for the B+ and also regulate the input triode voltage with a zener/Mosftet follower regulator. I'm also planning to use DC on the input triode heater as well.

The 6N1P heater runs at 6.3v and .6 Amps, which is equivalent to 21 ohms while hot. The heater circuit uses a bridge rectifier and a CRC filter to give me a ~9v tap and a 6.3v tap (with a little AC, but not much).

SE circuit.png PSU.png 12v PSU.png

The first step was to insulate the screws that hold the transformer together with plastic washers and heat shrink tubing (thanks Wavebourne!) to pevent eddy currents and vibration.

Then mark out the wood sides with a sharpie and pre-drill the screw holes. Best to do this before there's no room to work inside the case.

The first test of the top components. Two of the output transformers share a singe screw, and the choke on the underside shares one of the output transformer screws. I hope there's no magnetic coupling, but there's nothing to be done about it now.

transformer_screws.jpg sides.jpg initial_underside.jpg

Here's the first test fit of components. The Ground connection is using a piece of 14ga ROMEX as a bus bar.

I also decided to use solid state rectification instead of the tube rectifier. Because the transformer is a 300-0-300 meant for half wave rectification, the rectifiers will see at least a 600v difference. My Schottky diodes are only good for 600v, so I doubled them in series to give some overhead. They're soldered to a piece of PCB for mechanical structure and mounted to the side by the transformer (the right side of the picture).

hv_diodes.jpg initial_assembly.jpg
 

PRR

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Joined 2003
Paid Member
The 300-0-300 with two diodes is full-wave.

300VAC will rectify out to 420V DC. Your plan shows 350VDC. There's almost no drop in the too-small resistor or choke. Also 180u 10r 180u is almost no different than one 360uFd cap. I figure this resistor will end up nearer 500 Ohms. I'd start with two 500r 10W in series and see where that gets you.
 
If roughly stereo class A amp with 6L6 tubes would consume 120-140 mA of current, and 10% drop under the load would be around 40V, you get like 380V rectified, or about 375V on anodes, with voltage drops on a choke and output transformer primary. Minus cathode bias. If it is too much for 3K5 primary output transformers, then yes, the resistor for a power filter can be added and calculated. For 50V voltage drop and 140 mA current it should have 360 Ohm value. Power loss on it would be 7W, so 10-15W resistor would be fine.
 
The resistor in the CRCLC is primarily to reduce initial inrush and drop enough voltage to keep under 400v across the capacitors until the tubes warm up enough to conduct. The source follower in the driver stage at start up will have a load of about 16ma per channel until the tubes conduct, so hopefully this is enough load to keep the voltage down and the capacitors safe.
 

PRR

Member
Joined 2003
Paid Member
Voltage at first filter cap goes to 421V, no-load (or light-load), and near that at full load.

I need 500r in the middle to get 350V output at an assumed load. Another assumption would give Anatoliy's 350r. Otherwise way up at 400V.
 

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