This is my kit , (available now) pictures shown here are my prototype, it can deliver 2W of output power and also suitable for headphone. I'm designing this amp because some of the local DIYERs asking for simple and low cost but good sounding amplifier for their FOSTEX horn loaded speakers. The 6EW7 consist of one big and one small triodes in one envelope. It is a vertical deflection tube for early TV. 6EW7 have lower gain than 6EM7 but is in same family. I did try to source 6EM7 but quite hard to get this tube locally.
The first stage of this amp only have gain of less than 20 and the second one is around 5~6. The finished amp can only have 1~1.2W if directly input from CD player, Need a pre amp to drive into clipping. Someone tried his homebrew 12AU7 pre-amp for higher power but it sounds not as good as without one. I tested with my friend's 5687 pre-amp, it sounds clean and dynamic, it can play really loud when drive into near-clipping. I use one transistor for power supply to reduce noise and lower impedance. The supply voltage after the transistor is around 225~230V DC. Current draw at output stage is around 38~40mA per channel. I'm using PCB for easier construction, I heard from Audio Research, PCB without solder mask and flux will sounds better than computer- alike board, but this only happen on tube amps.
The driver stage is carefully tune the bias point to get the best result, and distortion measured is quite low, especially without any feed back applied. Because of low Rp of 6EW7, the damping factor also seems quite ok, fast bass can be easily heard compare to my previous EL84 SE amp.
The heat sink shown above is actually a mistake, I forgot to left some space for it. The PCB measures 12cmX 12cm.volume control is soldered on board, for some of DIYers who only request for half kit and test run without any chassis. This little amp sounds transparent, fast and dynamic. sound staging is ok with my TANNOY STERLING. Measured frequency response is 17Hz-24KHz -1dB when output power is 1W. The entry level 6W OPT's static response is around 16Hz-38Khz -1dB. not much different when running at 40mA of DC current. The primary inductance is around 18H and leakage inductance is around 20mH and some photos of square wave image below for reference. For schematic and distortion curve, please click links below:
DISTORTION CURVE at 1 KHz
100Hz at 1W output
1 KHz at 1W output
10KHz at 1W output
Another Silkscreen PCB that I had build for my kit set, with a few parts on it .
Here's a PCB after tin plating process. I'm etching PCB for this kit set. Wanted to reduce cost so that I can sell it much cheaper for beginner. This first batch of PCB cost me quite a lot actually. I need to purchase silkscreen material, Films, photoresist material etc.
Here's how my prototype looks like. I didn't fix the bottom cover . With this chassis, It sounds more clean and dynamic, don't know why but its true. Lazy to install power switch and indicator, I choose a pair of clear top 6EW7 so that the filament glows can be seen easily. The photo from rear view shows the fuse holder and IEC socket. For the last photo,sorry for it's dull color, I'm lazy to setup my external flash light for this last photo......
The transistor for power supply already modified to MOSFET for better S/N ratio and sound quality. I will update the new schematic here soon.
