Hello from Croatia,
BE CAREFUL INSIDE THIS PEDAL THERE IS 240VDC. Unplug it and leave it 5min before opening.
First thanks to Bajaman for his post about VT911 and Baja real tube driver.
That inspired me to write this post. On eBay or on Alibaba you can by OTD-100.
I bought it for 64 GBP with delivery from Hong Kong. Maybe for some of you it will look expensive but it has 12AX7 inside and 240V on plate. Similar pedals have price 400 USD or more.
It is quality built with quality components. Look at the picture of PCB.
From factory, without mod it sounds good. Very nice sound with my Fender Stratocaster 1978 and Fender Bassman 100W. The Box is from Spider Croatia with Eminence Texas heat inside.
However for my taste, this distortion needs more gain. It was good for riffs and chords but not for solo and Gilmour sound.
I couldn't find the schematic on web so I contacted Biyang by MSN messenger and they were kind but they didn't give me the schematic. Probably they want to protect it.
So I decided to reverse it and track the schematic.
Now I have schematic for complete audio path without output tone stack but I will track it too. I measured capacitors because there is no mark on SMD.
If you are interested I will post schematic in next few days. I am very busy man and I can't post all right now.
Also I have simulation in LTspice with graphs.
Take a look on PCB pictures.http://www.box.net/shared/vvyxr0x2ob
There is small PCB with SMD and 4558C OP amp. http://www.box.net/shared/u85dqsb787http://www.box.net/shared/mafbxfa3lq
The mod is to change cathode resistor on first triode. It is 10k and should be 2.2k or 1.5k.
You can add resistor in parallel with existing 10K like on picturehttp://www.box.net/shared/7lad5zt8qu
After this mod there is lot of gain and the sound is like Gilmours. Real tube driver sound. However I have in plan to put pot 10K in series with this resistor. This is because the sound without this mod is interesting. With mod it is good for solo but for riff and chord the sound without mod is better (more direct). I think that this pot will be something like bias on chandler tube driver. The pedal will be more versatile with this pot.
Schematic and more about this in next few days.
It will be more clear then.
Nenad Mihajlovic, Krizevci Croatia