freestompboxes.org

I was just noticing the board is all...I can't tell but almost looks like those valves sockets are ceramic, or are they just plastic to look ceramic? I also noticed those capacitors are WIMA from Germany..all and all seems very interesting, I heard some clips on it and it sounded great. Sorry, hard to grasp anything from China being of quality..I normally try to steer away from anything coming out of that particular part of the world if you know what I mean, but this little thing is a head scratcher for me but I'm not a tech. I just found this thread trying to find some information on Wangs

Yes, they are ceramic sockets and wima caps. In their advertisement they mention German wima caps and precision resistors. I am still working on reverse engineering the board. Got a little side tracked. I may just post the hand written version because it will take longer for me to draw it with a program. The input stage looks strange to me. Instead of being cascaded stages, the first 12ax7 is connected with plates and cathodes in parallel, then through volume tone circuit to a guess a phase inverter with plate and cathode going to output push pull configuration. I removed the ceramic sockets because I want to install my favorite sockets with support collar (for shield cover).

Here is a crude hand drawing made as I was tracing the circuit. I need to redraw it again. I am still verifying connections. This is just to analyze the circuit.

Hello mavrim

Since you removed the sockets, you could post pictures of the PCB, component-side? The other pictures were great, but there is no good photo, that side of the board.

Good job with the reverse engineering.

Cheers

Matec

I took many pictures to post here, but they are all over the 1 meg file size limit. I need to resize and or save as a smaller file (less quality).

Hey mavrim.

The pictures are great.
I'll try to make a schematic too, and then we will compare. With these images, will not have too many mistakes.

Thank you very much!

Matec

Thank you!

I don't think it is necessary to figure out a schematic from looking at these pictures. I have the board in front of me and I use my meter to double check connections and get resistor readings. Still have a few components to get value and to recheck. I plan to build this circuit minus the first tube configuration and use the input section as used on the mahaffay plexi which is cascaded.

mavrim wrote:I don't think it is necessary to figure out a schematic from looking at these pictures.

No problem. It's just from force of habit.

Cheers

Hey mavrim,
Great pictures. Thanks for posting. The PI looks like a cathodyne, except there is not feeback resistor to the grid like usually found in this design.

Hello mavrim.
As I said, I made a schematic of your mini amp. Follow here for you to compare.
It is almost entirely based on his scheme, but also has some of my observations.
There are parts I tried to see the value, and if you see that are wrong, please correct. It's a simple scheme, so there are solutions that may not be the best, but that form the tone of the amp.
If you can check the component reference nómeros and I thank you.

Cheers

Matec

Matec, Awsome job!

You saved me from having to make a schematic. I am not that proficient with tiny cad and it would have taken longer for me to do.

Yesterday I spotted a mistake with an extra 300 ohm resistor that I had shown between the two 470k divider network going to output stage. You are correct, the center goes to ground!

On your schematic:
R7 should be 47K instead of 470K
R14 should be 1K instead of 510 ohms
Led anode goes to pins 4/5 of the heater. Led cathode goes to a resistor that is grounded to power ground. Haven't checked resistor value yet.

I will recheck these two resistors, but I am pretty sure I had the correct values. I used an ohm meter and then checked with a jewelers loupe to see the tiny color bands to verify readings.