mercredi 27 août 2014

G9 51x DIY tube mic preamp

What is the G9 51x?

G9 51x tube mic preamp
I bought a PCB for a tube mic preamp some time ago: a G9 51X. The G9 is a famous tube preamp design from Gyraf Audio in Denmark. It has been tweaked so that it fits in a single 51x module. What a feat!

The 51x format is kind of an extension from the API500 format. 2 more pins which deliver +/- 24V. You can learn more about it here: GroupDIY 51X Alliance.
The 51x format is well regarded and used on the GroupDIY forum. You can find a list of 51x projects here: list of DIY 500 and 51x projects.

Build

I closely followed the advices from the designer of the board on GroupDIY. Those were important and helped having a smooth building process.
Basically, how to solder the little lugs so that your tube will fit when you insert it (you can ignore this and take the risk to break your tube of course), fitting from the elements on the faceplate, some mods are required so that the pre works better, how to solder the high voltage converter, and so on.

The build itself is easy if you proceed carefully. Solder from the smallest to the tallest elements. 
There is a vertical diode which is not silkscreened really clearly between the 2 caps C502 and C504.
For the HV converter, I would advice to solder C503, then D501, then the inductor L501 because D501 will get on the way of C503. Well at least keep an eye on C503 because you need to solder it flush to the PCB or you'll have clearance issues.

I used the OEP transformers because I had them laying around (well and because I couldn't afford to buy expensive transformers when I have cheap ones available).

Costs

I bought the PCB+frontplate long ago. I think I paid 25-40€. The faceplate is a new one from http://frontpanels.de which explains why it's different from the original faceplate.
Tubes Westinghouse NOS: 40€
Components: 61,30€ 
I used the BOM from the first post of the thread on GroupDIY about G9 51x. It was fine even though I had to find some replacement parts because some weren't in stock.
Transformers: roughly 30€
Total cost 150-200€
Well and I bought a 51x rack from Vandaal, so it's 550€ more expensive.

Pictures



You can use those pictures as a reference because the unit is in working condition.