6AU6 + 6GU7 Stereo Preamp
Here is a two-channel boutique ribbon mic preamp I made a couple of years ago and decided to make a little post about it. It Features a 6AU6 pentode pre-stage, along with a 6GU7 dual triode cathode follower output stage. The input and output transformers used to build this unit are Vintage UTC special series mic input transformers and FREED military grade output transformers.
This preamp is small and light, great for the field or live shows as well. I decided to add a filter circuit to this unit, which will be pretty useful. The filter section is placed in the feedback loop path and is a twist off of the baxandall style EQ.
Mystery Magicians 6AU6+6GU7 Mic Preamp One Off Build
There is also selectable VU metering for both channels via the mini toggle switch in front of the unit. The meter is buffered from the balanced outputs so it does not load down the output transformers or introduce harmonic distortion and other artifacts.
Overall it has a very rounded and balanced sound! I like it with Coles 4038 Ribbon microphones. As for the tubes, 6AU6 sounds good but you have to hand-select a good pair for very low microphonic, and rim lock tube shields are a must since they are not internally screened.
6GU7 Triode was originally designed for matrix circuits in TVs and general-purpose amplification. It is a medium mu triode like 12AU7 and 6CG7. it 6GU7 has an amplification factor of 17 just like 12AU7, but unlike 12AU7 it has a lower plate resistance of 5.5K vs 7.7K, which makes this tube an even better candidate for CF configuration and better all together.
A quick look at the triode curves of 6GU7. It’s a great little gem for audio applications.
6GU7 Triode Curves
