Jan 28, 2018

C3g Line preamplifier part 2

Power supply

From the beginning this preamp was intended to be an exercise in the 6J5 way of simplicity and I calculated bias such a way they could both share the same power supply. The two units are very similar in terms of HV and current requirements and it helps me to make a quick switch for comparisons.

Parts and construction

Even if the circuit is as simple as the 6J5 line preamp, implementation is different. All parts take place on a thick CNC aluminum sub chassis that fit in a much larger cabinet mainly due to transformers size.
High quality L pad attenuator (Siemens silver contacts rotary switch, Dale RL7R resistors) and other selected components to match these superb transformers. Loctal sockets are original Siemens ones and input rotary switch a Grayhill 44H.

Bias is set by Dralowid vintage German carbon resistors, they have a slight grain and a laid-back sound I like very much.

During my tests I noticed better dynamics and lower distortion increasing RL to 160 Ω. Also the C3g works better without feedback.
Be aware of possible oscillations with high Gm tubes. I used grid and anode ferrite beads soldered next to the socket lugs. Wires should be as short as possible and close to chassis (connected to ground and acting like a shield).

Construction details

pictures are worth a long story

almost done, still waiting for the silk screened front plate

On the bench for tests

Low distortion (0.08%@1Vrms) and good squares (100Hz and 10KHz) for a transformer coupled tube without feedback.

Listening report

The rectifier plays a considerable role in the result and among the many ones I stock I prefer, for this preamp, the Mazda GZ32. This is a great sounding tube and still a bargain for the price. My all-around favorite one is the above mentioned plus Philips 5R4GYS, Belvu 5U4G, RFT EYY13, Mullard GZ30, STC 5Z4GY/S depending upon use.

This preamp needs some burn in time to reveal its own character.
Sounded thin and obscure at the beginning, with “thump” bass, but after 2 weeks listening to different kind of music it gained in transparency and revealed a surprising analytical capability with lot of dynamics as said before. It is now evident that this preamp has a completely different character and appears to be detailed to the extreme (it makes me feel the 6J5 sound coarse, which is not). This restitution, like under a microscope, can be an inconvenient on poor or just average sources. I have some CD's, mainly ADD's, I can't listen to because I hear the surface noise of the original material! The C3g is an incredible information catcher but with very little soul.

This, to my ears, puts the preamp on the cold side of music reproduction and I will reserve it for string music, piano or small baroque formations. For me it is not an all-around preamp, but I have to say that my trials were made with the 801A amp/La Scala speakers, a very “clear” combo and probably not the best suited for this tube. I would certainly have a different perception using my Wharfedale Super 12 RS/DD and MC1/60 amp but with a very small listening room it is not possible to have the two systems at the same time.

Another interesting point is a great 3-dimensional restitution, almost like the E80CC SRPP. This peamp has a wider / deeper sound-stage that gives the sensation of being surrounded by music. The 6J5 is flat by comparison. If you like that kind of sound the C3g is the tube to go, taking in consideration that it can be the fact of the tube, the transformer or both...
However, ambiance is not rendered accordingly, and I miss the very living like 6J5 restitution, its "rough" sound gives a more human and pleasantly textured music.
Once again it is just a matter of taste and not a definitive judgment.

some CD's for this test


  1. I finished building up my own C3g preamp using similar topology. I'm using a Tango NP-126 OT (the input is through a Jensen JT-11P-1 at 1:1 and then onto a 10K potentiometer before the C3g grid). I too find the C3g very detailed. However, initially it was a bit too analytic, bordering on harsh.

    I lowered B+ to give about 140V on the anode and changed Rk to 130-ohms. This places the tube at 14mA with +1.8V on the cathode. I also re-wired the NP-126 primary from typical series (10K-12K) to parallel (5K) since the Rp of the C3g is about 2300 ohms (this also improves the current rating of the NP-126 from 10mA to 20mA).

    Now, the preamp is very smooth and 3D; the width and depth of the detail is astounding. I'm very pleased with the sound (my wife independently noticed the same improvements).

    Thanks for sharing your work,


    1. Hi Robert, thanks for this report and I must admit the C3g is certainly at its best with a lower load. I actually test a revisited LIE Belin with E90CC paralleled and the sound is much more appealing. Have to make further test but I think it is a better choice with these transformers