Gyrator Loaded 12b4A: The Latest Version.

[SGregory]

I know that 12b4A preamps are old news, but they are fun none the less and a good challenge with excellent sound quality. So here is my latest that I present for critical review and possibilities to improve. All comments are welcome as I have not put this in a chassis yet with the expectation of some ideas to improve it and/or correct my errors.

The goal, Low Noise, Low Distortion, Minimal Gain, High Bandwidth. No intentional Feedback. Also minimize the tube tendancy to be microphonic and vibration sensitive.

The Method To the madness. 250Vct, tube rectification, CLC filter with Hash element. 280Volts into a Salas HV Shunt Regulator holding 220V. Then to a Wavebourn Gyrator for a Load. From previous experimentation, I chose LED bias that is bypassed.

12b4 Operating Point. 130V Anode, 16V bias, with 15mA idle current. I chose this point based on a series of measurements previous to this build to identify a reasonable lower current, low thd operating point.

I also wanted to eliminate some of the issues of the 12b4 by mechanically isolating the tube from the chassis.

Attached is the schematic of the gyrator loaded circuit, followed by pictures of the etched and loaded pcb.

[SGregory]

I forgot to mention that the filament is regulated DC that is lifted to 20V.

In the following photos the stereo prototype test bed is assembled and commisioned.

As it turns out the heat sink wasn't vented well enough so it was raised off of the board by 1 cm to promote better airflow. In addition the current was dropped from 63mA to 45mA (which is still a little strong) in the shunt reg to reduce heat.

[SGregory]

It hit the limit of my current test set-up with respect to noise. In the following 1Vrms = 0dBs.

The first FFT is the baseline measurement with and without a 1kHz signal.

The second on shows a 1 kHz comparison versus a more conventional OD3-CCS loaded 12b4A

The third plot is a freq sweep of the Gyrator vs. the CCS.

Finally the fourth plot shows IMD of the Gyrator vs. CCS.

The Gyrator loaded unit measured 0.0117% THD with a s/n ratio of 78db.

[SGregory]

The following photos are of the square wave response at 1kHz, 10kHz, 20kHz, and 100kHz.
I tried to hit a perfect triangle at 250kHz but not quite there.

[SGregory]

As a comparison these are of the CCS loaded 12b4 that was replaced.

Obviously the original ccs unit was having a hard time at the higher frequencies

[SGregory]

In summary, the gyrator loaded 12b4 presented here is starting to approach my goals. Mechanically, I need to soften the mechaical isolators and possibly add some weight to the pcboard. It is a HUGE difference than from running it rigidly coupled to the chassis.

Electrically, it is certainly a huge step in the right direction. Without getting too far in explanation and descriptions, it is very musical, very quite, and very detailed.
I need to get back to listen some more ;-)

The question now is what else can I do to improve it?

[massimo]

Gregory, where did you source those blue grommets?
In order to add more weight, I would use a piece of copper bigger than the pcb, cut a hole for the tube socket, screw it on the top and then to the wooden pillars with the grommets.

[SGregory]

I purchased the gomments from Grainger. Look under Isolation Bushings. I will see if I can find them and post a link

In the final Chasis, the pcbboards will be suspended from the top plate, but your idea of the copper can be made to work.

Thanks.

Try This. It was from McMaster Carr and not grainger
http://www.mcmaster.com/#vibration-isolators/=aenvzf

[DualTriode]

Hello SGregory,
Good job. You have it all going on!
How does it sound?
DT
All just for fun!

[massimo]

Originally Posted by SGregory

In the final Chasis, the pcbboards will be suspended from the top plate, but your idea of the copper can be made to work.

That's my idea for a #26 line stage: copper sheet (1.5 mm thick I've in my workshop) with two UX4 socket suspended from the top with some elastic bushing or even spring. Still to be elaborated