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Tuesday, August 27, 2019

Tube of the Month : The 6BX7


In last months tube presentation of the 6BL7 I briefly mentioned this tube. Have a look at the 6BX7.

The 6BX7 belongs to a family of 3 medium mu double triodes along with the 6SN7 and 6BL7. They differ in amplification factor, transconductance and plate resistance.

All 3 of them share the same octal base. The 6BX7 has the lowest amplification factor of them with a mu of 10. But this comes with a low plate resistance of 1300 Ohm. Wire both triodes in the bottle in parallel and this halves to 650 which is in the territory of a 2A3 output tube. This is one hefty triode for such a small bottle. Its characteristics would also make this an excellent driver tube with quite large voltage swing capability. Besides a flea power tube amp it would also serve well in a headphone application or line preamplifier. Another tube which is rather forgotten but deserves more attention. I did not have any of these in stock, but after covering the 6BL7 last month I thought it would be a good idea to get some for completing the series of 3 cousin medium mu double triodes. To my surprise they were easy to find at modest prices. As always we will start with a look at the plate curves both from the datasheet and from a real life sample.

I only got some 6BX7GT from 2 different manufacturers. Here some made by General Electric:

From RCA I have samples with three different construction styles.

The first one has a clear top and the getter on the side.

Those little boxes on the top are cooling fins for the grids.

Next we have a coin base version:

And lastly a version with the getter applied in the top dome.

And now the tube in operation:

What a neat device. So many tubes, so little time to try them all.

Best regards


Wednesday, August 21, 2019

The SplitPath EQ Phono Stage


Since a few years I had an idea how to implement the LCR EQ in a different way, but kept it on the back burner due to other projects. Now I finally took a shot at what I call the SplitPath EQ.

This concept drastically reduces the signal path for part of the frequency spectrum. In my quest to improve sound quality keeping the signal path as short and direct as possible always was a key factor. But at a certain point there is a limit reached since total gain needs to stay within practical limits and I always want to keep output impedances low. I always feared that this new approach would run into various problems in an actual implementation but I am happy to present the first working prototype which was implemented with D3a tubes.

Above the phono section which like all my preamps has an outboard power supply. While the phono section received a new layout to adapt to the requirements of the new circuit, the power supply remained the same.

Some technical background about this idea. One of the main tasks of a phono stage (besides providing a lot of gain) is the EQ of the signal. When vinyl is cut, the bass frequencies need to be attenuated and the highs boosted. This is to avoid that the grooves would need to be cut with excessively large amplitudes for each bass frequency hit, while the higher frequencies usually are not having very big amplitudes. Below a simplified diagram which shows how the signal is attenuated relative to 1kHz.

Below 500Hz the amplitude is dropped off by 6dB/octave down two 50Hz below which the attenuation is stopped. At 2122Hz there is another corner above which the amplitude is boosted by 6dB/octave. In the phono stage this needs to be reversed. In a passive EQ this is done with a filter which attenuates the various parts of the spectrum accordingly. That means that the highest frequencies are attenuated by 40dB more than the lowest. This always bothered me since the signal needs to be amplified accordingly only to be attenuated again in the EQ. Now my thought was why not handle the part above 1kHz separately from the bass EQ. In simple terms what I did was to let the higher frequencies bypass an entire amplification stage and recombine them with the rest after the bass EQ.

So the frequencies above 1kHz pass through only a single gain stage in the phono, while the bass frequencies pass through 2 stages as in my previous phono circuits. The actual EQ is done with LCR networks. The challenge was to recombine the frequency parts with the right levels to have correct overall frequency response and phase behaviour.

Please understand that I do not disclose more details or schematic diagrams. This circuit contains all my knowledge and experience from decades of working on phono circuits.

Here we see the construction steps of the prototype.

Everything is placed such to have minimal signal path length and well defined and short ground paths.

Lots of trials, measurements and simulations were required to get things right. Here the completed phono:

How does it sound? Well I got what I was hoping for. Yet further enhanced resolution and transparency in the frequency range above e 1kHz which results in extremely natural voices and beautiful reproduction of ambience. Also the noise floor dropped even further.

As with all things in life, there is no free lunch. The overall gain is quite a bit lower compared to my current phono. So attention is necessary to integrate this phono into an existing system. In my system I can use it without issues with Lyra cartridges and the 10Y linestage. However I am at the upper limit of volume control range. To address this I am about to develop a new linestage which will have a dedicated input for this phono to provide a bit more gain.

This new phono stage will not obsolete my current one which has more gain and is more universally useable. Both versions reside alongside in my portfolio with this new one being the higher end alternative. Next step will be to implement this EQ approach also in the differential phono. Stay tuned.

Best regards