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Tuesday, April 25, 2023

Tube of the Month : The 6S4A


This month we have a look at a 9 pin miniature medium mu triode, the 6S4A.

This tube, like so many, was developed for use in the deflection system of TVs but can serve in other applications as well including AF amplifiers.

It is a single triode on a 9 pin base. The pinout is shown on the left. It has an amplification factor of 16.5 which comes with 3700 Ohm plate resistance. The plate can dissipate up to 8.5W which is not bad for such a small tube. As the 6 in the designation indicates it has a 6.3V heater which takes a moderate 0.6A of current. Like so many TV tubes you will  hardly see this one used nowadays although probably still easy to find. The low amplification factor and the fact that absolute linearity was not a priority when this tube was designed limits the usability somewhat. But I would see it working nicely in a linestage or as DAC or phono output stage. Check out the data sheet and decide for yourself if it is worth to give this tube a try. As always we compare the data sheet plate curves with those of an actual tube:

And here a few samples, starting with RCA:

Some close up shots:

Higrade branded 6S4A:

Westinghouse in military box:

General Electric:

Canadian Westinghouse:

And lastly a few photos of the tube in operation:

Best regards


Tuesday, April 4, 2023

The Compactron Preamplifier


In February I showed the prototype build of my new Compactron preamplifier concept. I am happy to report that it is finished in the series production chassis.

The prototype measured a whopping 65cm in depth. Clearly too much for most racks or side boards. So a lot of effort, layout and planning was needed to cut that number down. While the result still is not exactly small, it measures 434mm by 434mm now. Height is 100mm plus feet. Here is the final product:

Let me give some background about the motivation of this preamplifier which is the first of a series of different preamps I am going to offer. Many of my customers are vinyl lovers who have multiple turntables or a turntable with multiple tonearms and cartridges. So I have often been confronted with the request to allow multiple inputs and possibility to use both MM and MC cartridges. My D3a phono preamplifiers are simplistic designs optimised for shortest possible signal path. For that reason the MC step up is always integrated in those. In addition the D3a has rather high input capacitance which is no problem for MC cartridges but could be too much for certain MMs. 

So I decided to design a new circuit based on different tubes which offers the possibility to use MMs as well. At the same time I addressed the possibility to adjust input impedance and capacitance via switches on the front. Another criticism I receive often is the sheer size of my 10Y/D3a preamplifier combo which consists of 4 chassis total. So the result is a full function preamplifier with line and phono inputs. And the power supply is integrated as well. All in one single chassis. Here a peek into the chassis when it was fully assembled:

My biggest concern was to get it hum free with a sensitive phono section and power supply all in one chassis. To address that the signal tubes are all located at the back side close to the inputs. All switches are located close to the tubes for shortest signal path. Only the power switch and the volume control switch are directly behind the front plate. For the latter this is not critical because I decided to build this unit with a maxed out line section with transformer output and transformer volume control. The volume control receives a very low impedance signal which is not prone to hum pick up. The power supply is located at the front and I could keep tube rectification and choke input filtering. the latter even for the heater voltage.

I am happy to report that the unit is not just low hum but hum free. Even without using lesser quality toroidal transformers which I already got made just in case. I could use the same high quality power transformer as in my other preamps. Above a photo half in-between construction. Here you can see the horizontally mounted tubes which give the preamplifier its name: All tubes in this unit are 12-pin Compactron tubes. The tube lineup is: 6FM7 and 6FJ7 in the signal section and 6BE3 in the power supply.

Another highlight: I managed to avoid electrolytic caps in the high voltage supply alltogether. The only electrolytics are 2 EPCOS caps in the heater supply. And yet another feature for users of many different cartridges: The gain of the phono section is selectable in 4 steps. These are the gain numbers (total gain at line out):

low gain setting: MM 42dB, MC 60dB

medium gain: MM 48dB, MC 66dB

high gain: MM 54dB, MC 72dB

maximum gain: MM 60dB, MC 78dB

You asked for all the flexibility and adjustability, here you have it.

The gain switch was a late addition and it is a bit crammed into a corner on the backside. It could also be located at the front if preferred. This switch is not just selecting different voltage dividers but changes the actual gain of different stages by adding bypass caps to the cathode resistors.

Above the back panel. Gain selector on the left, then outputs (XLR and RCA). Next a Balanced (XLR) input followed by single ended inputs. A ground lift switch in case of a ground loop in the system. Then 2 MM inputs and 2 MC inputs selectable via a front knob. The MC impedance selector has a setting marked custom. This can be used if a more specific load resistance is desired. In that case the input can be loaded with RCA plugs with resistors soldered in. These plugs go into the right most jacks. Next the ground post, mains inlet and fuse holders.

Nowadays I see many amplifiers and preamplifiers with a lot of gimmicks on the front, with nicks, crevices or waves. But I wanted something simple and discreet. Just function. A bit of a reminiscence to classic gear from the 1980ies. The result is this preamp 25kg of pure analog delight. No PCBs, no microcontrollers, no LCD display, no relays. All hand wired.

As mentioned this is just the first of a series of Compactron preamps. If you are interested in something like this, you can specify the features you want depending on your need and budget. There will also be phono and line only versions. Above the layout for a simple phono without any adjustability. Or a phono only with the same functionality as above minus the line section:

Here shown with the gain switch moved to the front. Possible options for a phono unit are transformer coupled output or just RC coupled output. Also more inputs can be added and even several different pairs of selectable built in MC step ups. Or variable EQ to allow proper playback of historic records:

Or MM only:

Want it all? Can do, just need to double the height:

Or just a simple line stage only with input selector and volume control:

The linestage can be with transformer coupled outputs with volume control or a simpler RC coupled unit.

Or need more line features like Mode select, phase switch and tape loop?

As you can see the possibilities to customise are endless. Also headphone amps can be built. Hope you understand that it is impossible to list all features that could be done and calculate their price. If interested drop me an email with a list of your requirements and I will send a quote. As an indication prices for simple phono or line only versions will start at around 5000 Euros (EU price including VAT, 4200 export).

Best regards