The Compactron Line Preamplifier - part 1

Hi!

A Compactron Line preamplifier is in the making. Here some initial fotos.

This is a line only version of the previously introduced Compactron Preamplifier. It uses the same chassis style with brushed aluminum front and backplates and sandblasted top plate with my name embossed.

This time anodised in black, which was chosen by the customer who commissioned this preamplifier. He also decided the arrangement of the knobs on the front, features like the phase switch and of course the number and type of inputs.

The preamp will have one balanced and three RCA inputs.

Outputs are present in both balanced and SE fashion.

The chassis plates arrived and construction can begin.

The circuit is transformer coupled and volume control is done via a 24 step AVCs (volume control transformer) for low output impedance. This could also be used to drive high impedance headphones (300Ohm or more) but the customer did not require a headphone jack.

Black anodised knobs have been made for this preamp. But other colors can be chosen as well for some contrast:

Or stainless steel:

The internal sub modules are already assembled:

 

Power supply with two 6BE3 rectifier tubes and the signal tubes are two 6FJ7. The latter have been chosen for the required gain. Such a preamp can be ordered with different gains and also with different features like head phone jack, tape loop, etc.

Contact me if you want your own Compactron preamplifier customised to your needs.

Stay tuned for an update when the assembly of this preamp is finished.

Best regards

Thomas

The Compactron Preamplifier

Hi!

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

Thomas

Tube of the Month : The 6FJ7

Hi!

After the choice of the 6FJ7 as driver tube in the recently developed 6KN6 single ended amplifiers, it is probably no surprise that this tube is featured as tube of the Month.

The 6FJ7 is a 12 pin compactron equivalent of the 6DN7. It is a dissimilar double triode, which means it has two independent triode sections with different parameters.

The 6FJ7 shares all electrical parameters with the 6DN. It only differs in the base. The compactron base pin out is shown on the left. Most dissimilar double triodes have a low or medium mu section and a high mu section. But the 6FJ7 and 6DN7 have two sections with medium amplification factors. Section 1 has a mu of 22.5 with a plate resistance of 9kOhm and a maximum plate dissipation of 1W. The second triode has a mu of 15.4 combined with 2kOhm plate resistance and a whopping 10W maximum plate dissipation. The latter would make the second triode suitable as low power output triode as well. The combination of parameters made this tube the perfect choice as input/driver tube in the 6KN6 amplifiers. See that post for further details why this tube was chosen. All technical details can be studied in the General Electric data sheet. When I picked this tube as driver for the 6KN6 amplifiers I did not have
any of them in stock. So I had to buy a small selection to be able to show some variety of tubes in this post. This is a nice example that there are still many tubes out there waiting to be put to good use. Especially Compactron tubes keep being largely ignored by tube amplifier builders. So it is easy to source them. All of the tubes shown in this post have been bought within the recent weeks. Compactron tubes were manufactured at a time when the vacuum tube technology was very matured and they were produced in quite large numbers. As they were introduced when the transistor started to take over, thye did not get widely adopted but got made in large numbers, more than the market consumed. So tube dealer warehouses are still full of them.  Go get some and build tube gear with compactrons!.

As always let's start with the plate curves. Section one above and section 2 below.

And some real life curves taken from an actual tube with the tracer:

Now let me show you the small collection of 6FJ7 tubes which I acquired recently. Starting with General Electric, the inventor of the compactron base.

The tube from different angles:

Here we clearly see the higher power section on the left and the weaker triode on the right.

Some more views:

A different GE 6FJ7:

RCA:

Another RCA:

Sylvania of course also made the 6SF7:

Different style Sylvania:

Tung-Sol:

It is interesting that after decades of working with tubes you still come across brands never seen before:

El-Menco, obviously a rebrander.

International Servicemaster:

Lindal:

ITT:

So how does a 6FJ7 look inside? Let's find out by opening this one:

Glass removed:

Base removed:

The heater wires:

Both triodes separated from each other:

Section 1 cathode:

Section 1 grid:

Section 1 plate:

Section 2 cathode and grid still together:

Section 2 plate:

Section 2 cathode:

Section 2 grid:

These photos clearly show how far vacuum tube technology came along. Let's finish this presentation by showing some tube glow.

Ain't it pretty?

I hope you enjoyed this presentation of the 6FJ7

Best regards

Thomas