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Friday, December 15, 2017

Tube of the Month : The 6FW5


This month's tube is a nice low cost alternative to output triodes. The 6FW5.

The 6FW5 was designed as horizontal deflection amplifier for TV sets but it can be used for other purposes as well, for example as output tube in an audio amplifier.

The tube has an Octal base. The pinout is shown on the left. It is a beam pentode, which means the grid nr. 3 is a beam forming element. The beam forming plate is internally connected to the cathode. Unlike most tubes designed for this application it does not have a top cap. I have played around with this tube many years ago in a single ended amp and remember getting about 3-4W output power in triode mode. I have heard reports from people who got good results with the tube in screen driven mode. In triode connection the parameters are somewhat similar to the 2A3. In fact I heard rumours that Sylvania sold 6B4Gs in the 1980ies which were
6FW5s triode connected in the base and with the cathode connected to the heater to 'emulate' a directly heated triode. The absence of a top cap and rather modest heater current of 1.2A make this an attractive candidate for a low budget single ended amp. The plate dissipation is rated at 17.5W but as with many TV tubes this can probably be pushed a little beyond and will still last. The 6FW5 received some fame during the late 1990ies when JC Morrison published a circuit of a screen driven amp with it. But interest in this tube seems to have faded in the recent years. So it's time to give it some attention again. The complete technical data can be found in the General Electric data sheet. Let's have a look at the plate curves, pentode mode first:

And triode connected:

Looks quite good. Slap on a 6N7 as driver and you have a nice little two stage single ended amp.

All 6FW5s I have are branded International Servicemaster. Most likely made by one of the big manufacturers, RCA or GE.

The tube has a nice little bottle, which looks a bit chubby.

They look very well made.

The top:

Let's open one up to see the internal construction:

With the glass removed we have a better view of the details.

Now we pull out the heater wire:

The plate removed from the assembly:

Beam plate, grids nr 1 and 2 and the cathode:

In the close up it can be seen how well the two grids are aligned.

Grid 2 is in the 'shadow' of grid 1.

View from the side:

The cathode is internally connected to the beam plate:

The screen grid (grid nr 2):

It is carbon coated for better heat dissipation.

The control grid (grid nr 1):

It appears to be gold plated to suppress secondary emission.

Gold plating avoids that barium from the cathode can get deposited on it.

Beam plate and cathode without the grids:

The tube with the heater lit up:

A very well made tube with rugged construction which can probably take a lot of abuse.

Best regards


Thursday, December 7, 2017

845 Mono Amplifiers - Part 2


In part 1 about these single ended mono blocks with the 845 tube, I showed some first photos mainly of the chassis materials. In this part I will show the assembly steps.

The interstage transformer, decoupling choke of the driver stage and most important capacitor in the driver stage are mounted directly under the top plate.

This enables short signal path wiring.

The high power resistors are mounted on a separate aluminium rail close to where they are needed.  Here a detail of the mounting technique:

The output transformer, decoupling choke of the output stage B+ and final choke of the LCL filament supply are on the next level right below the top plate:

The most important capacitor of the output stage, in this case a NOS paper in oil type, is close to the output transformer to have a short signal loop. On the next level below there are the main capacitor banks for smoothing of the high voltage:

The cutout is there to leave room for another choke which will be mounted on the next tier below.

Another set of capacitors and the choke mentioned above:

Again a cutout, this time for the main power transformer which will be on the bottom tier.

The power transformers (separate for filament and high voltage) and the first LC section of the filament supply:

The filament transformer is not mounted yet, it will reside in the upper left corner. Next we see a photo of the finished amp without chassis which shows how all these modules are stacked together:

As you can see almost no space inside is left unused. Right side view:

I wanted to keep the height of the amp to my standard chassis size of 40cm (without tubes and feet), 30*30cm floorspace. So each layer had to be carefully planned to fit everything in. Left side view:

Testing of the amp is complete and assembly of the second mono block is underway. Stay tuned for an update with the finished units.

Best regards