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.
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.