After the presentation of the very early rectifier tube in last months ToM post, let's continue the rectifier theme. This months tube is a TV damper, the 6AU4.
I covered many TV dampers already, the 6BY5, 6BE3, 6CG3, 6CJ3, 6DQ4 and my favourite one, the 6AX4 and a 25 heater variant of it, the 25AX4.
I like to use the 6AX4 due to it's lower heater power. It's current handling capability is sufficient for most audio applications. If very high B+ current is needed, 6CG3 or 6CJ3 are well suited candidates. But they have different sockets. If an octal socket is preferred but more current is needed than the 6AX4 can handle, the 6AU4 might fit the bill. It shares the same pinout with other octal TV dampers. The increased current capability comes at a price, it also needs more heater current. 1.8A versus the moderate 1.2A of the 6AX4. Like the other TV dampers it contains only one diode. The only exception is the 6BY5 which has two diodes in a single bottle. This means two 6AU4 are needed for a full wave rectifier, which requires 3.6A heater current. Or 4 of them in an all tube bridge rectifier which would need a whopping 7.2A of heater current. Like the other TV dampers it has very high voltage capabilities and can also handle large voltage differences between heater and cathode.
This is limited to the heater negative to the cathode however which is allowed to go up to 900V DC. In the other case with the heater positive to the cathode themaiimum allowed is 300V DC. In actual implementations it is advisable to stay well clear of those limits. I typically use such tubes with the heater voltage referenced to ground and for DC output voltages up to 700V DC. Although the tube manufacturers never advised to use TV dampers for mains rectification purposes, they proved to be very well suited for such applications and have been working in the power supplies of my amps reliably for many years. I guess the tube manufacturers preferred to keep selling their more expensive rectifier tubes for power supplies. TV dampers were produced in the millions for TV sets and had to be cheap. Since they got produced in such large volumes they are still available in quantities at very low prices. The 6AU4 can be found as 6AU4GT and as 6AU4GTA. The GT simply stands for glass tube. The A was added after the tube was improved with slightly higher current handling capability.
Many of you are probably wondering where the name 'TV damper' comes from. So let's have a look at the actual application they were developed for. Before flat screens were introduced, TV sets had a cathode ray picture tube. These had an electron emitting cathode like other vacuum tubes. The cathode produced an electron beam which got modulated, accelerated and directed towards the front. In order to create a picture the beam had to move in a zig zag fashion from left to right and top to bottom. This was achieved by coil winnings arranged around the neck of the tube, which was called a yoke. Different yokes were used for the horizontal and vertical scanning. For this purpose the yolk had to be fed with a sawtooth current. This was generated by horizontal and vertical deflection circuits. Shown below is a very simplified drawing of the horizontal circuit. It typically used a beam power tetrode which was fed by a sawtooth voltage and coupled to the horizontal yoke through a transformer.
The problem with this is a parasitic oscillation set up in the transformer and yoke during flyback of the electron beam, caused by the sudden current cut off in the transformer at the end of each cycle when the magnetic fields collapse. This oscillation needed to be damped to avoid non linearities in the picture. This could be achieved with resistors or RC elements, but a more elegant way to do it is the use of a diode shunted across the secondary as shown in the next picture.
In later TV sets, the energy stored in the inductances of transformer and yoke was partially recovered. This was done by generating a boosted B+ voltage. Also the output transformer was usually replaced by an auto former.
The spikes generated by the collapse of the magnetic fields were used to charge a booster capacitor Cb and the voltage was added to the raw B+. Therefor TV dampers are also called booster diodes sometimes. The schematic above only shows the principle and is by no means accurate or complete. It is just meant to show the concept. In actual implementations this spike was also used to generate the very high ultor voltage of the picture tube through an additional winding which fed another very high voltage and very low current rectifier diode (not shown in the picture).
I hope this little excursion into TV sets enlightened the background of these tubes a little. For more detailed information you can refer to the website of the Early Television Museum. Now lets continue with some photos of various 6AU4 tubes, starting with Westinghouse of which I have various versions. One came in the beautiful boxes shown above and the other in rather bland packaging:
While the latter looks rather regular, the other with the getter applied in the top dome is quite different in construction.
A close up photo shows that it has a small glass piece at the top sitting above the mica disc:
I have never seen this in any other TV damper tube. Here a close up:
Probably done to get some extra strengthening. Here the same tube with power applied to the heater:
From another angle:
The top end of the other Westinghouse 6AU4 with more common construction style:
Another Westinghouse version came with the small 'coin base' as also seen on many other tubes. The coin base was introduced quite late and is a more modern construction style:
Here a comparison between coin base and regular style:
This one is in a Box marked Westinghouse Reliatron:
The plate has two extra wings for additional cooling
These are all the different Westinghouse 6AU4GT/A versions I have.
Up next let's have a look at Raytheon.
From this manufacturer I found 6AU4s in 3 different packagings and 6 different construction styles.
The first one looks quite ordinary like many other TV damper tubes. Clear glass top with the getter applied near the base, printing on the base.
The next one has a bit larger plate structure and looks more rugged.
This one is quite similar as the one before but different cutouts on the plate at the side and the glass is a bit taller:
Yet another minor difference in plate shape:
The next one has a very different plate structure with metal extending at 4 sides which should give better heat dissipation.
The getter applied at the top:
And lastly the one which appears to have the most rugged construction:
The top is also quite different with the spiral which isolates the heater from the cathode extending much further.
The spiral is also attached to a metal rod for some extra support.
And lastly two tubes branded Lindal / LTT. Apparently a japanese company but the construction is identical to one of the versions above. So quite certainly made by Raytheon and rebranded.
This sums it up for Raytheon.
Moving on to General Electric.
Most GE 6AU4 I have, look very similar with only minor differences.
Sometimes only the color of printing or position of printing is changed.
These two differ in the plate material:
One has copper.
This one looks quite regular.
A different version in another box style:
It is a bit longer than the other one.
Let's have a look at their heater current.
Apparently at the end of the tube era even the military had to accept large spec deviations:
This concludes the Sylvania section.
Of course no tube of the month post would be complete without RCA tubes.
All the RCA 6AU4 I have are of the GTA flavour.
They all have tall plate structures.
With slight variations in construction
Next a version with coin base which came in the later box design.
All the RCAs had a measured heater current well in spec ...
... as would be expected from such a premium manufacturer.