As promised in last months article about the 6SF5, this months tube is a very different one. A 9 pin miniature UHF mixer and amplifier triode, the 6AM4.
This is a tiny little tube. Certainly the smallest tube I have ever used. But it's quite impressive nonetheless.
As mentioned above it is a miniature tube with a 9 pin base, which is called Noval base in Europe. The pinout is shown on the left. As is quite typical for tubes which are meant for high frequency applications, it has the grid brought out to several pins. It was intended to be used in grounded grid configuration. It caught my attention when I browsed through tube manuals to find high transconductance tubes which might be usable in transformer coupled circuits to drive a 600 Ohm LCR RIAA. While it doesn't have super high transconductance values like a EC8020, it still has almost 10.000 micromohs. It's very high amplification factor of 85 makes it interesting for phono stage use. Amplification factor and transconductance result in a plate resistance of 8700 Ohms which makes it still usable with a interstage or line out step down transformer if it has enough primary inductance. The typical plate current of only 10mA allows the use of transformers with small air gaps which have sufficient inductance. Potential candidates are Lundahls LL1660 or LL1689. Also other transformers would work well like Tango NP16 or NP8. See the 6AM4 datasheet for all technical specs.
As mentioned, the dimensions of the tube are tiny. The glass body is about 20mm in diameter, which is typical for 9 pin miniature tubes, but the glass is only about 30mm high. This tube is a good example, that size does not always matter. What is more important are it's technical parameters and if they are suitable for audio purposes. Tubes which are meant for RF applications are often not very linear since that aspect is not very important for high frequency use. Let's have a look at the plate curves as shown in the data sheet:
As expected, the curves are not as evenly spaced as those of the recently presented tubes. But if the right operating point is chosen in the most linear region and if used to amplify the tiny signals of a phono cartridge, very usable for audio. Below the curves of an actual tube taken with a tracer:
Still looks pretty neat. Just pick a good op point. Give it some current and make sure the grid keeps some distance from 0V and stay away fro saturation. Perfectly fine for a phono stage and that is what I used it for. Below a photo of a two stage transformer coupled LCR RIAA with the 6AM4:
Those little tubes are almost lost on the preamp. Here a photo of a 6AM4 next to a EC8020 in comparison:
I have also used the 6AM4 in RC coupled RIAA stages with good results. A very usable tube. I'd only avoid applications which require large signal swing like power tube drivers and even line preamps. I prefer to keep them in phono sections only.
Availability of the 6AM4 is still good as it was manufactured by most of the big tube companies. Especially General Electric made them in some varieties.
These were packaged as service parts:
An older style GE packaging:
The military used the 6AM4 too, here a GE 6AM4 in plain military packaging:
Made by Sylvania:
A Zenith branded 6AM4 probably made by Sylvania:
Dumont, probably made by RCA or GE:
Here some shots of a GE 6AM4 showing some of the construction details:
Contrary to most tubes, the triode structure is mounted horizontally.
To get a better view of the internals, the glass has to be removed:
The photo above shows the connection of the heater to the pins. A close up of the tiny cathode:
The silver metal pieces on both sides connect all the grid pins to the grid:
This silver strap connects the plate:
The heater wire partly removed from the cathode:
The heater wire completely removed:
Unfortunately the grid got wrecked when I removed it from the plate:
Gold grid wires!
Note the shape of the plate, the middle section narrows to about 1mm. This was necessary to achieve the high transconductance and amplification factor:
The next photo shows the scale of cathode and grid:
This tube is a magnificent piece of precision engineering. Hope you enjoyed this tube presentation.