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Tuesday, July 22, 2014

Tube of the Month : The 958A


So far 38 different tubes have been presented in the Tube of the Month series. 4 types have been revisited for a total of 42 posts. I like to show tubes outside of the usual mainstream. Tubes widely ignored by amplifier designers yet very suitable for audio applications. While I was contemplating about which tube to pick for this month, I thought why not something very unusual. A tube type which probably many never have seen or even heard about. When rummaging through my tube stock I came across some acorn types and thought these are definitely worth writing about. There are about a dozen different acorn tubes. Among those some interesting triodes, like the 955. But while we are at strange tubes, lets pick a directly heated one. There are two interesting DHTs among the acorns, the 957 and 958A. I chose the latter one for this post. Here is the 958A.

Acorn tubes got their name from their small size and shape which resembles that of the acorn. These tubes got introduced in the middle of the 1930ies and were already obsolete about decade later, superseded by miniature type tubes.

Acorn tubes were developed for high frequency applications. The usable frequency range of a tube is limited by several factors. Among them the size of the electrode elements and their connections which determines inductances and capacitances. Acorns had an extremely small size and the connections were brought out at the side of the glass for very short signal paths and extremely low inductances and capacitances. Some of them also had connections coming out of the top and bottom of the glass. The pinout on the left shows  plate and grid are coming out of the glass on one side, the filament connections on the other. The negative end of the filament is connected to two separate pins. One for connecting to the filament supply and the other to serve as cathode connection.

The way the pins are brought out is probably the reason why the acorn type disappeared very quickly. By inserting the tube into it's socket, force is applied to the pins like to a lever, this could easily lead to breaking of the glass. Tubes with their pins coming out at the bottom are much less critical in this regard. But if handled with care this should not be an issue in audio applications. The drawings on the right show the tiny size of these tubes. The data sheet of the 958-A also shows the typical values for the use as audio frequency amplifier. With an amplification factor of 12 combined with a plate resistance of 10kOhm this tube could serve in a line preamplifier or as input tube of a 3 stage power amplifier. Filament voltage is only 1.25V and filament current is a very low 100mA which opens up some interesting possibilities. More about that later, lets first have a look at the plate curves:

These look quite promising, let's see how an actual tube looks on the curve tracer:

That's even better looking than the data sheet curves and promises excellent linearity.

Although acorn tubes have only been produced over a short period they can still be easily found at low prices. Here some made by RCA:

They came in beautiful small boxes with the RCA 'meatball' logo:

The tubes are held in place with an internal piece of cardboard:

The RCA 958A:

The exhaust tip at the bottom:

A 958A without manufacturer marking:

The packaging is less fancy than the RCA:

Probably from ex-military stock since the tube has the military designation VT-212.

An acorn tube in comparison to a 6SN7:

While the tubes are still easy to find, the sockets are a different matter. If you find them they will most likely be quite oxidised.

A tube placed in the socket:

Despite the low filament power, it lights up nicely:

So how could this tube be used in an audio circuit? As mentioned above, the very low filament current  opens up some interesting possibilities. I already wrote a post about the filament bias concept. This could be nicely applied to the 958A without the need to dissipate a lot of power in the filament bias resistor. We could even go a step further and use my DirectPath circuit which was presented in the article about the UX201A Sound Processor. Here is that circuit adapted for the 958A:

The 100k grid to ground resistor determines the input impedance of the circuit. A quarter Watt type can be used here. The 68 Ohm filament resistor will dissipate less than 1W. A 5W type would be recommended. About 10W are converted to heat in the 1k resistor so best would be to use a 50W type for long term reliability. The output transformer can be chosen according to the gain needed and desired output impedance. For example a Lundahl LL1660 would do here, wired in 4.5:1. The PP type could be used as this can handle enough DC current. The 958A draws about 3mA. For lower output impedance LL2745 or LL1689 could be used as well. A common supply for a stereo gain stage like this would need 200mA. This is easily obtainable from a pair of TV dampers like the 25AX4 which got covered last month. This is a circuit which is truly without capacitors in the signal path. Of course the 958A could also be used with more conventional circuits. For example it could serve as input tube in an all DHT power amp. I usually use step up input transformers in such amps to be able to stay with just 2 stages and still have enough gain. The 958A could provide that gain instead, for example to drive a 26 which in turn drives a 45.

I have never build the circuit above. B+ voltage and the resistors might need some tweaking to get exact filament voltage and operating point. But that would be easy to do and as usual with such circuits, things don't need to be trimmed to super exact values. I never tried this tube so I have no experience about it's micro phonics in audio applications.

If you are going to try the 958A in this circuit or any other, I would be curious about the results and would be happy to report your findings on this blog. I have too many projects on going and will probably never get around to give this a try. It would be great if someone breadboards this and reports back.

Best regards


Saturday, July 19, 2014

10Y Preamplifier 'Landscape Style'


The portrait style chassis with the controls at the front and the connectors at the back receive a lot of positive feedback. Yet there are many who prefer the classic landscape chassis style. Here are some photos of a new preamp built in this format.

It shares the same basic circuit design as the recently built portrait 10Y line stages, like this one. The difference is the chassis only. This particular preamp will be used with the no compromise 6CB5A amp which was built last year.

Therefore the same wood and color scheme is applied. The PSU also got some vintage panel meters for voltage and current.

This chassis style is more cost effective than the portrait style and assembly is much easier since the entire circuit is mounted on the top plates.

Best regards


Sunday, July 13, 2014

More 45 Bliss


After the series of posts about the humongous and expensive to build 'Tours Sans Fin' 211/211 amplifiers, here some photos of a more reasonably sized (and far less expensive) amplifier using the beautiful 45 as output triode.

Although they share the same mono design with external power supplies and chassis style as the over the top all silver transformer 'pure 45 bliss' amps, they use a simpler circuit and lower priced parts.

The circuit is basically the same as that of the 46 amplifier which I showed in the single ended amplifier concept series quite a while ago. The changes are obviously the mono design and the use of the 6A6 as driver tubes instead of the 6N7. Electrically these two tubes are interchangeable, they only have different bases.

A full wave bridge rectification is used in the power supplies instead of a full wave center tapped scheme.

Due to the available room in the mono chassis and power supplies a separate filament transformer has been used for the heating of the 45s. Also DC is used using my usual LCL approach for the filaments instead of AC. While AC is fine with most speakers, ultra sensitive speakers benefit from the use of DC filaments for absolute absence of hum. Sonically I even prefer DC supplies if they are implemented properly.

Hashimoto interstage and output transformers are hidden under the transformer covers on the amplifier chassis.

These amps show all he beauty in sound the 45 is capable of. With the use of the 6A6 as driver no input transformer is needed to get sufficient sensitivity and the amplifiers are easier on the preamp. They have a high input impedance wich allows the use of pretty much any preamplifier.

Best regards


Wednesday, July 9, 2014

Music : Brandt Brauer Frick Ensemble, Mr. Machine


Today I am presenting a record which I didn't know until recently. The guys with whom I exhibited at the High End fair in Munich brought this record to my attention. We used it a lot to demo and most visitors liked it a lot. Mr. Machine by the Brandt Brauer Frick Ensemble.

Brandt Brauer Frick is a techno/minimal music/experimental ensemble, founded in 2008. They combine electronic repetitive techno grooves with aspects of classical music. Classical interments are sampled and mixed in a collage with electronic sounds. The Ensemble consists of 10 people.

Mr. Machine was recorded 2011 and published on Studio !K7. Although the opening of the album seems very improvised and dis-harmonic, it quickly draws you in when the rhythm builds up. What at first seems difficult to listen to gets pure pleasure and you want to blast through all 4 sides of the double LP. The Brandt Brauer Frick Ensemble is often described as a modern version of Kraftwerk. For sure they have been influenced by that ground breaking band from the 70ies, which still needs to be covered in the music series of this blog. The title of the album which is also the title of the first track is probably a reference to Kraftwerk's Mensch-Maschine (Man-Machine).

We played this record a lot during the High End show and never got tired of it. Many visitors came back after their first visit and asked if we can play it again. Many photos have been taken of the album so people could buy it.

Of course I also had to get this record. It is available on vinyl and easy to find online.

Added 12-Jul-2014:

Just found this video on you tube in which a track of this album is played:

Best regards


Monday, June 30, 2014

Tube of the Month : The 25AX4


Last months tube was a rather large and fairly old transmitting type rectifier. This month we are moving to the other end. A much more modern diode designed for consumer applications. The 25AX4.

Readers of my blog know that I am very fond of TV Damper Tubes and use them almost exclusively as rectifiers in my power supplies. My favourite among them is the 6AX4.

The 25AX4 belongs to the same family. It only has a different heater which needs 25V instead of the 6.3V of the 6AX4. The current needed to light it up is a modest 300ma. All other parameters are the same as those of the 6AX4. It also has the same octal base and pinout which is shown on the left. What makes this particular version of this tube family interesting is the heater voltage requirement. It could be heated from a 24V transformer. This is still well within he limits for the tube to work well and 24V transformers are available cheaply. So if you are on a severely limited budget and $2-$3, which is the typical price of the 6AX4, is still too high, pick up some 25AX4 which can be found at even lover prices. Especially if purchased in bulk packs like this:

Each of the boxes contains 100 tubes, nicely packed in carton trays.

Like others this tube was build in various styles:

Three different versions with a regular base and one with a shorter one, also called coin base.

A closer look at the coin base tube:

Two 25AX4GT in front of a 'wall' of Sylvania boxes:

Since the only difference between the 25AX4 and the 6AX4 is the heater voltage and current, we would expect their construction to be quite similar except for the heater wire. Here is one of each from the same manufacturer and of the same coin base construction style.

No apparent difference between the 25AX4 (top) and 6AX4 (bottom).

The plate structures look identical.

In this case the only difference is the printing of the tube type on top of the 25AX5 while the other has none.

Opening them:

The difference becomes apparent, when the heater is pulled out of the cathode sleeve:

The 25AX4 (top) has a much longer heater wire which is folded several times to fit in.

Removing the heater completely, 25AX4:


Close ups:

Removing the heater wire from the isolation spiral. The 25AX4 heater is substantially longer allowing the large voltage across it:

The heater wire of the 25AX4 has 16 folds, while the 6AX4 has 4, so 4 times the length, which results in 4 times the resistance, and is equal to the 4 times difference in heater voltage.

The cathodes are identical, as expected:

A 25AXGT with the heater lit up:

A great tube, cheaply available and the warehouses of tube dealers are still full of them.

Best regards