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Thursday, March 22, 2012

New Lundahl Line Output / Interstage Transformer


In Lundahls line output transformers I always missed one which offers a ratio close to 6:1 step down. There is the excellent LL1660 with 4.5:1, the LL1692A offering 3.5:1 and the LL1689 with a high 9:1 step down ratio. After some email exchange Per Lundahl agreed to develop a transformer which fills that gap. Here is the new LL2745:

It has the same primary windings as the LL1689, only the secondary is wound such that it offers a 2.8+2.8:1+1+1+1 ratio. The transformer can be used for other purposes besides 5.6:1 step down. As with the other types, the winding arrangement can be combined in different ways, for example to get a 2.8:1 step down. It can also be used as interstage transformer, for example offering a 1:1.4 or 1:2.8 step up or step down.

I asked for this transformer as an ideal line output to go with tubes like the 26 or UX201A. For these triodes with a higher rp, the step down ratio gives a lower output impedance as the LL1660, but does not loose as much gain as the LL1689. Especially when a real 600 Ohm impedance should be driven for example wtih the 26, the LL1660 would not provide enough step down.

I got my first pair of the LL2745 gapped for 8mA, to be used with the 26. I will write about this project when it is getting built. Stay tuned.

Best regards


Sunday, March 18, 2012

Tube of the Month: The 6AH4


Last month we had the 10Y, a directly heated triode which I mostly use as line stage or driver tube. In those applications it requires special care in the filament supply for low noise and good sound. This months tube is an indirectly heated triode which is my favorite choice for such applications when the budget is limited or available space is constrained and does not allow for an elaborate filament supply. The 6AH4:

This is a single triode with a 6.3V heater. It can be heated with AC in the above mentioned applications without any hum. This reduces the effort considerably. I already used the 6AH4 in the linestage section of my Octal preamplifier Mk2. It serves very well in that preamp.

The 6AH4 has an octal socket. The pinout is pictured on the left. Amplification factor is 8, which makes it perfect for usage in a linestage. Most linestages have way too much gain nowadays. The 6AH4 when used with a line out transformer with a moderate step down ratio will yield an amplification factor of 3-6dB, depending on the transformer used. In my book this is the right amount of gain for a linestage to have some reserve for recordings with low levels and does not have too much gain so that the volume control will have a sensible usable range. It's plate resistance is a low 1780 Ohms. With a 3:1 step down transformer this is reduced to 200 Ohms. Perfect to drive an iductive volume control and to make the linestage immune against difficult loads like long cables to power amps or even passive line level crossovers.

The 6AH4 is only available in the GT form, which stands for glass tube, hence the complete designation of the tube is 6AH4GT. With it's maximum allowed plate dissipation of 7.5W it is actually a small power triode. It would deliver about 1.5W in single ended mode. But I have never tried that. It's power and high plate voltage rating would also make it very usable as driver for large output tubes like 300B or 211. It could even deliver enough voltage swing to drive a 845. Again I have not tried it as a driver yet, but definitely will in some future project. For all technical details see the datasheet. All the mentioned technical parameters make this tube perfect for any case where more drive capability at a lower gain and much lower output impedance is needed as the widely used 6SN7 offers. That's also why I replaced the 6SN7 when redesigning the Octal preamplifier to the Mk2 version. Another important factor is the linearity of the tube if it's meant to be used without negative feedback. These are the plate curves as shown in the datasheet:

How does that compare to the actual curves taken from a real tube? See the screenshot of the curves below.

The 6AH4GT was offered under all the major brands like GE, RCA, Sylvania and many others:

The tubes from the different manufacturers have quite different construction details and sizes:

Long glass bottle, short bottle, side getter, top getter, large base, short base, etc. also the internal electrodes have different shapes. Here is a photo showing a General Electric tube:

This tube has a short glass bottle and a rather large base. The getter is placed on the side of the glass. The photo on the right shows a close up of the top of the General Electric 6AH4. The small fins attached to the rods which extend through the mica are cooling fins for the grid. The tube was designed to work as vertical deflection amplifier in TV sets which requires a very sturdy tube. Other brands come with a rather long glass tube and have the entire upper dome covered with getter like this sample from the Zenith brand:

This has a nice large getter which makes a perfect mirror:

The next photo shows a RCA 'short bottle' 6AH4 in operation in the Octal preamplifier:

Two shots of a Sylvania 6AH4 powerd up showing the glow of the heater:

A close up which pictures the hot heater extending out of the cathode which glows red:

Although the tube comes in many shapes, I have no particular preference. As long as the tubes measure good they are all very well usable and sound differences are more a matter of taste.

A stand alone linestage with the 6AH4 is already in the making. I showed some parts of it in one of the recent articles about the modular preamplifier concept. Stay tuned for an update which will show the completed 6AH4 preamp.

Best regards


Thursday, March 15, 2012

Music: Yello, Pocket Universe


Yello is one of my all time favorite bands, so it is about time I write about them. During their career which is lasting already more than 30 years, they produced many albums.

Yello is a swiss band, actually a duo which consist of Dieter Meier and Boris Blank. They use a lot of synthetic sounds. All of their records are of very good recording quality. It is debatable if they are of any use for system testing but nonetheless fun to listen to. They typically have a huge holographic soundstage, albeit completely artificial. The recrodings always have lots of detail and a balanced frequncy response. No harsh mids or overly emphasized treble like many other pop albums.

Their style of music is very unique. Although each Yello album is different, all of them are immediately recognisable as beeing produced by Meier and Blank. A lot of guest musicians appeared on their records. For example the amazing Shirley Bassey on the hit 'Rythm Divine'.

Some of their more popular and well known records are 'One Second', 'Flag' or the latest album 'Touch'. Today I'm writing about one of their lesser know albums called 'Pocket Universe'. Of this album only a few vinyl copies have been pressed, but it is easily found on CD. It opens with the mystic piece called 'Solar Driftwood' which consist of 'sci-fi' like sounds and lyrics. A theme which is maintaned throughout the 4 sides of the vinyl discs. Visitors of the European Triodefestivals in Langenargen will remember this record since I enjoyed playing it at wall shattering levels there on this system:

The audience applauded when the first track opened with the lyrics 'The big bang...'. Seemed quite appropriate for the bass capabilities of this speaker with 8 15 inch Altec woofers.

If you can find this album on vinyl, grab it and savor it!

Best regards


Sunday, March 11, 2012

Filament Bias, Part 1: Concept


It seems that filament bias became very popular recently. There is a long thread on the DIYaudio discussion forum about preamps with the 26 triode and filament bias seems to be used a lot there. So I thought it would be a good idea to write a few articles about this biasing method. This one starts with the basic introduction of the concept.

I came up with the idea more than 12 years ago when I spent a lot of effort to minimize the capacitors in the signal path. I do not claim that this is an original idea. I have seen similar schemes in old textbooks especially with battery DHTs.

The most conventional and in my opinion also one of the best biasing methods is good old cathode bias. In this scheme the plate current is returned to ground through a dropping resistor which is placed between cathode and ground. In case of directly heated triodes, cathode and filament are the same electrode. A point needs to be chosen as the cathode connection. In case of AC heated filaments this can be the center tap of the filament transformer secondary or the wiper of a hum pot between the filament terminals. Filament bias is only relevant for DC heated triodes. In this case many people still put a hum pot across the filament and use the wiper as connection point. I'm of the opinion that in case you heat with DC, it should be as clean as possible so that no hum bucking is necessary and no ugly pot distrubs the signal path. I commonly choose the negative filament as the cathode point. From there you connect your cathode resistor to ground.

If the full amplification shall be utilized or if the triode is transformer coupled, it is advisable to bypass that cathode resistor with a capacitor so that the cathode has a low AC impedance to ground. Otherwise the internal resistance of the tube would be increased by roughly the cathode resistor value multiplied by the amplification factor of the tube. This would move the internal resistance into a region which is not usable with transformer coupling in most cases. In case of directly heated filaments the bypass cap will also shunt any noise voltage which might be present between the B+ and filament supplies. Such noise voltage can build up if transformers without electrostatic shields are used. Obviously this cap is in the signal path and has a great influence on the sound of a gain stage. So how can we eliminate it?

In the case of cathode bias, the plate current through the cathode resistor generates a voltage drop which elevates the cathode to a positive potential, which in turn translates to the grid being negative with respect to the cathode. The beauty of this scheme is that it lets the tube find it's own operating point. As the tube ages and emission drops, the bias voltage decreases which counteracts the aging effect somewhat. Also in case of some fault the cathode resistor acts as a safety mechanism. In order to eliminate the cthode bypass cap we also have to eliminate the cathode resistor or reduce it's resistance to a very low value compared to the plate resistance of the tube. One way to do that is fixed bias which feeds the grid with a bias voltage from a separate supply. Obviously that additional supply is in the signal path, but we wanted to minimze that. Filament bias utilizes the filament current in addition to the plate current to generate the bias voltage through a cathode resistor which now can be much smaller.

The picture above shows conventional bias vs. cathode bias. The difference is minor, the negative end of the filament supply moves to ground rather than the negative filament terminal of the tube. Now the filament current flows through the cathode resistor. With filament currents typically in the range of one to several amperes, this means we can develop 10s of volts for the bias with resistances in the 10s of Ohms. This is about a factor of 100 below the typical plate resistances and won't hurt much if left unbypassed.

Now the filament and B+ supply are referenced to the same ground, no danger that any electrostatic noise voltage builds up between the two. Of course the filament supply needs to be very clean and well filtered since everything present on top of the filament voltage will be amplified. In the schematics above the filament voltages are supplied through chokes to the triodes. This is to isolate the filaments from the supply and capacitance therein. Instead of the choke the filament bias voltage can also be supplied through a constant current source which seems quite popular lately. I still favour the passive way with good iron though.

This scheme works nicely and is proven in various configurations. It simplifies the signal path and ties filament and b+ supply nicely together. Of course the self biasing and safety aspect of cathode bias are lost. This basically acts like a fixed bias stage now. There is another disadvantage: The cathode resistor needs to dissipate a lot of power. This calculates as filament current times bias voltage. So in case of a 26 DHT we will dissipate about 10-15W in each filament bias resistor, depending on the operating point which is chosen. Worse in a 801A or 10Y: Here we get 20-50W depending on the operating point. This is some hefty power dissipation which requires serious resistors with proper heatsinking. I still use filament bias, especially with triodes like the 26 or the UX201A which only needs 0.25A filament current. With these the heat dissipation is managable. With other triodes I mostly use a scheme called ultrapath which is another way to get the cathode bypass out of the signal path. In some cases I even combine them both.

I will go a bit more into deatils in future articles and also show some actual implementation in a new preamp which I plan to build. There will also be an article about a further development beyond filament bias which I named DirectPath. This eliminates the last remaining cap in such a stage, the last B+ filter cap. Stay tuned!

Best regards


Thursday, March 8, 2012

Tube Box Art, Part 5: de Forest, Cunningham, RCA


Today I want to show the beautiful tube boxes of the de Forest company and also the boxes made by Cunningham which had close ties to RCA. I also found some more very old RCA boxes which did not make it into the first article of this series, but which are worthwhile to show.

Pictured above are boxes of the de Forest 427 and 499 tubes. These are equivalents to the UY227, an indirectly heated triode and the UX199, a directly heated detector tube.

I have two versions of the 427 boxes which differ slightly in the printing on the top:

The sides showed patent information and the specs of the tubes (quite convenient):

The 499:

Lee de Forest invented the Audion in 1906. He founded a tube manufacturing company which was later sold to RCA.

Another company which had close ties to RCA was Cunningham:

A detailed history about Cunningham and the relationship to RCA can be found here.

Three versions of boxes with the CX301A and UX201A which are the same tube:

The CX301A box was made with some variations:

Two versions of packaging of Cunningham 56 triodes:

The boxes on the next photo  show the close relationship to RCA, they have both their own logo as well as the RCA 'meat ball' printed on them:

Quite interesting also this early box from RCA for the UX201A:

Contrary to other early tube boxes these were rather simple in design. In the early days, the tube boxes had very informative inserts with lot's of information about the specs, application hints and even circuits:

Best regards


Sunday, March 4, 2012

The Modular Preamplifier, Part 8 : Line Stages


Some progress on the line stages for the modular preamp concept. As mentioned before, two different versions are planned. One with a directly heated triode, the 10Y and one with indirectly heated triode, the 6AH4, which will be basically the line section of the Octal Preamplifier Mk2.

The photo above shows the sub assemblies of the 6AH4 line. It will use Slagle TVCs and Lundahl line output transformers. 5 inputs, one of them will be transformer coupled with an input transformer with a 1:4 step up ratio.

The power supply for this line stage is already finished and tested. It just waits for the wooden enclosure:

The circuit is the same as the line section in the Octal pre Mk2.

While waiting for the next batch of wooden enclosures, I started assembly of the directly heated linestage:

This is a bit different from the 6AH4 linestage. There will be a metal plate on top on which the tube sockets, output transformers and capaitors are mounted. These parts would not all fit inside, so some had to go on top. And it would be a shame not to show the beautiful 10Y tubes.

A close up of the top plate:

The tube sockets are mounted on vibration damped sub plates:

Different look this time, no screws visible from the outside.

The power supply sub assemblies:

Again some parts had to go on the top side, the power transformers, one of the B+ chokes and the rectifier bridge which will be built with 4 6AX4 TV dampers. The transformers and choke will later dissapear under transformer covers. Inside the chassis will be the second B+ choke, filament chokes, filament rectifiers and B+ smoothing caps.

Best regards


Thursday, March 1, 2012

Beware of Visseaux 6N7 failures


I received several reports of Visseaux 6N7GT failing. Up to now 3 people who used these in my circuits had failures, one person had 2 tubes going bad, that is a total of 4 failed tubes.

This does not seem like a coincidence any more. I have zero reports of 6N7 from other brands going bad. I never used the Visseaux myself so I have no own experience with them. I'm just relaying what I hear from others. Failures happened in The Octal Preamp Mk1 and in two different power amps which use the 6N7 as driver tube.

Apparently they tend to develop heater to cathode shorts. In all my designs the voltage between heater and cathode is far below the maximum allowed. Also plate voltages and dissipation are within the allowed limits.

Those people who used the Visseaux all liked their sound a lot, but with these repeated reports of failures I'd discourage everyone from using Visseaux 6N7 in my designs.

Best regards