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DISCLAIMER: Vacuum tube circuits work with dangerously high voltages. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

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Monday, December 31, 2012

Happy New Year!

Hi!

Another year comes to it's end. This has been the second year since I started the VinylSavor blog. It has been great fun and once again I'd like to thank all my readers. Not all projects which I planned got started, for example the mighty 851 triode which I plan to use for a 150W SE amp is still lying dormant in my basement. But other projects came to life, which I didn't think of a year ago. For example the modular preamp concept and especially the directly heated phonostage.




In last years year end post, I showed some photos taken in my home town Lindau. This year I'd like to entertain you with some photos of the magnificent 866A mercury vapour rectifier tube.

This tube was already covered in a Tube of the Month article last year. In the meantime I improved my photographic skills and took some more photos of this beautiful tube. The first photo above is a close up shot of a 866A in operation. The drops are condensed mercury on the glass wall of the tube. There is a certain fascination and magic about the blue glow of this tube. This glow combined with the rather impressive size of the tube is quite a show. The intensity of the glow depends on the current that is drawn from it. For these photos I made a special rig which holds a single tube. The current is adjustable with a rheostat. A close up shot of the label on the glass:




Different views of the entire tube:





A close up of the glass stem which holds the electrodes:




Eerie looking, isn't it? Going closer to the 'eye':



Almost looks like an alien, no? Another view of the glass stem, bathed in blue light:




Another close up shot:






Ok, enough close ups, let's take it outside and have a look at the 866A at night with an almost full moon:





The 866A with lake constance, mountains and the city of Bregenz in the background:




The 866A competing against the moon:




The moon behind some clouds:




Two more shots:






Blood red moon:




I hope you enjoyed these photos!

All the best for the year 2013 to you and your families!

Thomas



Tuesday, December 25, 2012

Listening to the all DHT amplification chain

Hi!

I have been extensively experimenting with the DHT gain stage in the recent days and would like to share some listening impressions.





As an initial test it replaced the output section of the E55L LCR phono stage which I've been using recently. The E55L input stage feeding the 600 Ohm LCR RIAA EQ, from there straight into the DHT gain stage. Since it has quite a lot of gain, no step up transformer at the input of the DHT stage. From there into the 6AH4 line stage and then into a 10Y/45 power amp.

Although this was a strange mix of IDHTs and DHTs it gave a good first impression about the suitability of the 841 and 801 in a phonostage. Microphonics were surprisingly low due to the use of vibration damped tube sockets. Noise and hum also very low. Initial sound impressions were excellent. Very smooth and detailed.

But the real question was, would this DHT gain stage also be usable at the input side of the phono, enabling an all DHT amplification? Or would noise and microphonics render it impractical? I hauled the 10Y linestage up from the basement. To give an all DHT amplification chain a try.




The result is amazing. No appreciable microphonics. Noise and hum levels absolutely ok! Sure when you tap the first tube in the chain you hear it ringing in the speakers. Hum and noise are not as low as with the E55L or EC8020. But absolutely not objectionable at all.

This is an amplification chain now which is purely built with directly heated triodes: Lundahl LL1933Ag silver MC step up transformer wired 1:16 feeding the grids of the 841 tube. The 841 is DC coupled to a 801 which drives the Tango LCR EQ through a 4.5:1 step down transformer. The output of the LCR EQ is connected to a transformer input of the 10Y linestage with 1:11,2 step up. The linestage uses a Tribute line output / transformer volume control. Connected to the linestage is a power amp with a 10Y  as driver and 45 as output tube. Directly heated all the way and thoriated tungsten filaments up to and including the driver stage of the power amp.




Anybody who has experienced the difference in sound when changing a tube from indirectly to directly heated has an idea of the sound of this set up. It is extremely detailed without a hint of annoyance which high resolution systems often have. The DHTs sound very smooth and musical. Almost scary lifelikeness.



 
 
These initial listening tests had been done with fairly regular tubes. Hytron VT51 (military number of the 841) and Taylor VT62 (military version of the 801). Both with metal plates.
 
In the linestage I usually get best results in terms of microphonics from graphite plate and ceramic socket 801s. So I was curious how such tubes would perform in the phono. Surprisingly the graphite plate 841 from Mazda were more microphonic than the metal plate Hytrons. Still usable, but microphonics at a level as one would except in the phono.
 
 
 
 
 
My favorites! Surprisingly low in microphonics and noise. Very detailed. Voices are so beautiful through these triodes! Almost unreal!
 
Does this render the IDHT phonostages obsolete? No, certainly not! A properly implemented LCR phonostage is reproducing at a very high quality level. A DHT version is for the DHT lovers who seek that special magic only directly heated tubes have. A IDHT phonstage is far more practical. A DHT phonostage will always remain rather insane and demand more maintenance. Tubes require some selection for low microphonics, any tiny problem with a tube becomes very obviously audible.
 
Gain in this set up is on the low side, but sufficient for most of my listening situations. With other power amps or recordings with a low level another amplification stage might be useful. Or when the volume shall be really cranked up. Say a single stage 801 with input and output transformer giving another 12dB. But I have been happily listening to it as is. I even switched to the lower step up transformer input at the linestage (12dB).
 
The power supplies got a new look. I decided to experiment with wooden transformer covers:




Best regards

Thomas

Thursday, December 20, 2012

Tube of the Month: The D3a

Hi!

After the series about transmitting tubes from the 800s during the last 4 months, I will present a very different and more modern tube today: The D3a pentode.





I'm constantly asked for suitable alternatives to the hard to find EC8020 for use in transformer coupled LCR phonostages. Various pentodes work very well in this phono stage topology when triode connected. I already presented the E55L as an alternative which I used in two different LCR phono stage builds. The frist one can be seen here and another one here.

The D3a is a quite modern tube developed for wide band amplifiers for telecommunication applications. It has a nine pin miniature base. The pinout can be seen on the left. Triode connected it has a very high transconductance almost approaching that of the EC8020. Amplification factor is 77 at a plate resistance of 1900 Ohms. See the Telefunken datasheet for all electrical parameters also for the pentode connection. The low plate resistance makes this tube very suitable for transformer coupling. With a step down transformer, for example Lundahl LL1692A in 3.5:1 configuration or LL1660 in 4.5:1 the tube is perfect to drive a 600 Ohm LCR RIAA network. In such a configuration there would even be some gain left after the EQ network. With a step up transformer after the RIAA, for example Lundahl LL7903 in 1:4 confguration this could be boosted up for further amplification in a second D3a stage using another step down transformer for low output impedance. With a Lundahl LL2745 wired 2.8:1 at the output such a 2 stage phono preamplifier would have a MM gain of about 46 dB with an output impedance of less than 300 Ohms. This gain can be brought up to suitable levels for moving coil cartridges with a MC step up transformer.





Although signal levels are very small in a phonostage we want the amplification device to be as linear as possible. These are the plate curves for the triode connection from the datasheet:




This looks promising! Let's see how this keeps up with reality. Here are the plate curves taken from an actual tube with a curve tracer. The scale is 5mA and 50V per step, 0.5V grid voltage spacing:





Looks perfect for audio applications.

The D3a was made by various european manufacturers: Telefunken, Valvo, Siemens, Philips. Although it has an american designation, the industrial type 7721, I'm not aware of any US based company who made this tube.

A Siemens D3a:



This tube has been manufactured by Siemens in the former western part of Germany. It has the characteristic pinched glass around the electrode structure.



Siemens purchased tubes from the former eastern Germany in the 1970ies. These D3a are probably relabelled, but I'm not 100% certain:





A comparison between pinched and straight tubes:




The boxes of the older Siemens tubes had a seal to prove that the tube is unused:





A closer look at the tubes:






Zooming into the internal structures:




This photo gives already a hint at the delicate grid structure inside. Another view.




In order to get a better view of the internals, we have to take a tube apart. First the glass has to be taken off:




Now the details are better visible:





A view of the top showing the ring which held the getter material before it got flashed during the manufacturing process:




This ring is often wrongly referred to as the getter, while it only held the getter material before it got flashed and condensed as the silvery mirror on the inside of the glass. After the flashing of the getter this ring has no function any more. It especially has no influence on the sound of the tube as some audiophiles seem to believe.



The ring and upper mica disc removed:




This gives a nice view of the heater wire inside the cathode sleeve:





The heater in the cathode, as seen from the other side:





The base removed:




This shows the details of the internals nicely. The plate actually consist of two pieces at each side of the grids/cathode assembly. Grid number three actually consist of two beam forming plates, the silvery frames between grids and plates.

A close up of the grids:




This gives already a hint of the control grid which is visible below the screen. To get an even better view we must take the tube further appart.

Removal of one of the mica discs which aligns the electrodes:





Removing the anode to give a good view to the beam forming plate:



With the beam forming plates also taken away we see the grids and cathode. of course the alignment is screwed up now without the mica holding everything in place:




Removing the screen grid:




The ruler gives a feel of the scale. Now with the actual control grid is exposed and we can have a close look at it:



Removing the cathode is impossible without destroying those extremely thin grid wires:




Another zoom in showing the tiny control grid wires placed on the ruler:




The black lines on the ruler are 1 millimeter apart!

Now some pictures of a D3A when it's lit up:




Closer up:






I hope you enjoyed this months tube presentation. A LCR phonostage with the D3A is already in the works. Stay tuned!





Best regards

Thomas