DISCLAIMER

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.

All rights of photos and text reserved. Usage of photos or text from my blog on other websites or for any other purpose only with prior permission. If you want to use any material from my blog please contact me by email.



Monday, January 30, 2012

Tube Box Art, Part 4: Sylvania

Hi!

Sylvania was one of the major tube manufacturers in North America, not quite as dominating as RCA or General Electric but definitely one of the major players. So their tube box art deserves an own article.




The company started under the name Hygrade Sylvania Corporation which was formed by a merger of three lamp manufacturers. Incandecent and flourecent lamps were a major part of their business and today lamps are still produced under the Sylvania brand. Besides lamps they also produced vacuum tubes and later even transistors.

Like all tube manufacturers they had nice colorful boxes for their tubes like this package for a 01A:



The top of the box bears this nice leaf logo:



The Sylvania tube box art followed a similar path as others, the boxes got more simple and less colorful. The next generation of tube boxes shows this trend already, a box of a 26 tube:




Sylvania went through several changes throughout the decades and the tube boxes changed along:






Towards the end the boxes had a more 'modern' but also bland and boring styling:



In the 1980ies Sylvania stopped making electric and electronic products. They were not acquired by Philips as I wrongly wrote in the article about Philips tube boxes, Philips bought the rights on the brand name for consumer electronics.

Best regards

Thomas

Thursday, January 26, 2012

The Modular Preamplifier, Part 4: Phono Stage PSUs

Hi!

After the introduction of the 6BY5 tube in the previous post, I will write about a power supply that uses it in this article. As mentioned in the 6BY5 tube of the month post, two of them will be very well usable in a bridge rectifier configuration for preamplifier supplies. I'm using them this way in the power supplies for the phono gain stages of the modular preamplifier.




In this modular preamplifier, the phonostage is split up into two separate gain stages (input and output stage), separate LCR RIAA module and MC transformer. Each gain stage has it's own separate power supply for maximum isolation and flexibility. The gain stages will use the E55L which was presented in a Tube of the Month article in last November. They will be run at around 150V B+ and about 35mA. Taking some voltage drop into account in the output teransformer primary and local LC decoupling which will be in the gain section, a voltage of approximately 175V is needed.




The schematic shows the high voltage part. The two 6BY5s are wired as a bridge, both heaters fed from the same winding and referenced to ground. About 300V AC are needed from the transformer to get the 175V out with the voltage drop in the chokes and rectifier. Final voltage will be determined once the PSU is tested under load. The transformer allows adjustment of the voltage.

Heaters will be supplied by DC, rectified with Schottky diodes and powered by a separate transformer. A choke input filter is used for low switching noise. About 12Vs AC are needed to get 6,3V out with the losses in the rectifiers and filter. Again the final voltage will have to be trimmed when tested under load.




The 100mA B+ transformer and low voltage transformer for DC filaments are used from my own range of custom wound power transformers. Details about these transformers can be found here.

All elements of the modular preamp will use the same chassis style as shown in the posts about passive line stage and MC transformer. The circuit is divided into sub modules which are premounted on metal plates:



All these plates are assembled in sandwich fashion:



The complete assembly slides into the wooden enclosure: There are holes on the top and bottom of the enclosure to allow airflow around the tubes.


A rotary on/off switch was chosen so that the same knobs can be used as on the passive line stage



Usually the glow of the tubes are good enough for me to work as on/off indicator. Since they are mounted inside, a LED was added to the front:


The PSUs for phono input and phono output stages are identical. Assembly of the gain stages themselves will be shown in the next post. Stay tuned!




Best regards

Thomas

Sunday, January 22, 2012

Tube of the Month: The 6BY5

Hi!

Many DIYers discovered that TV dampers are well suited for rectifier use and outperform most of the 'standard' rectifier tubes. Their main drawback is the need for two tubes for a full wave rectifier since they usually only come with one diode inside. There is one exception however: The 6BY5.



This tube has two individual diodes in one bottle. Common rectifiers have two plates and a common cathode while in the 6BY5 the cathodes of the two diodes are also separated, only sharing the heater.

The 6BY5 has an octal base. It has a moderate heater current requirement of 1.6A. It is not as tough as other TV damper tubes since it has lower peak inverse voltage and heater to cathode voltage ratings. The original version 6BY5G was rated at a peak inverse voltage of 2500V. This rating is for TV damper service only. The Tung-Sol datasheet also states the peak inverse voltage for mains rectifier service at 1400V. This is the only case I know when a manufacturer also stated the value for mains rectification. Later datasheets dropped this value since they didn't want these tubes to be used for power supplies. They preferred to keep selling their more expensive rectifiers which were marketed for that purpose.

The 6BY5G was revised to the 6BY5GA which got an increased peak inverse voltage rating of 3000V for damper service. A similar derating can be assumed for mains rectification so the 6BY5GA is good for up to 1680V. Another important parameter which needs to be observed is the heater to cathode voltage limit which is about half that of other TV dampers like the 6AX4. This value is 450V heater negative with regard to cathode and 100V heater positive to cathode. It is important to keep these voltages below the limits, otherwise the tube will develop a short between heater and cathode. In a full wave rectification scheme with center tap on the transformer secondary, this can be easily done by connecting the heater to the B+. Simply connect one of the heater terminals to the cathodes. A separate heater winding is needed for this scheme. A single 6BY5GA can be used this way in power supplies delivering up to about 450V DC with choke input filter. Somewhat higher if a cap input filter is used.

So far I have only used the 6BY5GA type and not the 6BY5G because of the higher peak inverse voltage rating. A complete datasheet can be found here.
The 6BY5G comes in the older 'coke bottle' style. Here a comparison between the two versions:


The 6BY5s unique feature of separate cathodes makes it attractive for full wave bridge rectifier schemes since only two of them would be needed to build a 4-diode bridge. However the rather low heater to cathode voltage limits restrict it's usefullness in full wave bridges. No matter how they get connected there would always be the case that in at least one of the tubes there is a voltage difference between the cathodes. Since the cathodes share a common heater, this voltage difference needs to stay within the allowed limits. Best would be to use a single heater winding and reference it to ground. This allows a bridge with 2 6BY5s to be used with transformer secondaries up to about 300VAC. Interesting for preamp power supplies or small power amps for example.

These close up photos show the independent diode structures of the 6BY5GA and 6BY5G:



Photos of the upper mica supports of the GA and G versions:



I already showed the use of this tube in the power supply of a 6CB5A amp in an article about the single ended amplifier concept. I plan to use two of them in a full wave bridge scheme as mentioned above in a phono stage supply which will be part of the modular preamp concept.

Best regards

Thomas

Thursday, January 19, 2012

The Modular Preamplifier, Part 3: passive line stage

Hi!

The next module is finished, the passive unit with transformer volume control and input selector for line level signals.



A bit more work to wire this one up compared to the MC transformer shown in the last post, but still a rather quick job.


Slage autoformers are used for the volume control, ELMA switches for input select and attenuator. All wired up with solid core silver.



For best sound the autoformer needs to be driven from low output impedance sources like the phonostage modules which will be built to complement this modular system. Separate buffer stages will be used for sources which do not have the drive capability. The resulting sound can be tuned be using bufferstages with a specific sonic charactder to complement the source. For example a UX201A buffer stage for CD sources which have a tendency towards the harsh side.

The line control unit has the same size as the MC transformer:


This unit is not to be confused with the usual passive resistive controls which tend to have a high output impedance and thus are influenced by cable parameters and input impedance of power amps. It will be used with low impedance sources of 300 Ohms and less only. So the worst output impedance will be 300 Ohms. Much less at lower volume settings.




The only reson I put it into a separate box is to adhere to the modular approach and to be able to experiment with different gain stages without the need to put a volume control in each of them. This will allow to mix and match different gain stages according to taste and amplification needs.

This approach will be helpful to experiment with all directly heated preamp stages and to see how far upstream DHTs will be practical. Starting with the output section of the phono preamp, mixing it with an indirectly heated input stage. Later also a DHT phono input stage will be added.

I also plan to build a linestage in this chassis style with input selctor and volume control built in. This will use one of the longer chassis shown in the previous post.


Best regards

Thomas

Tuesday, January 17, 2012

The Modular Preamplifier, Part 2: MC Transformer

Hi!

The new Lundahl silver MC step up transformer LL1933Ag is the first to get a chassis in the style for the modular preamplifier concept.



Not much in there, so it was a quick job, just the transformer capsules wired to the inputs and outputs. Solid core silver wire has been used for the wiring.






More photos catching a bit of the evening sun before it sets:




My carpenter already finished more enclosures for the phono gain stages, power supplies, LCR EQ and volume control:



Stay tuned for updates as soon as the stuff which goes inside these is finished.


Best regards

Thomas

Sunday, January 15, 2012

Making of a Field Coil Speaker, Part 1: First Trials

Hi!

This weekend I finally had some time to start experiments with my Kilimanjaro Series field coil drivers. For these I prepared a new listening room which I will use to demo my gear. Of course this room needs an adequate speaker.


I have various horns in my collection. One of them is an Iwata horn made of die casted aluminum. This is one pair of a few which got custom made by a group of people in Berlin. One of the members of the Munich Triode Mafia bought a pair years ago. I found them in his attic and we set up a deal. Unfortunately the horns were poorly painted in a horrible blue. So I brought them to a car paint shop where they got sandblasted and repainted in a special color which is used by Mercedes. the color is called 'designo graphite green'.

I used these horns a long time in my speaker installation in Munich, which is now dismatled:


In this system the Iwatas replaced TAD4001 horns which I had before. First I used them with TAD4001 drivers. After I tried Wolf von Langa's A288 field coil drivers, the TADs went out and I was bitten by the field coil virus. The horns were used with various combinations of Altec drivers for the lower mids and bass: 414As, 515Bs and 416As. During the time of the High End fair 2010 Wolf and myself set up a demo at my place for which we replaced some of the Altec woofers against his A515 field coils. After that session it was clear to me that one day I need to have a full field coil speaker system.



Initially I planned to have two woofers per side, A416s and A515. For the first trial I got simple open baffles made from multiplex wood for just a single woofer. Since my new listening room is fairly small I will initially go with with one per side. If the A515 needs some sub bass support, I will add the A416, kicking in at low frequency.


A close up of the horn driver mounted to the Iwata horn:


The horn drivers have a high voltage field coil. They will get a tube rectified and choke filtered power supply which still needs to be built. The speaker will be set up as an active system with separate amps for bass and horn driver and later also for a tweeter if needed. Frequencies will be divided passively between linestage and power amps. The system will get a LCR type crossover.

For a first listening the woofer had to be operated full range with the field coil powered from a lab PSU since crossover and high voltage field coil PSU still need to be built. Of course this is no meaningful listening test but it already gave an hint of the incredible tone these field coil woofers are capable of.

I also consider to give Wolf's enclosures a try if they fit into the room. Once the final enclosure is decided upon it will be made in some nicer wood.


Stay tuned for updates as the speaker development progresses.

Best regards

Thomas

Thursday, January 12, 2012

Tube Box Art, Part 3: Philips and Valvo

Hi!

Today I'm showing some european tube box art from Philips and Valvo. Philips used to be a major tube manufaturer in Europe. They acquired numerous other companies during their history.

2A3 boxes from the Philips 'Miniwatt' series:



Quite nice how they play with blue and yellow on these boxes:


Beautiful, but check out this Philips box with 'Radar Man':




Look closely, radar man wears earings.


A stunning box, isn't it?



Here a AZ1 box made by Valvo which became a part of Philips at some time:




Later the Valvo box design became more simple and bland. A ECC88 box:



Another company acquired by Philips was the tube manufacturing branch of Sylvania which was renamed PhilipsECG. This was towards the end of the tube era, and the box design followed the trend :


Older Sylvania tube boxes will be covered in another article. Stay tuned!

Best regards

Thomas