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.

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Wednesday, September 28, 2011

Cool Links: www.intactaudio.com

Hi!

I haven't presented a cool link in a while. After I received a delivery of swedish iron a few weeks ago, I got some US made transformers this week. From Dave Slagle's intactaudio. So it is about time that I write about his website.

I know Dave from the early days of Sound Practices. We both participated in a discussion forum related to the magazine, the JoeNet. It was not like todays forums but a mailing list. The JoeNet still existsts in it's original form today. Those were really exciting times for tube audio enthusiasts. In Sound Practices and the JoeNet ideas and thoughts about things like directly heated triodes and single ended circuits were shared. Back in the early 90ies this was pretty uncommon in the western world.

Out of that mailing list the idea for a meeting for tube enthusiasts came up which resulted in the NYnoise which was hosted by JC Morrison in New York City. At one of the early NYnoise festivals I met Dave personally the first time. This was 1999.

Back then he was already into magnetics and wound his own stuff. He put a lot of effort and energy into developing transformer volume controls. These efforts resulted in excellent sounding TVCs. More and more people got aware of Dave's work and asked him to wind stuff for him. So some years later he founded the company intact audio.

Since I'm an avid user of all things wound on iron or other metals, especially volume controls, Dave was the natural choice for supplying my needs when I started to go commercial with tube audio. I get all my volume controls from Dave. Most of them are custom made to my specs, but occasionally I use his standard TVCs too. Dave also winds inductances for me for LCR RIAAs and other stuff.

Here is a recent delivery from Dave:


Volume controls on the left side, EQ coils in the diagonal line. These are intended for the variable EQ mono phonostage I wrote about in an earlier post. More about that phonostage soon! On the right are 8 1dB/step volume controls which will be used in a passive line level crossover for fine adjustment of each channel.

On the left a close up of a 28 step TVC, with 2dB per step. Actually they are correctly called AVC for autoformer volume control. They have a single continous winding with the taps logarithmically spaced for 2dB steps. As you can imagine, these are quite tedious to wire up to a switch. Therefore I developed a solution which is relay controlled on a PCB. This version uses Dave's simpler 14 step AVCs. These are made with custom step size for me. Through alternate switching of input and output 24 2dB steps can be had from a 14 tap AVC. With the correct control scheme for the relais, this can be done with a single 24 position switch. The photo below shows such a volume control PCB:



The 14 step AVCs are wound on the same core size as the 28 step version. Pins are only located on one side. Soundwise this solution is very similar to the one with the 'big' AVC.



The core material used in the AVCs and coils is nickel. Dave's preferred material. The coils make use of the available pins on the bobbin. These are used to bring out some taps for fine adjustment of the inductance. Very simple and effective. Besides this the inductance can be controlled by simply changing the airgap or by rearranging the way the laminations are stacked. Dave can supply paper pieces of different thicknesses for precise adjustment of the airgap.

There is also a forum on Dave's site. Here you can ask questions about his stuff, place inquiries for new things and you can also find discussions about a lot of audio stuff. Check it out! Besides this site Dave also runs a blog. See my blog roll on the side bar for HiFi heroine. There you will find posts about various stuff, not only related to tubes or transformers, there is also the occasional hilarious post just about anything.

It is great to have Dave as a reliable source for my special needs for anything which needs wire to be wound on exotic materials in a special way. I also consider him as a good friend. The photo on the right shows the man himself. Keep up the great work Dave!

Best regards

Thomas

Friday, September 16, 2011

Gallery: SE 45 amplifiers

Hi!

Here are some photos of two recently finished single ended 45 amplifiers. Both with different driver tubes, and different color schemes.


This one uses a directly heated triode as driver. Transformers are Tango. Black anodized metal plate, handles also in black. Capacitors are ASC X386S series, varnished in metallic bordeaux red.



The wooden frame is made of birch wood.



The power supply is external in a chassis in the same style:



The high voltage power transformer and capacitors in the power supply are hidden under aluminum covers varnished in a metallic graphite tone.




External power supplies offer the advantage of good isolation between the signal section and PSU. But there is also a very practical reason for such a two chassis solution. In a single chassis the amplifier would just be too big and heavy with all the transformers and chokes. This amp has 6 signal transformers: output, interstage and input transformers. The input transformers are placed inside the chassis. There are 4 power transformers in total. One for B+, one per channel for the DC filament supplies for the driver tubes and one for the AC filament supplies for the output tubes. The AC filament transformer is placed in the amp chassis, close to the output tubes. There are 10 chokes in total for the smoothing of voltages. Two for each of the driver tubes, for the filaments (LCL) and 6 B+ chokes. Two of the B+ chokes are in the PSU chassis, a common LCLC filter for both channels. In the amp chassis the B+ voltage is split between the channels and between drivers and output. Each of the 4 signal tubes is fed by it's own LC segment for good isolation.


A 10 wire umbilical connects PSU and amp. It hooks up to heavy duty Amphenol screw jacks. A female jack on the PSU side and a male jack on the amp. This way no voltage carrying pins can be touched even if any end of the umbilical is not connected while the PSU is turned on.

The PSU side is shown on the right. The screw type connectors and jacks provide a good and tight connection.

The photo below shows the connector on the amp chassis.



The umbilical:




The varnishing of the caps was done at a car paint shop. It requires several steps to get this kind of surface. First the caps get cleaned and the surface smoothed. Then the varnish is applied in 2 colored coatings, followed by a third clear coating for the shiny look. The photo on the left is a close up showing the metallic texture. The next photos show different close up shots of the caps.


Next some photos of the amp with indirectly heated driver, 6A6. It comes with a silver metal plate. The same ASC caps but with a matt silver coating. Same Tango transformers:



Also with external PSU. This time only with one cover, the caps are visible:




Both amps have switchable output sections. A switch placed nearby the output tubes selects between 45 or 2A3:




The amp with directly heated driver has also the input tube switchable, between 26 and 10Y (801A can be used as well). The switch in the middle is the ground lift:


When it comes to tube preference, I like the 10Y as driver best. It combines the resolution of the thoriated tungsten 10Y with the sheer magic of the 45. An ideal combination in my opinion. As output tube I prefer the 45 over the 2A3. The option to switch to 2A3 offers the possibility to boost the power to 3.5W compared to the 1.5-2W of the 45. This helps if less efficient speakers need to be driven. Among the 45s the globe versions are my favorites, like RCA UX245 or Cunningham CX345.



The picture on the left was taken in the dark to show the glow of the tubes. The amp is equipped with 10Y driver tubes and UX245 globes. See how much brighter those thoriated tungsten tubes glow compared to the oxide coated filament of the 245.

The photo on the right shows the PSU with the 6AX4 TV damper tubes in operation.





Best regards

Thomas

Tuesday, September 13, 2011

Swedish Iron

Hi!

Just received a delivery from Sweden. Plate chokes and transformers from Lundahl.



Nobody does a better packaging job than Lundahl. Each piece is individually secured with foam to avoid any potential risk of damage during shipping. The heavier items come in wooden crates.

This delivery contains plate chokes, line out transformers and MC input transformers for a couple of Octal preamplifiers MK2 as well as material for various amplifiers with 45, 46 and 6CB5A output tubes.

Several versions of 45 amps are in the planning, from single chassis (stereo amp with PSU in one chassis) with 6N7 or 6A6 drivers to two chassis amps with directly heated driver tubes like 26 or 10Y and input transformers.

I'll post some pics of newly finished amps soon.



Best regards

Thomas

Tuesday, September 6, 2011

Tube of the Month: The 45

Hi!

So far mainly indirectly heated tubes and rectifiers have been covered in this series. The only directly heated tube presented was the 46. But that is not a 'real' triode but a dual grid amplifier tube. Today I will write about the first directly heated triode in this series, the 45.



The 45 is an absolutely magnificent tube. It is among the 2 favorite directly heated triodes of mine. When it comes to colors, emotionality of reproduction and sheer beauty of sound, the 45 is unbeatable. It is the queen of tone. When the 45 was introduced, it was quite a step in achievable output power. Back then it required high plate voltages to get more than 1W out of a single triode with tubes like the 10. At lower plate voltages tubes like the 71A would yield a mere 0.7W. With the 45 up to 2W can be obtained with plate voltage below 300V, making the power supply easier. Compared to nowadays standards 2W seem like a joke. The 45 requires a suitable speaker to be able to show it's full capability. Speakers with sensitivities of 100dB or more should be used. It is possible to use less efficient speakers if you don't need to play at very loud levels or if you have a small room. Many people are surprised how much volume they can get from those 2W even with moderatly sensitive speakers, when they listen the first time to a single ended 45 amp. As was once written in the Sound Practices magazine: Don't think two Watts, think two thousand milliwatts!

The 45 shares the UX4 socket and same pinout with most of the famous directly heated triodes for audio frequency amplification. It has an oxide coated filament which requires only 2.5V. Due to this rather low filament voltage the 45 can be operated almost hum free even with AC heating. This together with the low plate voltage requirement of 250V typical, 275V max makes the 45 quite easy to use. For further details refer to the datasheet. So why does this tube sound so good? In fact I prefer it to all other directly heated tubes with oxide coated filament. Only when it comes to absolute resolution and neutrality, I prefer thoriated tungsten filamentry triodes. But as mentioned above, no other tube I know can beat the tone of the 45. This makes it easy to forget about the last bit of resolution and neutrality. Back to the question, which properties of this tube are responsible for it's sound quality? Have a look at the 45s plate curves which show their exceptional linearity:


The load lines drawn into the curves represent the manufacturers suggested loads which are more optimized towards maximazing power output. I like to use the 45 with higher impedance plate loads like 5k or even 7k Ohms. This improves linearity further and gives a better damping factor. With such an output transformer the 45 will not only have beautiful midrange and smooth highs but also a well defined and solid bass. No woolly lows as are sometimes heard from SE DHT amps. The next photo is a screen shot from an oscilloscope showing the plate curves of a real 45 tube taken with a curve tracer:


I have measured many 45s and when the tubes have no defect and the emission is still ok, they all show this exceptional linearity.

The 45 has been manufactured by many companies. Here is just a small selection of different 45s in the coke bottle shape:



And a selection of boxes from manufacturers like RCA, Sylvania, Raytheon, National Union, Fivre and many others:





Not all of these brands had their own manufacturing. As with many tube types, cross branding was a common practice. So you will find 45s with identical internal construction but with different brand names on them.

I'm often asked which brand I prefer in certain tube types. I'm not really a tube roller. As long as the tube is physically intact and measures ok, sonic performance is on the same level. Electrodes can sometimes be misaligned from transport or careless handling. This can lead to non linear plate curves. Especially very old tube samples can have this problem. This is why I use a curve tracer to check all tubes and match them to create perfect pairs. Although many brands just relabelled tubes from the large manufacturers, also many companies built their own tubes. So the 45 can be found with different internal construction, like plate profile and internal support. The next picture shows the different upper support structures which align the electrodes with a mica plate:



However as mentioned above I do not think the differences between various ST shape 45s are significant. It is more important to find tubes which measure well.

I do care for the sound difference between the ST or coke bottle shape and the earlier globe shape of the 45. These can reveal even more of the magic the 45 has and have a smoother even more involving sound. The globe 45, was actually named UX245 (RCA) or CX345 (Cunningham). While the ST shape 45s are still fairly easy to find in good quality, it gets increasingly difficult to find globe shapes which are still good. But I think worth the trouble finding them. The coke bottles can still even be found as NOS samples. This is almost impossible with the older globe shapes. Even if they come in seemingly original boxes, I have yet to come across a genuinely NOS globe shape UX245. Used ones can still have a lot of life left in them. With the globes it is important to be able to thoroughly check them. I think a curve tracer is mandatory to make sure the tube is ok. I have seen tubes which would measure ok on a static tube tester but reveal distorted plate curves on a tracer. Especially the globe tubes are prone to electrode misalignment. A reason for this is the lack of any physical support of the internals at the top of the tube.


They lack the mica plate which is used in the ST shape tubes to align the electrodes and give them support in the top end of the bottle. On the other side there have been speculations that exactly the lack of mica support in the globe shape tubes are one reason for their good sound. The mica plates can release gases over time and have a negative impact on the quality of the vacuum. Indeed most of the old globe shapes I have come across had excellent vacuum.

The globes are quite a bit larger compared to the coke bottles:



As mentioned above, the linearity of the tube is probably one of the most important factors which contribute to the good sound. Another one is the low filament voltage which makes the use of AC heating possible. DC heating can have a very negative impact on the sound if not done right. I have used AC with 45s succesfully, without any hum issues. Another and in my opinion also very important point is the modest drive requirement. The grid of a 45 is a fairly easy load to the driver. It needs a modest 35-40V RMS to drive it to full power. This makes 2 stage amplifier concepts feasable. A good driver tube for the 45 is the 6N7 which I introduced already in previous posts. Below are some pictures of an amp with the 6A6, the predecessor of the 6N7, as driver. The first one shows the amp equipped with globe shape UX245:



The next photo shows the amp with ST 45s plugged in and together with the external power supply:



I hope you enjoyed this first Tube of the Month article about a directly heated triodes. Stay tuned for more DHTs to be covered in upcoming posts.


Best regards

Thomas