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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Thursday, October 31, 2013

UX201A Sound Processor, Part 1 : Circuit


In the Tube of the Month Article about the UX201A I already mentioned it's unique sound qualities. This is the first post of a series of articles about a device which I call a 'Sound Processor'. It uses a single UX201A triode per channel as active element. The purpose of the device is to present the sound of the tube itself in it's purest form with as little coloration as possible by other components in the signal path. This is the extremely simple circuit that does this:

As you might have noticed this circuit has no capacitor in the signal path at all. There is also no hidden one in the power supply since that is decoupled through a choke from the circuit.

To achieve this the circuit uses an enhanced version of filament bias. Filament bias effectively removes the need of a cathode bypass capacitor which would be in the signal path. It pulls the entire filament current through a resistor to ground which thus becomes very small in value (22 Ohm in this case). So small compared to the plate resistance of the tube that it can be neglected and does not need a bypass capacitor.

But the usual filament circuit still has a capacitor in the signal path on the B+ side. In case of a transformer coupled stage this is the capacitor from B+ to ground (or B+ to cathode in case of the ultra path connection).

In case of the circuit above this capacitor is replaced by a resistor, the 500 Ohm resistor from B+ to the filament. 500 Ohm is quite small compared to the plate resistance of the tube to which it is basically connected in series. I coined the name 'DirectPath' for this circuit since it DC couples the B+ side of the transformer primary to the filament. It also sounds very direct, removing the last capacitor from the circuit and thus revealing the tube sound in full.

This 500 Ohm resistor serves another purpose at the same time. It derives the filament voltage for the UX201A from the B+. Since the UX201A only requires 250mA filament current, this is doable in this way. Nothing is free however and this approach has a big disadvantage. The 500 Ohm resistor dissipates about 30W. This requires a massive resistor on a heat sink. Also the power supply needs to be quite substantial. It has to deliver 500mA for a stereo unit. Luckily the UX201A runs at low plate voltages of 135V max. In this circuit the plate voltage of the tube is about 125V. The B+ which is fed to the circuit needs to be somewhat higher to allow for bias voltage and the drop within the choke.

The 100k resistor in the circuit serves as grid to ground resistor to ensure the grid always has a defined potential. The cold end of the secondary of the output transformer is connected to ground through a 1M resistor to avoid any possible static charge build up in case the output is unconnected.

I have tested and used this circuit successfully many years ago. I am now working on a new implementation using Tango NP8 line out transformers. The assembly process and power supply will be covered in upcoming articles. Stay tuned.

Best regards


Monday, October 28, 2013

ELROG 211 now with silver wiring


After some discussions with ELROG I managed to get my tubes custom wired with solid core silver wire for all socket connections. This is the wire from the base pin to the metal connection which extends out of the glass.

A limited quantity of the silver wired tubes are available, currently only 211 triodes. Some 211 and 845 are left in stock with the standard wiring. These are available at a discounted price.

Drop me an email for a quote. Contact details can be found under the link Impressum / Contact at the top of the left column of the blog site.

Best regards


Thursday, October 24, 2013

The Integrated Amplifier


I often get complaints that my electronics are too big and consist of too many chassis. So I came up with this integrated amplifier. It is not only a full function amplifier with volume control and 3 line inputs, it also has a phono section on board with built in MC step up transformer.

This requires a lot of tubes. 10 Signal tubes in total. the phono stage is implemented with 6SF5 and 6J5 triodes. The circuit is exactly the same as in the Octal Phono Preamplifier. The 6GL7 serves in the linestage which shares the same circuit with the Octal Line Preamplifier. Basically the preamp section is the same as in the Octal Preamplifier MK3. To make the amplifier complete a stereo version of the low cost 6CB5A amplifier was added.

The PSU got it's separate chassis to keep the chassis size and to avoid interference with the phono section. To fill up the PSU chassis it got a full wave bridge rectifier with 4 6AX4 TV dampers and a choke input filter. Two LC section provide the smoothing of the B+. Inside the chassis is a filament transformer which is part of a DC heater supply for the preamp section. The power amp section is AC heated.

Some photos of the assembly steps. The top plate with capacitors and output transformers mounted:

The output transformer had to be mounted sideways so that they fit under the 100mm transformer covers.

The preamp tube sockets are mounted on rubber dampers:

Heater wiring and some of the coupling caps:

Adding solder strips:

Adding all the remaining parts:

Prewiring the back plate:

Ready to be mounted into the wooden frame:

Just two controls, input select and volume:

The backside is quite crowded:

Starting from the right: Phono MC inputs, Ground post and ground lift switch underneath. Next two switches for selecting the step up ratio of the build in MC transformer. Followed by 3 line inputs. The 5th pair of RCA jacks is a line out. The signal after the line stage section is routed to these so an additional amp can be connected or the component can be used as a preamp. The speaker jacks on the bottom and the power supply connector on the left.

The entire amplification chain for an analogue system in just two boxes. Five Amplification stages. All Octal tubes. No more complaints about size please ;-)

Best regards


Tuesday, October 22, 2013

Cool Gales Bath AudioFest 2013 photos.


Last weekend I exhibited some of my new preamplifiers on the Cool Gales Bath AudioFest which is hosted annually. Ivan Kursar invites a few selected manufacturers each year to show their equipment. The event was held in two locations within walking distance. I was lucky to get a room in the prestigious Bath and County Club:

I had the pleasure to set up a system together with Ulla Scheu of Scheu Analog. I was very glad to team up with Ulla since I am using Scheu turntables since many many years. We used a Classic Line turntable equipped with the Tacco tonearm along with my Octal Phono Preamplifier and Octal Line Preamplifier:

Ivan's two chassis single ended 6CB5A amplifier served as power amp driving Audioarum's LaGrande speakers:

Despite the rather large room the moderately sized speakers had no problem to fill the room with sound, powered from just 7W.

Also on display, the full function Octal preamplifier:

And Ulla's gorgeous Diamond turntable with a stunning base in pink:

We had a lot of fun during the exhibition, playing various pieces of music and chatting with the visitors.
Thanks a lot to all the visitors for the nice feedback. Many thanks to Nicolas of Audiaurum for providing these great speakers. And last but not least to Ivan and his family for organising such a high quality event.

See you again next year for the Cool Gales Bath AudioFest 2014!


Wednesday, October 16, 2013

Tube of the Month : The UX201A


This months tube is a very special directly heated triode which is among my few all time favourites. Since I started this blog, I was looking forward to present this tube, so get prepared for many photos. It is a very old tube, which was introduced in the 1920ies. The UX201A.

The UX201A was among the very first tubes which got marketed by RCA. They started with the UV200 and UV201. The latter was replaced by the UX201A in 1925.

The UX201A differs from it's predecessors in the base. It uses the standard UX4 base. The pinout is pictured on the left. It has a thoriated tungsten filamentary cathode which runs at 5V and 250mA. The UX201A was used as detector and amplifier tube in radio sets. See the datasheet for all technical parameters. With it's low transconductance of a measly 800 micromhos combined with a low amplification factor of 8 it seems rather useless in tube audio circuits nowadays since there are plenty of more modern tubes available which have much more interesting technical properties. But used in the right way this tube can produce some very detailed and marvellous sounds. Eric Barbour once wrote an interesting article in the Glass Audio Magazine about the use of the UX201A in line stages. He wrote a very true statement: 'In a sane world there would be more tubes like the UX201A'.

The datasheet reveals some very linear plate curves as is typical for directly heated triodes:

A cross check with an actual tube on the curve tracer confirms the linearity:

I have used this tube in line stages or simply as buffer stage. It has some very special sound characteristics. Definitely on the euphonic side but in a subtle way which does not impact resolution. More about this in upcoming articles about a project with this tube.

Also check out the Reverse Time Pages for additional information about this family of tubes.

From the 1920ies to the early 1930ies the tube was built in the beautiful globe shape. There had been a myriad of small tube manufacturers who built this or similar tubes. Some had chosen slightly different type nominations. Construction styles varied from manufacturer to manufacturer. Some had short bases some taller bases. Later the tube was manufactured in the more modern ST shape like other tubes as well. While the globe versions where rather small compared to output triodes like UX210 or UX245. The ST versions had the same size bottle as the 45 output tubes.

Let's have a look at some tube samples from different manufacturers. We start with the RCA Radiotron UX-201-A:

These got packaged in one of the most beautiful tube boxes. The tube is wrapped for additional protection inside and comes with a paper insert with information:

The dark spot with the rainbow colored border is very typical for these tubes. This is no sign of a worn tube and is present in many unused tubes. This was caused by the getter activation process during manufacturing.

The RCA Radiotron UX-201A in a different style package:

Beautifully engraved base:

The top flaps of the boxes:

Cunningham named the tube CX-301-A:

All sides of the Cunningham box:

These also came with a paper inserted.

The tube in all it's glory:

In the old days, tubes often got stickers from the shop which sold them:

Or from testing:

The Cunningham logo on the top:

The tube can be found in different sizes, even from the same manufacturer:

RH-201-A from Perryman:

Triad T-101-A:

This one got plastered with labels:


This one has the entire glass covered with getter:

The base:

Arctutus 101A with blue glass:

The boxes:

The engraved base:

Let's have a look at the more modern types with ST-glass. The naming was changed to 01A when the globe shape got dropped.

Philco 01A:


Another Sylvania in military JAN packaging:

National Union 01As in pristine boxes:

Engraved base:


RCA 01A:

You can be sure the tube is NOS when it has the box still sealed like this:

Guarantee, licence notice and patents printed on the inner protective carton:

Engraved base:

Tube type printed on the glass:

Some photos of the predecessor of the UX201A, The UV201:

These had the older UV4 base with shorter pins than those on the UX4.

These tubes were marketed by RCA under the Radiotron brand. The manufacturer was General Electric. The GE label is barely visible:

Now let's examine how the tube is constructed. Of course I used a non functional tube with open filament. First the base is removed:

The cement which glues the glass into the base is often loose in such old tubes. Desoldering the pins and a little force gets the glass body off.

Breaking the glass dome:

These old tubes used a magnesium getter which unlike the barium getter which was used in later tubes reacts more slowly. So the silver coating does not turn white immediately as with other tubes when the glass is broken.

The internal structure:

Surprisingly the alignment of the structure is done with a triangle shaped mica disc. Other tubes of this era used glass stems.

The small wire in the middle at the top holds the filament. Since the filament was already broken, none of it is visible in this photo:

Close ups of the filament hook:

Removing the plate:

A close up showing a remainder of the filament. It is very thin, much thinner than the grid wires:

No wonder that the most common failure of UX201A is open filament. This is a very fragile tungsten wire. I opened another tube sample to get some better pictures of the filament and how it is welded to the support rod:

Another view:

And now some photos of glowing tubes:

Since most of the UX201A tubes have almost the entire glass covered with getter, glow is often only visible near the base.

This one has a bit less getter:

A glowing ST 01A:

View from the top:

ST 01A at night:

The blue Arcturus 101A in operation:

UV201 in operation:

The older UV201 had pure tungsten filaments which glowed almost white.

A view from the top:

At night the logo on the top is beautifully illuminated by the filament:

And the label as well:

I hope you enjoyed this tube presentation as much as I did when I wrote it. Of course a project with this tube will be shown soon. Stay tuned.

Best regards