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Friday, February 28, 2014

The Remote Controlled 10Y Preamplifier


I am not really a big fan of remote controls. I prefer to physically interact with the audio gear in the same way as I prefer the interaction with vinyl as a medium over digital formats. Whenever someone asked for a remote controlled preamplifier I refused to do it usually with a little joke that the exercise of getting out of the chair to change volume is rather healthy. But then came this nice chap who already has a 10Y/45 power amplifier and convinced me that remote control for the volume is essential for him. It would have been a shame if that 10Y/45 amp would continue to be driven from a different preamp. So here it is, my first remote controllable line preamplifier.

The circuit, parts and design are the same as those of earlier implementation of the 10Y preamp. The line stage can be equipped either with a 10Y (10, UX210, VT25, VT25A) or the slightly uprated transmitting tube 801A (801, VT62) which will be covered in a future Tube of the Month article. Volume control is done with a Tribute line output transformer which has a tapped secondary.

So how to implement the remote control? A solution would have been a PCB with a bunch of relays which are then controlled by the remote. But the preamp should still have that classic old school look and feel. Which means a solid volume control switch which always indicates the volume setting rather than a digital LCD display. Although my professional background is in digital microelectronics I chose rather not to reinvent the wheel but use an existing solution. Bent Audio offers a remote controlled stepper motor along with the IR receiver and a nicely made metal handset. So I emailed John Chapman of Bent Audio and ordered a set. John is a very nice guy who was very helpful to sort out some minor mechanical issues to combine the remote control system with an appropriate switch.

The motor is mounted on the back of the switch and turns it up or down as the remote control is used. The switch can also be turned manually. So the marker on the volume knob always indicates the volume setting. When the preamp is turned off and back on the volume is at the same setting as before without the need of any memory storing the value or special relays which hold their setting.

This is the motor mounted to the switch which has already the teflon insulated silver wires attached which will get soldered to the transformer volume control:

The preamp has two single ended inputs and two balanced inputs with separate input transformers. The input transformers are mounted right behind the input selector switch:

The stepper motor and it's control electronics only need a single supply between 9 and 12V. Quite convenient, but the motor needs to have considerable torque to rotate the switch. When in operation it consumes current peaks up to 1A. Although the digital control circuit goes to sleep mode and turns it's clocks off when not used, I didn't want to supply it from one of the filament supplies. So it got it's own independent supply with a separate power transformer. This power supply is placed in the external PSU chassis along with filament and high voltage supplies:

The remote control system PSU even got it's own smoothing choke.

The top plate of the signal section with all the signal wiring in place:

The inside of the completed signal unit:

The completed preamp with power supply:

The front:

The two balanced inputs have separate sets of input transformers. One wired for 1:4 step up (+12dB) and one wired 1:1. In addition there are two regular single ended inputs.

The back sides:

The umbilical between PSU and preamp needs 9 separate wires for all the different supply voltage and safety earth. A 10 pin umbilical is used.

The preamp section from the top, with the remote control:

Preamp equipped with globe tubes:

View from the side:

The globe in these photos are the magnificent 1602 triodes. A special low microphonics version of the 10Y.

Best regards


Thursday, February 20, 2014

211 Drives 211, Part 4


First sounds from one channel of the 211/211 amplifier!

After the first amplifier section was finished, I did the assembly of it's power supply unit which is built in the same circular style, with several round plates carrying submodules. Starting from the bottom, the first one holds the main power transformer for the high voltage and the connectors for umbilicals and mains, which plug in from the bottom:

A set of bleeder resistors is mounted on the same sub assembly:

The next level holds the filament transformers and chokes:

The rectifier bridges for the filament voltages are mounted on solder terminals in the middle, stacked on top of each other:

Then the smoothing caps for the filaments:

The two resistors on top of the stack are bleeder resistors to ensure the filament chokes will carry critical current for proper choke input operation even when the amplifier is disconnected from the PSU.

Chokes and large paper in oil caps for the smoothing of the high voltage supplies for output tube and driver:

And finally the top plate which holds the sockets for the rectifier tubes:

Rectification is done with a full wave bridge which can either consist of a pair of mercury vapour 866A, supplemented by a pair of 6CG3 TV dampers, or alternatively a bridge of 4 6CG3s.

The completed power supply:

View from the top:

The blue glow of the mercury rectifiers:

First test of the amp with the PSU, here with 836 instead of 866As:

The capacitors lying on top of the amp is a last minute tweak. The amp is playing nicely, tweaking is finished and the assembly of the second channel can start. Different versions for the housing are also in preparation. Stay tuned!

Best regards


Monday, February 17, 2014

ShellacSavor, Part 2: The Circuit


In part 1 of the series about Shellack reproduction, I covered the basics about the different EQ curves used.  In this post I will show the circuit of a variable EQ preamplifier for Shellac playback.

In the first part the faceplate of the preamp was already shown with 12 settings each for the turnover frequency and 10kHz roll off. Turnover and 10kHz roll off shall be implemented passively and separately so that they do not interact with each other. This requires a 3 stage design. A first stage which drives the turnover EQ network and a second stage to drive the high frequency EQ. Followed by a first stage which should provide low output impedance.

Although I am in favour of LCR EQ networks, implementing a total of 24 would require a lot of space and become impractical. Therefor the EQs will be implemented with RC networks. The bass EQ will require 3 passive components. An input resistor followed by a RC segment to ground which will determine at which frequency roll off will start and stop. A fixed 100k resistor can be chosen as input feed and a selectable RC segment based on each setting.

The high frequency EQ only requires a capacitor after the input resistor. The same 100k is chosen followed by selectable capacitance to ground. The flat (0dB) position will not require a capacity so that position of the switch will be left open.

For the equalisation to remain fairly constant with ageing of the driving tubes, it is desirable that the tubes chosen have a plate resistance which is substantially lower than the input resistance of the networks. Ideally less than one tenth. The output resistance of the driving stage is in series with the EQ input resistor. This way a 10% change in plate resistance would translate to less than 1% change of the EQ input resistance. So we would want tubes with plate resistances well below 10kOhms.

Another factor which determines the tube choice is the overall gain needed. MC cartridges for Shellac records usually have a rather high output voltage compared to regular cartridges. I have chosen the Ortofon SPU MONO CG 65 which has an output voltage of 1.5mV at an internal resistance of 6 Ohms. So with a step up transformer this will deliver 10-20mV depending on the step up ratio used. The loss in the EQ networks will vary depending on the EQ setting. This can be anywhere from around 20dB to a bit over 32dB loss. The preamp shall be used with an active line stage, so a sensible raw gain (excluding step up transformer) would be in the range 60-70dB which would leave 30-50dB after the EQ, depending on the EQ setting. Adjustment of the gain could still be done with choice of MC step up transformer.

In the last tube of the month article I already presented the 6SN7, which seems perfect for the job. It has a mu of 20 and a plate resistance of 7kOhm. With resistive plate load this would give a gain of about 15 at an output impedance of around 5kohms. The two 6SN7 stages (one triode system per stage) would result in a raw gain of 225.

To bring that up to the desired total gain we need another factor of 5 or 6. A 6AH4 seems perfect for the job with it's mu of 8 and plate resistance of 1780 Ohms.

This is a sketch of the circuit:

The values of the resistors and capacitors in the EQ networks might need some tuning.

The prototype build of this circuit will be shown in part 3. Stay tuned!

Best regards


Wednesday, February 12, 2014

Tube of the Month : The 6SN7


Most tubes which are presented within the Tube of the Month series are outside the mainstream and not well know to most tube amplifier builders. This month is an exception with the presentation of the 6SN7.

The 6SN7 got very popular among tube amp builders during the 1990ies. In fact a real hype developed around this tube. It was very often used as driver for the 300B which also had a big rise in popularity during that time. As usual with any type of hype, things got exaggerated and some people even thought the 6SN7 is the best tube ever with exceptional linearity and sound. Every hype reaches it's zenith at some point and draws some haters. Especially after commercial amplifier manufacturers jumped on the single ended DHT amplifier train, the 6SN7 was used on an even broader basis. During the last decade the 6SN7 got some bad rap from the haters with claims of poor sound. Again overly exaggerated. While the 6SN7 is not the super tube it was claimed to be, it is not bad either. It deserves a spot in the Tube of the Month series.

The 6SN7 is a medium mu double triode. Two identical triode systems in a common bottle with an Octal base. The pinout is shown on the left. It makes perfect use of the octal base since it utilises all 8 pins. Even pin 1 is used for an electrode. Pin 1 is usually reserved for connection to the metal shell for metal envelope versions of octal tubes. It is left unused in glass versions. Since there never was a metal 6SN7, pin 1 is used for the grid of triode system 1. Each triode has a raw amplification factor of 20 at a moderately low plate resistance of 7700 Ohms. Complete data of the tube can be found in the General Electric data sheet. Since the tube was so popular, it got used in many amps and preamps regardless if it was really suitable or not. And this is probably the reason why the popularity dropped rapidly after some years. If used as a RC coupled driver for a 300B in a zero feedback single ended amp (which is a very common use of the 6SN7 these days) it's gain is not enough to stick with just a single driver stage. So most amplifier designers just add another 6SN7 stage in front, using the second system in the envelope. The problem with this approach is that the total gain of such a 3-stage design is too high, resulting in an overly sensitive power amp. I have seen such designs in which the first stage was run at verily low voltages, around 50V and low current. Probably to deliberately add some 'tubey sound'. No wonder it received some bad reputation with many such amps out there.

Let's have a look at the claimed superior linearity. These are the plate curves of the tube from a data sheet:

Not bad at all, but some care needs to be taken to avoid the knees of the curves. Actual plate curves taken with a curve tracer look even a bit better:

Great if it is used in a linear region and some substantial current (8-10mA) so it stays away from the knees. If used at the mentioned low plate voltage and at only a few mA the tube will contribute significantly to the distortion behaviour of the amp. It will also react sensitively to minuscule changes in operating points. And of course whenever the tube is rolled, each sample will sound differently.

As mentioned above, the 6SN7 was never made with metal envelope as most other octal tubes, maybe besides some initial prototypes. So the tube is only available as 6SNGT type, the GT suffix indicating a glass tube. There is also no coke bottle version of this which would have been called 6SN7G. Other letters in the naming like the W on the photo above indicate a special ruggedised military version.

Suffixes 'A' or 'B' at the end have been added as the tube specifications got changed. The original 6SN7GT had a maximum plate voltage of 300V and maximum dissipation of 2.5W per plate. After World War II the tube got uprated to the 6SN7GTA with an increase of the maximum plate voltage to 450V and 5W plate dissipation (limited to 7.5W for both plates combined). This was necessary to enable the tube to be used in TV sets as oscillator and amplifier in the deflection unit. This is important to consider if you have an amp with 6SN7. If it operates the tube outside the limit of the original version. Make sure to only use 6SN7GTA or 6SN7GTB.

The last upgrade to the 6SN7GTB only changed the heater. In the 'B' version the heater has a controlled warm up time to enable the use in series connected heater strings. Again a requirement for TV sets.

The 6SN7 has an interesting history. It is at the end of a development of medium mu indirectly heated triodes which started with the UY227, the globe ancestor of the 27. This was one of the first indirectly heated triodes which found broader commercial use. The UY227 is the first tube on the left in the picture below:

It was followed by the 56 and 37. These were initially also made with the beautiful globe glass. The second tube is a Cunningham 56 ans the blue one a Arcturus 37. With the jump from 56 to 37 the heater was changed from the 2.5V of the 27 to the most widely used 6.3V heater voltage. This had the advantage of lower current. Next to the Arcturus are ST shaped 'coke bottle' 27 and 56. The last tube in the row is the 76. All these tubes had the 5 pin UX5 base.

The development continued with the introduction of the more modern 8 pin Octal base. The first derivate which could be regarded as an ancestor of the 6SN7 was the 6C5 shown on the left in the next photo:

While the development from the 27 to the 76 only brought moderate changes in amplification factor and transconductance, the 6C5 made the jump to a mu of 20 which is the same as that of the 6SN7, but it had a bit less transconductance. The 6C5 was not very successful, some of the early 6C5s were just internal triode wired 6SJ7.  The 6P5 (second tube in the photo above) had a lower amplification factor again, similar to that of the 56. The first successful tube of these octals was the 6J5, which was already covered in it's own Tube of the Month post. The three tubes on the right are variants, a 6J5G, a 6J5GT and a metal 6J5. None of these tubes made use of all pins of the octal base. The 6SN7 packs two triode systems into one bottle, both with the same specs as the 6J5. There had also been some variants with grid caps like the 6C8 and 6F8. Most of the tubes mentioned above are very interesting and will be covered separately in future Tube of the Month posts.

Since the 6SN7 was the most widely used of all of these, it was made by many manufacturers and in different variants. It found wide use not only in audio amplifiers and TV sets but also in military equipment and even computers. Lets have a look at some different 6SN7 variants. The first photo above shows a beautiful Ken-Rad 6SN7Gt made for the military, here with the box:

It has a distinctive dark coating which covers almost the entire tube, except the top:



General Electric 6SN7GTB:


This is a tube with a so-called coin base. Largely ignored by audiophiles for claimed inferior sound. Which is absolutely not true if used at decent operating points. A closer look at this coin base:

Another coin base Sylvania made for the military:

A Zenith, made by Sylvania:

A PhilipsECG 6SN7GTB:

These have distinctive triangular shaped plates:

These are only a few of the abundance of different 6SN7s which had been made. With all those variants, the 6SN7 is a tube rollers dream, or nightmare. Some people spent years hunting the perfect 6SN7 for the amplifier. Some specific variants skyrocketed in price while others remained reasonable.  While there are differences in performance, these do not justify the price difference which can bee seen on the market. If used reasonably well, any of these will give good performance without the need to worry if the tube should be rolled.

A special version of this tube was hyped among tube amp builder especially after it had been briefly covered in the Sound Practices magazine. The industrial version 5692, one of the three tubes in RCA's special red series:

The tube has a red coloured base which gave the series it's name. It had a guaranteed lifetime of 10000 hrs. But this was probably just reached since it's specs were down rated from the 6SN7GTB.

The 5692 had extra bracings inside to make it usable in harsh conditions where it is exhibited to shock and vibrations like in space applications. Wether this is of any advantage in an audio application is questionable.

A beautiful tube nonetheless especially with the unique 'ox blood' coloured base.

When the prices for 6SN7s started to set off in the 1990ies, smart DIYers used the 12SN7 instead. Simply provide a 12.6V heater instead of the 6.3V and the amp can use these. For a long time prices of the 12SN7 were very low.

But these had also been discovered by the mainstream at some point and cannot be found at rock bottom prices any more. They are still reasonably priced though.

I did use the 6SN7 in the line section of the Octal Preamplifier Mk1. There it works quite well and provides a reasonable gain when used with a 4.5:1 step down line out transformer. The next use of this tube will be in the Variable EQ preamplifier for Shellack reproduction.

Best regards