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, March 25, 2015

Tube of the Month : The 816

Hi!

In the previous post about the latest build of the Octal Preamplifier, I showed a power supply with a hybrid bridge which uses two 816 mercury vapour rectifiers. Since this was the first time I used these, it is a good opportunity to write a post about the tube.




The 816 is a smaller version of the 866A with less voltage and current handling capability. The 866A is way oversized for most audio projects so the 816 is a good alternative if you must have that magic blue mercury glow.

The 816 has the same pinout as the 866A. An UX4 base, of which only two pins are used for the filament. The plate is wired to a cap on the top. While the 866 has a medium size cap, the 816 has a small cap. A pair 866A can deliver 500mA when used up to it's peak inverse voltage limit and a whopping 1A at peak inverse voltages up to 2500V. A pair of 816 'only' handles up to 250mA (125mA per tube). Still plenty and more than enough for the preamp it has been used in. The peak inverse voltage rating of the 816 is an impressive 7500V. The big advantage of the 816 over the 866A is the filament current. It 'only' needs 2A at 2.5V while the 866A uses 5A.
This makes the use of the 816 a lot easier compared to the 866A. You get the same beautiful blue glow from the 816 but in a much smaller bottle. Which in turn can be an advantage since it does not need so much chassis space as the impressive 866A. The ST12 bulb of the 816 is about 10cm high and has a diameter of about 4cm. The dimensions of the 866A's ST19 bulb are 50% bigger. See the photo below for a size comparison between these two mercury vapour rectifier tubes. For further technical specs of the 816, please refer to the datasheet.





Both the tubes above have been made by RCA. They used to package their mercury tubes very careful with soft padding inside the box:






This one contains quite a lot of mercury.




The mercury is plastered all over the inside of the tube. This is the reason why they require a special conditioning when used the first time in a power supply or after they have been transported. Running only the filament for 30 minutes vaporises the mercury which then condenses at the base. After such preconditioning the tubes can be used with high voltage. This procedure prevents that any mercury drops are still inside the electrodes and could create a short.

The base of an RCA 816:




Another RCA 816, labelled as JAN type for the military:




General Electric GL-816:



The base:




There seems to be much less mercury in this one compared to the RCA shown above.




The mercury deposited on the metal stems:



Close Up:



More GE 816s:




These have a warning label on the base regarding X-rays. However such tubes only produce X-rays when operated close to their peak inverse voltage rating. At the typical voltages in audio amps this is not of concern.




Raytheon RK816:




Sylvania:




United Electric:




CBS Hytron:




Westinghouse:




Due to the mercury content, I am not going to open a tube to show the internals. Here a close up showing some mercury condensed on the inside of the glass:




Instead some photos showing the beautiful glow:













The intensity of the blue glow depends on the current drawn. At low current it is very dim:




Becoming more visible as the current goes up:




At medium current the glow starts to fill the entire bulb:




Full current:



It should go without saying that such tubes ought to be handled with utmost care. Mercury is very toxic. Only use these tubes if you know what you are doing.

Best regards

Thomas




Saturday, March 21, 2015

The Octal Preamplifier Mk2

Hi!

Since I just finished another build of an Octal Preamplifier in the Mk2 version, I thought this is a good opportunity to write again about my Octal Preamplifier series.




The Development of the first Octal Preamplifier started back in 2008. After I published the 6CB5A amplifier concept on a german tube forum, the question about a matching preamp came up. Since the 6CB5A amp got built in various incarnations including some no compromise versions, The question about an improved Octal Preamplifier came up as well. This led to the Mk2. The details about the design can be found here.




After all the line stages with directly heated tubes and LCR phono stages, it was refreshing to work on this design again. The circuit is unchanged from previous versions. Also the chassis design is virtually the same classic landscape layout. With just the exception that the tubes on the signal section are now on a suspended plate to reduce microphonics.





The use of such high mu and low transconductance tubes as the 6SL7 or 6SC7 as in this version, as first tube in the phono section gets unjustified criticism by some tube amplifier builders recently. The claim being that only high transconductance tubes will yield a low noise design. But in real life the few dB difference in noise performance which can be gained by using high transconductance tubes is rather minor. The 6SC7 performs nice and quiet in this design. But it doesn't have the diva like behaviour which tubes like the D3a or WE417 can show. If not taken care of they can burst into self oscillation which can cause rather harsh sound.




The 6SC7 is complemented well by the 6N7 which is used as second stage in the phono section. RIAA equalisation is done by split passive RC networks. The beefy 6AH4 is performing in the line stage. It drives a Slagle/Intactaudio AVC through a step down line output transformer.


The volume control is done by a 24 position Elma rotary switch which selects the taps on the AVC and provides 2dB steps. There are three line inputs and one phono input for MM cartridges. MCs can be connected through an external step up transformer. The MC transformer can be integrated as well but the person who is going to use this preamp already has a selection of step up transformers.





Power is provided to the signal section from a PSU in it's own chassis. As a special touch the power supply has a hybrid rectifier bridge using two 816 mercury vapour rectifiers in conjunction with two 6AX4 TV damper tubes.




The 816 is the 'small sister' of the 866A and provides a similar eerie blue glow in operation. More about this tube in the upcoming tube of the month post.





Due to the 6AX4s controlled warm up, no separate filament and high voltage switches are needed. The high voltage comes up gradually as the heaters of the 6AX4s warm up. This has the nice visual effect that the blue glow appears gradually. This is illustrated in this short video clip:


video





How about the sound of this preamp? As it is playing right now I am enjoying nice vocals, beautiful tone colors and a smooth sound. The 6SC7 and 6N7 combo in the phono section are performing without any harshness. No listening fatigue. The overall representation tends a very slight bit to the caramel side compared to other preamps. But in a very subtle and nice way. The line section brings all the advantages of a low impedance transformer volume control. High resolution and transparency. The low output impedance can drive any cable lengths and pretty much any power amp.





So why bother with separate phono and line stages, LCR EQ and directly heated triodes if this design performs so well? Of course such separates bring yet another level of performance. A D3a or EC8020 phono does have a more quiet back ground and improved articulation of sibilants. A direct heated line stage does give a higher level of resolution and stronger tone colors. But all that at 2.5 - 3 times the cost or more depending on some choices and twice the rack space needed. If the budget does not allow for a full fledged LCR phono and DHT line stage, the Octal preamplifier is a smarter choice, rather than trying to take short cuts and choosing a cheap implementation with LCR and DHTs.




The photos above do not show the final wooden enclosures. These are still in the making. The wooden frames on the photos are just temporary and the chassis will teals get some feet to rest on. This preamp will be used with a 6CB5A amp for which it was originally designed. The 6CB5A amp is also finished und currently undergoing testing. It will be shown in an upcoming post. Stay tuned!

Best regards

Thomas

Saturday, March 14, 2015

The Stereo 300B Amplifier, Part 2

Hi!

In this post I will show the assembly steps of the recently announced Stereo Amplifier with the Elrog ER300B triodes.




The top plate with sockets, resistors, capacitors and output transformers mounted:




Heater and B+ wiring in place as well as most of the signal wiring:




Chokes and interstage transformers are mounted on the next tier:




The inside of the completed PSU which has it's own chassis:




The finished amplifier:



Front view:



Stay tuned for part 3 with a report about the sound.

Best regards

Thomas