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, February 21, 2018

The Octal Preamplifier

Hi!

since I mostly write about my separate phono and linestages, many people are not aware that I also have a full function preamp in the offering which has a phono stage integrated. The Octal Preamplifier.




The Octal Preamp has a transformer coupled lien stage with auto former volume control. In this Mk2 version the linestage uses the 6AH4 triode.




The phono section is a LC coupled circuit with passive split RC RIAA.




6SC7 is used as input tube and 6N7 in the second stage.




Signal section and power supply are in separate chassis.





In this case the preamp is built in the classic landscape chassis but could be done in portrait stye as well.








A lower cost alternative to my D3A and 10Y combo.




Best regards

Thomas




Friday, February 16, 2018

Custom Line Preamplifier

Hi!

A while back in a post about a set of custom 300B amplifiers I showed the kind of adaptation which is possible to my regular designs. Here is a custom line preamp which will be used with those 300B monos.




The concept is based on my 10Y linestage but with different transformers, in this case a set of line output transformers supplied by Monolith Magnetics.





For volume control Sowter TVCs have been chosen which allow some additional gain which in this case is used for separate balance adjustments.




The knobs which are different style from my usual ones were also customer supplied.




The power supply comes in a separate chassis.




The PSU uses a 5U4G full wave rectifier tube. the switches on the PSU allow some adjustment of the filament voltage which can be monitored through the jacks in front of the tube sockets on the line stage.




While I usually offer a variety of options, such special requests can also be fulfilled.

Best regards

Thomas




Monday, February 12, 2018

The 46 drives 46 Amplifiers - Part 1

Hi!

After the excellent sound achieved with the 46 amplifiers I built so far, I always wanted to make a more elaborated one with a second 46 as driver tube.




Finally somebody commissioned a set of mono blocks with this configuration. Here are some first photos of the initial construction steps.




The top plate only has the tubes exposed on the top. the two 46 and a row of 4 6AX4 or similar TV damper tubes for a full wave bridge rectifier. some components are directly mounted on the underside of the top plate for short signal paths:




This is how the top will look like with the tubes plugged in:





The amp will be constructed in the tower style similar to the recently finished 845 monos.





As in those the parts are mounted on separate sub modules which get stacked inside the chassis. Above the filter section of the high voltage supply, filament chokes (all filaments will be DC) and the Tango output transformer. The power supply section, with main B+ transformer and separate filament supplies for the two 46:




Stay tuned for updates as the construction of these progresses!




Best regards

Thomas




Thursday, February 8, 2018

News from the ELROG Tube Factory 9

Hi!

It's been quite a while since I published an update about the Elrog tubes. We had been busy catching up with orders and with the development of the new tube types.





The development of the ER50 and ER801A took more effort than expected. I am happy to announce that we have first functional prototypes.




It's always the details which cause throwbacks. What first seemed like a rather straight forward job by making the necessary adaptions to the 300B design, ended up in a complete redesign of the entire structure.




But it was worth it! The first 801A prototypes are shown in the pictures. amplification factor is a bit low but will be adjusted to reach the target value in production.




The filament spec is spot on at 7.5V/1.2A.




Some of the lessons learned from this development will also be used to improve the existing 300B design.




Here a screen shot of the prototype on the curve tracer:




It turned out as linear as a DHT can get. Exactly what we wanted to achieve. But what about the sound? Can it compete with the old production tubes?




First test in a linestage:






I am always reluctant with sound descriptions. All I can say is that I am very happy. Immediate emotional impact with the prototype tubes which are not even properly burned in. Unbelievably transparent and detailed.




Similar results in my 801A drives 801A power amps.




We will now make the first batch. It will take a couple weeks until they are finished, burned in, tested and printed. Then those who preordered will receive their tubes.





And now a few impressions from the production floor. Here we see a 300B on the sealing station. Here the glass bottom gets sealed to the tube.






These video clips show the process, here we see the internals getting inserted into the glass tube:



The sealing:




Then the tubes go onto the vacuum pump:




They are evacuated over night with the lid down and heated to a high temperature to drive all gasses out.





An ER242 fresh from the vacuum pump and powered for the first time:




ER845s in the burn in station:




Stay tuned for further updates from the production floor.

Best regards

Thomas




Monday, February 5, 2018

Tube of the Month : The 82

Hi!

This months tube is a mercury vapour rectifier, the 82.





The 82 is the small sister of the 83. While the latter is quite popular, the 82 seems to be completely forgotten.

The main reason for that might be the lower current handling capability of the 82. It can only deliver about half the current of the 83. But that would be sufficient for low power single ended amplifiers using tubes like the 45 or 2A3. The 82 has the same pinout as the 83, see diagram on the left. It also requires the same hefty 3A filament current but with only 2.5V, half the 83s filament voltage. The 82 is also physically smaller, while the 83 comes in the ST-16 bulb, the 82 has a ST-14 bulb like the 45, which again would make it a suitable choice for such an amp for physical appearance. It could also serve as rectifier in preamplifiers of course. Like all mercury vapour tubes the 82 is preferably used with a choke input filter. Although it can be used with cap input as well if precautions are taken to limit the peak current to the maximum value of 400mA as specified in the RCA data sheet. Although I never used the 82 in a project, I acquired a small stash of them over the years. Let's have a look at some of them.




First some samples made by RCA.




In this type box the tube is held in an extra card board sleeve inside.




The tube has the typical full wave rectifier structure with the two plates mounted in parallel inside the bulb.





Close up:




The top:





Slightly different style RCA packaging:








Close ups showing the two supports on either side at the base which held getter material and the mercury during the manufacturing process of the tube:




The typical getter holder on the right and the little pouch on the left held the mercury.





Raytheon also made the 82:





This tube had a clever packaging which ensured that no used tubes were resold in the box.




The tube could not be removed without tearing the inner carton.




And the packaging allowed to test the tube while it remained in the box. The inner carton can be pushed out a little so the tube pins are exposed and can be plugged into a tester.




These Sylvania boxes had a similar sealing:






Since the seals on these boxes are still intact I did not want to remove the tubes.





Older style Sylvania boxes without seals:






A 82 made by General Electric:






Here a 83 and a 82 to show the size difference.




The 82 was also made in the ballon shape glass, here an early RCA:






The tube is securely wrapped inside the box:






And comes with a spec sheet.





Gorgeous globe shape:






Unlike other globe tubes this one does not have a three digit designation.






Close ups:










Another globe 82, branded RCA Cunningham:








Now let's see how it glows. When the heater voltage is applied the mercury starts to vaporise:




And when high voltage is applied the blue glow appears inside the plates:




View from the bottom:






An interesting small rectifier! I hope you enjoyed it.


Best regards

Thomas