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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 30, 2015

211 -The Next Generation- Part 11 : 801A/211 tower, amp assembly

Hi!

The next set of 801A drives 211 amplifiers shares exactly the same circuit with the already finished amps. But it will be in a different chassis in 'tower style'.




Everything will be mounted on metal plates, stacked above each other:




The top plate carries the two tubes and the signal transformers:




The bottom side of the top plate showing al the signal wiring:




The small transformer on the top left is the input transformer which is wired 1:4. This way the amp has a sensible input sensitivity with only two stages.

Handles and vertical mounting bolts added:



Capacitors and chokes are mounted on a sub assembly and pre wired:




The sub assembly added to the amp:




The amp ready for testing:




In the next part I will show the assembly of the power supply.

Best regards

Thomas

Friday, September 25, 2015

Tube of the Month : The 76

Hi!

After the presentation of the types 27, 37 and 56 of the early indirectly heated triodes, the logical next candidate for the tube of the month is the 76.





The 76 is basically an equivalent to the 56 only with different heater specs. It needs 6.3V at 0.3A.

All other parameters are identical to those of the 56 which of course includes the same UX5 base and pinout and also the same dimensions. See the RCA data sheet for a complete set of technical data. So unlike the 37 which received slight bumps in amplification factor and transconductance over the 27, the 76 is exactly a 56 with 6,3V heater. If these kind of triodes are appealing to you but the 2,6V of the 27 or 56 are inconvenient, this tube might be the right choice. I have not used it myself yet so cannot share any actual experience. But that might change some time in the future. As always lets start by looking at the plate curves of the tube:




And actual curves from the tracer:





Another shot with different spacing for the grid voltage steps:




As expected these look identical to the curves of the 56. It is also very similar visually:





The tube above is from the Wards brand







RCA 76 still sealed in the box:






Different style RCA box:




Insert with floating tube removed:




The tube:




Another view:





Like the 27, 37 and 56 the 76 has circular grids and plates which are arranged in a concentric manner around the cathode:




The top:




Another RCA 76 with 'meat ball' logo on the base:



Triad 76:



The top:



Plate structure:




76 made by Sylvania:






The top:



The 76 shows a similar glow as the 56 when in operation.




A close up to the heater as it enters the cathode:




This post concludes this mini series about the early indirectly heated triodes. I hope you enjoyed it.

Best regards

Thomas




Monday, September 21, 2015

211 -The Next Generation- Part 10 : 801A Drives 211 glowing

Hi!

Here some photos of the latest 211 amps in operation.




The set of 4 chassis glowing away in the dark:




Close up to one of the amp chassis:





The driver tube: 





The Power Supply:





Amp viewed from the top:




Some photos taken in day light:






Another amp based on the same circuit is about to be finished in a different chassis style. Stay tuned!

Best regards

Thomas



Thursday, September 17, 2015

Guest Post : Olivers Amplifiers

Hi!

Oliver who built some amplifiers based on kits from me was so kind to share his experience...


Hi, I'm Oliver and I live in Brooklyn, NY. Some time ago Thomas asked me to write about my experience building a 10y/45/50 amplifier, finally here it is!

I've always been a music lover/collector but discovered tubes accidentally when I inherited a pair of Altec Model 19 speakers along with some old Quad 2 amps and matching 22 preamp. Like most children of the 80's I had no idea what those strange light bulb things were but they sure sounded nice so after some research and soldering practice the Quads were restored and my first tube guitar amp built.

Pretty quickly I was hooked! I discovered the long out of print Sound Practices magazine which helped feed my tube addiction and I began building stereo amps of various kinds but especially single ended types with the wonderful 45. I also became aware of Thomas from the Sound Practices 'homebrewer of the month' feature where he detailed very intriguing 211/211 amplifier with an elaborate 3 phase power supply, this captured my imagination and I immediately hunted to find some of the Tango NC-20 interstage transformers mentioned.




A few years and many amplifiers later I stumbled upon the Vinyl Savor Blog when Thomas first posted his 45/2a3 amplifier, As soon as I saw it I knew I had to have one. Before mustering up the courage (and finances) to build that particular amp however I decided to start with one of the simpler circuits generously published as the Single Ended Amplifier Concept. The basic design appealed on many levels, it was elegantly simple, had no cathode bypass capacitors, lots of iron including choke loaded power supply and was versatile in driver tube choice between 6j5/6p5/6n7, of which I had many examples. Another huge bonus was that I already had most of the Lundahl iron from a previously aborted project so the additional investment wouldn't be that large.

The Lundahl power transformer I used didn't have a centre tap on the high voltage winding so I settled on a hybrid bridge rectifier with 6ax4 and BY229 diodes and in the end chose 45 over 46 in the output stage, otherwise it was simply built as drawn. All the fabrication was done the old fashioned way (no CAD or CNC) with ruler and pencil, it was time consuming but relatively simple and all carried out in my garage with rudimentary tools. I tried as much as possible to remain faithful to Thomas' aesthetic and layered sub chassis approach using standoffs and aluminium angle stock to secure the many pieces of iron, it took a lot of planning the first time but after the layout was finalized and all the iron mounted completing wiring was very fast and now I am a big fan of this layered approach.




As often happens there was some trouble shooting to get things working correctly but once the problem was found (incorrect connection of the output transformer secondary) I was immediately impressed by what I heard. It seemed to magnify all the strengths of 45 tube and I couldn't find any audible weakness, especially impressive was the rock solid low end and the sense of focus and composure on more complex music.





Before long my friend Daniel heard the music at my place and decided he needed to build a Thomas Mayer 10y/45 amp too. After the initial planning stage all the necessary iron was ordered from Thomas and we sourced the passive components ourselves, soon packages full of exotic parts started arriving from around the globe. Since I had some 50 and VT-52 triodes I wanted to try in the output stage I added a few features to the basic design including a switch arrangement to increase/decrease the B+ and 2 additional passively filtered DC filament supplies for the output tubes, also switched, to alter filament and bias voltage. Since I had larger Tango XE-60 output transformers I ended up with a larger footprint and made some slight changes to the existing layout, all the capacitors were moved to the top plate and the 6 additional transformers and necessary passive components were mounted inside their respective chassis. I also really wanted the impressive blue glow of mercury vapour rectifiers so with some help from Thomas we were able to devise a way to do this using 816a and the filament windings on the existing power transformer. The power supply also got a little extra reservoir capacitance and a dual switch arrangement for preheating those precious antique triodes, although the high voltage rises slowly enough with the 6ax4 damper diodes.





Again all the construction was slowly done by hand, so slowly that I was able to hear Daniels' finished amp before my own. Needless to say we were both extremely pleased by the results, the transparency and air that the 10y contributes to the top end is fantastic, realism and tone abound and it uncovers all kinds of previously hidden details. To me this is the ultimate realization of a 45 amplifier and something I will never let out of my hands.






Since it was made with ease of modification in mind I've been able to try many combinations of DHT in both driver and output sockets and one of my favourites has been 801a/801a, although not an ideal match for the output transformers it was a very pleasant surprise and so lifelike the equipment simply disappeared leaving just music floating in space. Definitely something I want to investigate in the future. Also possible for the future is trying out the Elrog 300b which will fit right into the circuit with only the slightest modification.




Lastly I wanted to share a recently finished project which is a redux of the initial 6n7/46 build. This time I used a set of NOS Tamura iron and decided to go with the separate bridge rectifier power supply for a no compromise approach. One drawback commonly mentioned of the Tamura transformers is the slightly narrow bandwidth but I certainly don't notice this, very clear and extended with simply beautiful tone. Next is a matching LCR d3a phono stage built up in the same style, can't wait to get my soldering iron in it's heated state again.



 
Big thanks must go to Thomas for sharing his wonderful designs and providing the critical parts to make them a reality. Always willing to answer my questions (of which there were many) and a great source of inspiration. I'd encourage anyone out there considering building one of Thomas kits to go for it. A great learning experience and a truly world class finished product, I simply can't imagine building anything better!

Many thanks to Oliver for sharing his story!

Best regards

Thomas