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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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Thursday, August 27, 2015

211 -The Next Generation- Part 7 : 801A Drives 211 Assembly

Hi!

Assembly of the second set 211 mono amplifiers of this year has been progressing well. The amps are now under test and will receive the final wooden enclosure when testing is finished.




Here a few photos of the assembly steps, starting with the top plate of an amplifier section:




All transformers, tube sockets, resistors and some of the caps are already mounted and wired up. All remaining parts are mounted on a second level below the top plate:




The completed amp without wooden enclosure:





Testing is done on the open amp to have access to all relevant points for measurements. The terminal plate is provisionally attached for the tests:




The power supply sections are constructed in a similar manner.




Mounting of the power transformers:




Two of the transformers are mounted in cut outs and a bit recessed below the main plate since they would others be too high to fit under the transformer covers. Wiring of the high voltage power transformers and tube sockets:





All chokes  and the remaining capacitors are mounted below the main plate. Here the finished power supply ready for testing and before the transformer covers are added. Quite a mountain of iron!





The amps running in the system for an initial listening test.




Stay tuned for photos of the finished set!


Best regards

Thomas

Saturday, August 22, 2015

211 -The Next Generation- Part 6 : 211 Drives 211 : Finished Amps

Hi!

The 211/211 amps have received their wooden bodies and are undergoing final testing.




One of the amps in the final chassis:





The entire set of amps and power supplies:





In these photos the power supplies are equipped with 1616 rectifiers.




Besides these 866A and 836 can be used as well.




A couple views of the amplifier set :








The backsides showing the terminals with RCA and XLR input jacks, WBT connectors for the speakers and separate jacks for plate and filament voltage inputs.




So far measurements and sound are excellent. Stay tuned for more photos.




Best regards

Thomas





Wednesday, August 12, 2015

Tube of the Month : The 56

Hi!

The last two Tube of the Month posts covered the 27 and 37 which are the earliest indirectly heated small signal amplifier triodes in a long row of tube development. This month we see the third one, the 56.




In the 56 transconductance and amplification factor got increased over the 27. Therefor the old tube data books list it as 'Super-Triode'.

Overall it is very similar to the 27, sharing the same UX5 base, pinout and physical dimensions. It also has the same 2.5V heater voltage. But the efficiency of the heater was significantly improved, consuming only 1A, almost only half of the 27s 1.75A. Transconductance and amplification factor both saw an increase of about 50% over the 27. Now with up to 1450 micromohs and a mu of 13.8. Typical plate currents and maximum plate voltage stayed the same. For complete technical data refer to the RCA data sheet. The 56 would drop into most circuits designed for the 27 without any or just some minor changes. A line stage for example could be designed to accept both. This way it is easy to boost gain by just changing the tubes. As expected they show a nice set of quite linear plate curves:




Real life curves from a tube sample look even better:




Here with a different setting of the grid step voltage:




Very nice set of evenly spaced curves.

Like the 37 the 56 was initially made in globe shape and later mainly in the ST 'coke bottle'. They never had the 3 digit designation with the UY-prefix like the UY227. Here some beautiful globe shape 56 from Cunningham:







The electrodes are fixed at the top with a small piece of mica:




Helix wound heater:




A cute little bulb!




A later version from Cunningham in the coke bottle shape:






Testable in box without taking the tube out:




The tube is floating in a carton inlay:




Cunningham 'C'-logo on the base:




Philco 56:




Crosley 56, made by Ken-Rad:




This one has very distinctive mica spacers in the top:






Never used 56 triodes made by Raytheon in perfectly sealed boxes:




Bottom of box:






For testing the tube can be pushed down:




The tube:








RCA 56:





The top with a large mica disc:




The base:




Close up to the plate structure:



Like the 27 and 37 all 56s have nice round plate structures. Cathode, grid and plate are concentrically arranged. The heater wire:




RCA Cunningham dual branded 56.







Globe RCA 56:




Tung Sol:





Due to the reduced heater power the 56 exhibits less visible glow than the 27, but a bit more than the 37:




Zooming in:




In some tube samples there is only a small glowing dot visible from the top:





Here a globe version in operation:




A different one:



The 56 is yet another almost forgotten triode which is very well suited for audio applications. As time permits I will develop a line stage for 27/56.




Stay tuned!

Best regards

Thomas