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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 17th December 2017, 07:54 PM   #1
Tubelab_com is offline Tubelab_com  United States
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Default Tubelab Universal Driver Board, 2015 version

The Universal Driver Board (UDB) was a concept that was underway in 2009 when the 6L6GC in AB2 thread appeared. The design concepts are discussed throughout that long thread. The circuit features two cascaded LTP's made from dual triode tubes. I have made several different versions of a similar design with 9 pin miniature, and octal tubes. The Octal version was used for Chrish's amp design, and for my test breadboard. Several different types of output tubes have been tested through my breadboard. The thread is here:

6L6GC AB2 Amp

Shortly after Chrish's amp was done, it became evident that my engineering career was in jeopardy, and I had to devote most of my time to keeping it alive, so my amp never got finished. In 2014 my 41 year engineering career ended, so we packed up everything and left south Florida. The "6L6GC" thread fell into a 4 year sleep during that time.

It turned out to be a hasty exit and I had to sell, give away, or trash over half of my stuff. What was to be a long careful packing, labeling and moving job got compressed into 3 weeks, so things were essentially randomly packed. The breadboard built during that amp design effort has never been found, so I assumed that it was lost, and set out to build a new one.

I laid out a new board that used 9 pin tubes since there are far more different flavors to choose from. I had a batch of the boards made by a commercial PCB vendor, since they were to be the next Tubelab product. That was early in 2015. About this time my daughter returned home with 4 kids and a husband, which we are supporting, so Tubelab again got pushed back into the corner. That situation has still not been resolved, and often requires most of our time. In the exit from Motorola and south Florida, I also lost access to my web site. I am working on that as well, but that's also why there hasn't been any recent updates.

The batch of UDB's sat in their shrink wrap on the shelf, until we moved again into a bigger house. In late 2015 Chrish woke up the original thread, which prompted me to build a pair of my UDB's and a new breadboard. First seen in Post #630, the board, and new breadboard finally saw life in early 2016, post #651. There have been a few random tests since then, but nothing too serious, just 250 watts from some transmitting tubes.

I have ideas on building a few amps using these boards, and I have sold 4 of them, so it's time to put together some documentation. There will be more posts in this thread in the coming days with real data on test amps. The initial choices for output tubes is KT88's, and the amp has already been fired up on bench supplies.
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Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 17th December 2017, 08:03 PM   #2
Tubelab_com is offline Tubelab_com  United States
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Here is the schematic for the not so new board. Being a "universal" board requires a different parts list for each application, and some parts are not used in each application. A generic parts list is done, but it had NOT been optimized for any specific application. Specific versions are coming soon.
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File Type: pdf LTP_driver_board.pdf (24.6 KB, 444 views)
File Type: pdf Univeral Drive Board generic BOM.pdf (191.9 KB, 221 views)
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Old 17th December 2017, 08:11 PM   #3
Tubelab_com is offline Tubelab_com  United States
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Random Pictures of the KT88's cranking out 50 watts in triode from 450 volts. The audio analyzer reads 2.8% THD.

Off to take the grandkids to a Christmas show...one of them has a part in it. Maybe some more info later tonight or tomorrow.
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File Type: jpg P1000322_x.jpg (903.9 KB, 1195 views)
File Type: jpg P1000324_x.jpg (191.8 KB, 1151 views)
File Type: jpg P1000325_x.jpg (695.8 KB, 1145 views)
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Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......

Last edited by Tubelab_com; 17th December 2017 at 09:04 PM.
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Old 18th December 2017, 01:26 AM   #4
mchrismag is offline mchrismag  United States
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I will be following this thread with much interest!
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Old 18th December 2017, 01:28 AM   #5
Tubelab_com is offline Tubelab_com  United States
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Here is the complete generic BOM in zipped excel format. It contains the note that were chopped off in the PDF version.

Why would someone want to use this board, what can you build with it, and what else do you need?

This board is a single channel driver circuit for a push pull vacuum tube amplifier. I have used it (or it's predecessors) to make P-P amps from 20 WPC to 250 WPC. It will drive small tubes, the typical audio tubes (6L6GC, EL34,Kt88...), DHT's (45, 2A3's, 300B's), TV sweep tubes in conventional G1 drive, screen drive, and both grids (dual drive), large transmitting tubes like the 811A, 813, 211 845, 3D21, and well just about anything that can be used to make an audio amp.

To make a typical stereo amp, you would need a power supply with the necessary transformers, two driver boards, 4 output tube sockets, 2 output transformers and the usual chassis or cabinet.

The power supply needs to generate the heater voltages for the pair of tubes on each board (typically 6.3V at up to 600 mA each) and heater power for the output tubes. You will also need a negative voltage supply of at least enough voltage to fully cut off the output tubes being used at 2 X the B+ voltage being used, and a positive supply of at least 50 volts or so for conventional G1 drive. Screen drive systems use a lower negative voltage supply (about 50 volts), but somewhere in the 200+ volt range for the positive supply. You will also need a B+ voltage for the output tubes, which will also be used for the driver tubes.

A power supply PC board had been laid out, but not yet built and tested. The same power supply board can be used with the TSE board to improve its performance and reduce heat.

The only components needed around the output tubes is a small resistor in the cathode connection for easy bias measurements, and possibly a stopper resistor on the grid leads. These are usually mounted on the tube sockets, so a PCB may not be needed. Some universal output PC boards have been laid out, and proto versions are being tested with these driver boards. I have also used turret boards from AES for easy - swap output boards.

1st picture, 4D32's mounted on bare sockets, cranking out 250 watts.
2nd picture, a pair of $3 25L6's (like a 6W6GT) mounted on AES boards, making 40 watts on 400 volts.....yeah a bit over spec.
3rd picture, a shiny new board before I even used it.
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File Type: jpg 4D32_bigOPT_x.jpg (706.0 KB, 1085 views)
File Type: jpg 25L6_CrazyDrive_x.jpg (872.8 KB, 1039 views)
File Type: jpg Universal Driver Board_x.jpg (580.3 KB, 344 views)
Attached Files
File Type: zip Univeral Drive Board BOM.zip (8.8 KB, 100 views)
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Last edited by Tubelab_com; 18th December 2017 at 02:04 AM.
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Old 18th December 2017, 03:55 AM   #6
zman01 is offline zman01  Bangladesh
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Good stuff George, keep 'em coming.
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Old 18th December 2017, 05:51 PM   #7
invaderzim is offline invaderzim  United States
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I've been scouring schematics for a 6l6GC PP amp that isn't so complicated that it is beyond my ability but still sounds good (and won't cost a fortune) so I'll be watching this thread with a lot of interest.
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Old 18th December 2017, 06:06 PM   #8
akimmet is offline akimmet  United States
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This amp looks very interesting. I have built an SPP with one of your boards in the past and was impressed by the design.

Hopefully you will be able to sell these PCBs soon. I know I am interested.
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Old 19th December 2017, 01:55 AM   #9
colnago55 is offline colnago55  United States
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I recently received two Universal Driver Boards from George and am hoping to build an amp to drive some vintage AR3a speakers. I've built 5 of George's boards and am looking for something with a little more power than the SPP which is currently driving those speakers.

Per George's suggestion I'm thinking of KT88's for power tubes.

I'm looking forward to this project.

thanks George.
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Old 19th December 2017, 02:11 AM   #10
Tubelab_com is offline Tubelab_com  United States
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I put together the parts list based on what parts were in the board that I had been using last, but that was two years ago. After playing with it last night I believe that it was last used for a screen drive, or dual drive amp.

Some of the resistor values that are on the current BOM will be changing. I have three of these boards built up, and they are all different. I think I will fire each one up tomorrow to see which parts do what.

I hope to have the list correctly filled in for a generic driver board that will work on 350 to 450 volts in a few days. That should cover most use cases.
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