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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 25th January 2020, 03:27 PM   #371
dady is offline dady  Balearic Islands
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Send a message via MSN to dady Tubelab Universal Driver Board, 2015 version
Hi George, thanks for your effort, I was in the USA and acquired a set of boards both TSEII, pushpull and Universal driver. A set for another member of Slovenia to which I sent them after we bought them together.
All good, the boards very well finished and referenced. Thick and with a generous copper layer. Then I packed them and brought them to Spain.
Here as I said I want to build an AB2 prototype and try RS1003 and E130L. I did not have many references in the precise instructions concerning the supply voltages of the board, at the end after reading all the thread, I knew it was +150 -150volt.
I was seeing the schematic and it is the condensation of all the solutions that I had imagined with my mind, something narrow but pretentious. I think the thing can make it work well.
But I have some questions which can be answered perhaps by rereading the thread or directly from you. The first is,
I do not have many 6CG7 but I have 12BH7 about 50 units or maybe more,
Then the other question related to the mosfet transistor I have 2SK2700 which I obtained many years ago after seeing your Screen drived pushpull circuit with sweep tubes and 6AV5GA, apart and in your driver schematic A2 of the SE. Right now that I am here already in Spain, a wonderful place on my island Menorca but, I notice that now you use other transistors which surely, you have replaced the previous ones due to their poor attainability, the question regarding this, It is necessary that you get the Are the SK2700 new or better?
Many greetings to all the colleagues who are in this thread and a special one, full of thanks for you Tubelab
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Old 25th January 2020, 03:57 PM   #372
Tubelab_com is offline Tubelab_com  United States
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Quote:
I have 12BH7 about 50 units or maybe more
The 12BH7 should work fine. I have used it with a 5751, 12AY7. or 12AX7 in the first tube when I needed more gain. All three heater pins are brought out to the rear of the board so you could run them from 6.3V or 12.6V but both tubes must have the same heater wiring. This means that the 12BH7 will work fine with other 12 volt tubes that have the heater tap on pin 9 but it won't intermingle with 6 volt only tubes like the 6CG7 and 6922 / 6DJ8.

The2SK2700 mosfet is extinct, but I never updated the library part in Eagle. There are lots of high voltage N channel mosfets in the TO220 package that will work. I like to use ones that are insulated to avoid having high voltage on the metal tab. Any of the mosfets used in the TSE or TSE-II will work fine since that part of the circuit is the same:


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The 2SK2700's have been extinct for years. Some of the parts seen on the web are likely fake. In today's constantly evolving world of electronics and parts a new part can be introduced, and be extinct in a year or two if it doesn't find a place in a high volume product. The part that I have been using and recommending has been the On Semi NDF04N60Z, but it has vanished from the face of the planet. The last two TSE-II boards I built have used the STF3LN80K5 part from ST microelectronics. They work fine and are in stock my the thousands at Digikey and Mouser....for now.
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Old 30th June 2020, 08:24 AM   #373
dch53 is offline dch53  Australia
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Default MOSFET source resistors

After a couple of years of working on other people's projects I've finally got around to my search for an output stage for my Universal Driver Boards.

WRT the MOSFET source resistors, R30 and R38, the original BOM, which I've attached for ease of access, stated:

"Sets the idle current through the MOSFET. You want 10mA to 20mA at idle, less if there is a secondary current path like a screen grid"

Given a secondary current path, like a screen grid, how do I work out the value of the resistors?

I'm currently running 150V MOSFET supplies with 12k resistors which gives me a current of 12.5mA with ~2W dissipation in both the MOSFETs and the source resistors.

In my simulation at least, reducing the MOSFET idle currents doesn't have much impact on the load distortion. I haven't played around with the source resistor values in my test board yet because I don't maintain a large stock of 2+W resistors.

Interestingly, the original schematic (also attached) showed 82k source resistors. Assembled PCB picture too.

Thanks.
Attached Files
File Type: pdf LTP_driver_board.pdf (24.6 KB, 12 views)
File Type: pdf Universal Drive Board BOM original 18122018.pdf (30.9 KB, 7 views)
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Old 30th June 2020, 12:11 PM   #374
Tubelab_com is offline Tubelab_com  United States
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The Gm in a mosfet will change with the current through the fet. Some mosfets can emit a burst of high frequency oscillation at turn on or turn off if they transition through cutoff during an audio cycle. This can also occur if the load on the mosfet goes through a rapid change in impedance, such as a control grid transitioning through zero in an AB2 situation. It is desirable to keep some current flowing through the mosfet at all times, even if the tube it's driving is cutoff.

I was doing a lot of high power screen drive experiments with this board and it's predecessors. Here the screen voltage is driven highly positive (+250 to 300 volts) during peak current on the driven half cycle. It will swing almost as far negative when the other output tube is at full conduction. To avoid the condition of mosfets switching on and off, I tied that source resistor to a negative 400 volt supply, hence the 82K resistor. Note all the unusual jumpers in the first picture. The bias was adjustable from zero to a POSITIVE volt age in the 100 volt range for screen drive.

When I transitioned from screen drive experiments to conventional G1 drive the 82K resistors came out, and 12K 3 watt Panasonics went in. They were carefully chosen for the criteria of "I have a big bag full of them." Many other parts were changed for a negative bias voltage at idle.

This test setup is the one that I have been using to test lots of different tubes in ordinary G1 driven AB1 and AB2 push pull. It uses a +/- 150 volt supply made with an isolation transformer. The source resistors seen in three years worth of pictures are those blue 3 watt 12K Panasonics. I have found no reason to change them despite feeding everything from tiny 50C5 radio tubes to big fat transmitting tubes. If I needed to deviate from the +/- 150 volt mosfet supply, I would change the resistor accordingly.
Attached Images
File Type: jpg P1000322_x.jpg (894.7 KB, 91 views)
File Type: jpg P1000335_x.jpg (923.9 KB, 89 views)
File Type: jpg P1000462_x.jpg (873.3 KB, 86 views)
File Type: jpg P1000468_x.jpg (918.8 KB, 85 views)
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File Type: jpg P1000714_x.jpg (952.7 KB, 22 views)
File Type: jpg P1000748_x.jpg (928.7 KB, 34 views)
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Old 1st July 2020, 01:42 AM   #375
dch53 is offline dch53  Australia
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Default MOSFET source resistors

Thanks George. I understand now.

If there is a secondary current path like a screen grid the resistor needs to be smaller to get a higher MOSFET idle current.

I'm doing some THD vs power measurements on a pair of 6HJ5s in crazy drive configuration with 500V HT and 150V.

With 12k source resistors in the simulation at least the driver tubes start to clip long before the MOSFET current gets below 7mA. I can offset that by changing the operating point of the driver tubes.

I have to say that even at 1W, the simulation suggests THD for this configuration of ~7% and that's about what I'm measuring.

I'll play around with the Rg2g1 and Rg1 to see if I can improve the situation before moving on to evaluating other output stage configurations.

I'm not keen on going to 600V+ HT as others have suggested for screen drive.
Attached Images
File Type: png 1W 1kHz THD.png (68.8 KB, 21 views)
 

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