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Old 12th March 2012, 05:20 AM   #1
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Default 12BK5 UL tracing gone boosted triode?

I have been playing with my curve tracer from DIY Curve Tracer and decided to try getting some UL curves for a 12BK5.

I used a voltage divider (1 meg pot) and a separate supply to feed the screen very similar to the drawing in Help with setup to trace UL curves


Click the image to open in full size.

The interesting thing was that the normal taps did not straighten the Pentode curves too nicely. I kept going until i reach one side of the pot (100% of the Plate ac component back to the screen). When I did this the curves straightened very nicely. I adjusted the screen supply to around 120V and took the pictures below. First one is with Screen supply off (triode).

Click the image to open in full size.

Click the image to open in full size.

I think by coincidence I replicated a "boosted triode" method published some time ago.Boosted Triode Article

I believe the article was not so well received. I have a bunch of the 12BK5 that I got for $1. How much does it affect the longevity of the tube? The tube seemed to take it well, nothing glowing. May give it a try in PP mode and see how long the last.

Alfredo
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Old 12th March 2012, 06:04 AM   #2
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Nice curves Alfredo!

Similar effect can be achieved if to add positive voltage on the 3'rd grid. Gu-50, for example, has 3'rd grid that is not connected internally to cathode, it can be used to squeeze a bit more power from it without going to A2 region.

It is easier than power supply between anode and screen grid.
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Old 12th March 2012, 01:24 PM   #3
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We get EDN magazine delivered to the engineering lab where I work. Several people showed me this article when it was published since tubes are rarely seen in a mainstream electronics magazine. When I read the article I had doubts since I know what happens when you put 500 volts on the screen of a 6L6GC. I tried the circuit anyway and aborted the test long before I got to the voltages published in the article because the screen grid was glowing red. I believe the author did the entire experiment in a spice simulator and never tried it in a real tube.

I haven't tried the 12BK5 in any experiments yet, but I have some 6BK5's and they are on my list.

I am about halfway done with a new universal output stage breadboard that can test any tube up to 50V 10A heater, 1000V 0.5A plate, 500V 0.2A screen with independent drive level and adjustable (-200 to +500V) bias applied to any two grids. The grid drive can reach 300+ V P-P and the board can operate in P-P or SE. The plate supply can be supplied externally just in case I need more power!

I am making this to test my theories on dual grid drive with some types of tubes. Needless to say some little (and not so little) tubes will be seen melting in quest for the ultimate output stage design.

There was a short thread about the original article here. It might have merit with some tubes, but not a 6L6GC at 400V plate and 500V screen.

Boosted Triode mode?
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Old 12th March 2012, 02:58 PM   #4
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Thanks for the added input. The more I think about it the more I think it has no practical use. The added complexity of two floating screen supplies for just a bit more power and potential premature damage to the tubes is not worth it. It is much easier to parallel connect these very inexpensive tubes in triode mode and get more power that way. I think the curves as just eye candy on this one!

Alfredo
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