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Old 7th November 2013, 10:52 PM   #11
roline is offline roline  United States
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Add Mosfet grid drive on the outputs to prevent blocking distortion....
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Old 8th November 2013, 06:08 AM   #12
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It is not necessary in this case.
If AB2-drive is used intentionally, then several other details must be redesigned.

By the way; how do you define "blocking distortion" ?
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Old 8th November 2013, 06:16 AM   #13
roline is offline roline  United States
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Blocking distortion is cross over error from the coupling caps taking on charge should the grids go Plus from being over driven. Mosfets are just to prevent. I added them to my last PCB design where they could be left out and jumpered instead. I left them in.
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Old 8th November 2013, 06:54 AM   #14
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Quote:
Originally Posted by roline View Post
Blocking distortion is cross over error from the coupling caps taking on charge should the grids go Plus from being over driven...
That is what I thought you to mean.

In typical AB1 pentode connected output stage, the output tubes are saturated well before the grid - cathode voltage reaches zero and grid current begins.

This means that the output signal is badly distorted before any grid current begins to flow.

The full advantage of grid current drive can be achieved if the operating conditions of the output tubes are such that positive grid voltages cause linear anode current increase.
This requires that the anode impedance should be lowered, for example.

Anyhow, there is no disadvantage to use such low impedance driver in general.
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Old 8th November 2013, 06:43 PM   #15
45 is offline 45  Italy
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Quote:
Originally Posted by artosalo View Post
That is what I thought you to mean.

In typical AB1 pentode connected output stage, the output tubes are saturated well before the grid - cathode voltage reaches zero and grid current begins.

This means that the output signal is badly distorted before any grid current begins to flow.

The full advantage of grid current drive can be achieved if the operating conditions of the output tubes are such that positive grid voltages cause linear anode current increase.
This requires that the anode impedance should be lowered, for example.

Anyhow, there is no disadvantage to use such low impedance driver in general.
How do you find these 6F12P in terms of general performance and quality? I have no experience with these but have been thinking for a while to buy some.
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Old 9th November 2013, 05:09 AM   #16
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6F12P is very versatile and well performing tube.
Both sections have high gm and are capable to operate at relatively high anode current (22 mA). The triode side has also hi mu (100). The max. power dissipation - 5 W for pentode and 3,5 W for triode - makes it easy to use this tube for many designs.

I once bought a lot of these tubes with the same date code.
All what I have used have been very similar in actual performance and I have found no faulty units.

One essential detail I have observed is to allways use 1 k grid stopper at triode side.
Without it the tube is prone to high frequency oscillation.





http://www.mif.pg.gda.pl/homepages/f...12/6/6F12P.pdf
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Old 9th November 2013, 01:28 PM   #17
45 is offline 45  Italy
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Quote:
Originally Posted by artosalo View Post
6F12P is very versatile and well performing tube.
Both sections have high gm and are capable to operate at relatively high anode current (22 mA). The triode side has also hi mu (100). The max. power dissipation - 5 W for pentode and 3,5 W for triode - makes it easy to use this tube for many designs.

I once bought a lot of these tubes with the same date code.
All what I have used have been very similar in actual performance and I have found no faulty units.

One essential detail I have observed is to allways use 1 k grid stopper at triode side.
Without it the tube is prone to high frequency oscillation.





http://www.mif.pg.gda.pl/homepages/f...12/6/6F12P.pdf
Thanks. I know the specs. My concern was about consistency, reliability and performance. I suspected that such "extreme" specs could lead to oscillations but was note sure if this could be cured easily or not.
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