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Old 28th February 2005, 10:06 AM   #21
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What if i connect the screen grids to the UL taps on my output transformer ?
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Old 28th February 2005, 10:10 AM   #22
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Then you'd have an ultra-linear (distributed load) output stage...

You'll need a bit more drive, but there is more local feedback at the output stage so distortion there should decrease a little.

Have a look at the original "Ultra-Linear Amplifier" by Hafler and Keroes (which, incidentally, used 6L6s )
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Old 28th February 2005, 10:14 AM   #23
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Here we go, An Ultra-Linear Amplifer, David Hafler and Herbert Keroes, Audio Engineering, November 1951
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Old 28th February 2005, 10:18 AM   #24
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There's also the Improving the Williamson article, which will be of interest for finer tuning of the circuit.
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Old 28th February 2005, 10:52 AM   #25
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Now i dont have the components to build those, would it be mutch difference ?
The problem is that the voltage drop from the anode voltage of 400V to your screen voltage will not be constant as the screen current varies, at low signal you will have high screen voltage and at high input signal the screen voltage will drop, this introduce distortion.

In many older amplifiers the screen and anode voltages are the same for this reason, the alternative would have been to build a voltage regulator which is more expensive.

What you can do is either use a regulator or use the same voltage at anode and screen and get rid of most of the problem, another alternatives is triode coupling ala Williamsson but that give slower output power or a compromise ultralinear coupling which give output power nearly as tetrode but lower distortion but you need an output transformer with screen connections for that.

If I where you I would probably build an ultralinear amp copied from an old design but maybe that is too boring, incidently my first real power amp was class AB2 with 807's and 750V at anodes and regulated screen voltage and a driver arrangement which was almost identical to yours, that was however 35 years ago, I wouldn't build that amp today.

Regards Hans
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Old 28th February 2005, 12:08 PM   #26
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Okay, could i connect the screens directly to the plate supply ?
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Old 28th February 2005, 12:44 PM   #27
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Okay, could i connect the screens directly to the plate supply ?
Yes, if you increase the bias voltage somewhat it should probably be OK, see here for an example of operating point with anode voltage 450V and screen 400V, that is very close, 55W at 1.8% distortion not bad! http://www.triodeel.com/6l6gc_p4.gif

Note 6L6GC can handle up to 500V Vg2 but ordinary 6L6 and 6L6G only 270V

Regards Hans
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Old 28th February 2005, 01:56 PM   #28
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What capacitances do you suggest in the ps ? i only have some 400V 100F, 150F and 330F, also a bunch of 200V 330F and 470F that i can connect in series.
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Old 2nd March 2005, 02:14 PM   #29
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Heres a new update: The amp is playing!!! Thanks for the help guys
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Old 2nd March 2005, 03:54 PM   #30
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Heres some pics of the amp:
Click the image to open in full size.
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