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Old 18th January 2013, 09:46 PM   #21
es345 is offline es345  Germany
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Join Date: Jan 2012
Location: Ulm
Direct coupling allows to go into AB2 operating mode, driving the power tube grid into positive voltage against the cathode. You will get grid current and as a consequence a lower saturation voltage between anode and cathode. This leads to more clean power. In my case that mode has increased the clean output power from 300W to ~ 350W.

The second effect is that you will have a stable bias for the power tubes.

You can see here how I have implemented the direct coupling: Bass Power Amp with GU50 . It includes also a limitation of the maximum positive grid voltage to protect the grids.
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Old 20th January 2013, 10:17 PM   #22
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Location: Chicago IL, Long Beach CA, Vienna VA
the MOSFET will deliver more of the rail voltage too. But I don't have the knowledge or experience to do that yet. Tubes can't conduct the other way, so I guess I'd have to have an interstage transformer.
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Old 20th January 2013, 10:19 PM   #23
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I see high-voltage regulator kits of various kinds. I also see high-voltage grid bias supplies that monitor the cathode current. They react slowly, not to the signal, supposedly. Seems to me the grid choke is a good alternative, because it makes such a high impedance to the grid at audio frequencies. I kind of like that, and it should be a good thing with 6 output tubes in parallel. I will probaby start with changing from the stock half-wave diodes to a full-wave rectifier and bigger cap to just one choke, then a resistor and bias pot for each tube. If the choke causes it to lose bass, maybe I can just add a carefuly-tuned cap after the choke before I resort to multiple chokes.

Last edited by cyclecamper; 20th January 2013 at 10:43 PM.
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Old 25th January 2013, 10:30 AM   #24
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Location: Chicago IL, Long Beach CA, Vienna VA
So I could use some advice here. Assuming I want to try a grid choke on the outputs for the super twin (6 6L6GC) how many should I use? One with 6 resistors, two with 4 resistors each, or six (one for each tube)? I'd probably add trimmer bias pots for each tube in addition.
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