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Old 14th November 2010, 02:24 AM   #11
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Quote:
Next to digest is PowerDrive.
PowerDrive simplified:

Part 1:

The CCS provides the driver triode with the ideal load. This lowers its distortion. Use a resistor load if the driver is a pentode.

Part 2:

The mosfet seperates the output tube from that troublesome coupling cap. The mosfet has a very high input impedance. The impedance is mostly the resistor connecting to the bias pot. Adjust the resistor value to make the driver happy. The driver and the coupling cap now see a constant impedance regardless of signal level. They are happy.

The mosfet feeds the grid of the tube with a very low impedance drive voltage. It can source the grid of the output tube with all of the current it can eat if driven hard, and it can deal with Mr. Miller in triode mode.
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Old 14th November 2010, 08:09 PM   #12
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Hello tubelab and All,
I sat down with your printed posts and PowerDrive from your web site. I made some observations that were interesting to me. TubelabSE has grid bias and Simple SE has cathode bias.
A SET with cathode bias can also have itsí own version of bias drift. If the output goes into cutout the cathode voltage falls towards earth. I can imagine non-musical shoot and overshoot during the readjustment of the equilibrium of the interstage coupling and cathode bias capacitors. None of this shows up on theoretical load lines or in the ideal world of spice.
Thanks for tickling my brain George.
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Old 14th November 2010, 11:45 PM   #13
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A SET with cathode bias can also have itsí own version of bias drift.
So can a cathode biased P-P amp. The cathode bypass cap can be made excessively large which will effectively remove bias shift due to cathode voltage change though.

I have seen small changes in the cathode voltage in the Simple P-P when driven to extreme overload.

Another "bias" issue is unregulated screen grid voltages in pentode amps. The screen current can vary a bunch in some tubes as the drive is increased. If the usual resistor - capacitor screen supply is used, another time constant is created along with the overload and recovery issues.

Any and all of these describe possible modes for overload and recovery issues. More than one of these can be at work in any given amp.
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