by Ty_Bower » Thu Feb 11, 2010 10:42 pm
Triode strapping the finals is not the same as running pure class A. Triode means you're going to tie the screens of the finals to the plates. Class A means that both halves of the push/pull output stage conduct through 100% of the cycle, with neither tube ever going into cutoff.
The datasheet you are looking at is describing a circuit where the tube is cathode biased. That's an effective technique for biasing a tube, but really has nothing to do with either triode strapping or class A operation. It's also something you probably don't want. The Stereo 70 has a bias supply which provides a fixed negative voltage (fixed bias) to the grid. In this amp, I think you will find the fixed bias topology sounds better than cathode biasing.
I think what you want is to triode strap the output tubes. Simply disconnect the screen taps of the output transformers. They are the ones going to pin 4 of the tube sockets (should be green and green striped). Carefully insulate the bare end of the lead and neatly tape it back. Connect pin 4 of the socket to pin 3 using at least a 2 watt flame-proof (carbon or metal oxide) resistor. The value of the resistor should be between 100 ohm and 1K ohm.
Leave the bias circuit alone, and bias the tubes the same as you normally would. Do not install 220 ohm resistors at the cathodes of the power tubes.
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