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In Reply to: RE: Schematic shows posted by PeterI on June 14, 2017 at 07:28:48
"The schematic showed -45V at the bias source AND -45V at the grid which must be wrong, after passing thru resistors. "The grid draws no current. The two voltages should be the same. The factory simply had this amplifier set well below what would later become more typical for AB1. Many so-called AB1 amplifiers are in fact closer to Class A. This one really is AB1, where only a small percentage of total current flows without excitation. I would recommend caution before increasing the current with stock components, as the power supply and output transformers might not be up to the task.
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Buy Chinese. Bury freedom.
Edits: 06/14/17 06/14/17Follow Ups:
I think you meant Class AB2. More like solid state. With the tubes barely on.
The "2" has nothing to do with where you bias the tubes.It has to do with how hard you plan to drive them.
When the driver stage is up to the task you can drive the output tubes into grid current (grid positive WRT the cathode). That's the "2" part.
"With the tubes barely on" is Class B (or at least what they call Class B).
The "barely on" part is just to keep the notch distortion down.
True Class B (rarely used for audio) is when the tubes are both in cutoff (not just in the cutoff region but in real cutoff ie; no current flowing) in the absents of input signal.
Now, just to start a flame war, in true Class A the tubes are never allowed to enter the cutoff region (the region approaching cutoff where the linearity of the tube goes to sh_t or the non-linear region close to saturation).
Class of operation is more than just the bias point. It's how the tube is biased, how the tube is loaded and how hard the tube is intended to be driven.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/16/17
Nothing to do with SS, have no idea what you mean. What I said is accurate.
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Buy Chinese. Bury freedom.
nt
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