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03-17-2020, 12:55 AM
Hi Noel
If you revert the amp to stock you can measure the voltage across the bias filter cap, since this is used to feed both the output stage and the splitter. This gives a reference for sensible mods.
The cap is unprotected from over-voltage, say if the splitter tube is removed. It would be good to add a zener across the cap that allows there to be somewhat more negative voltage than is present now. This will give you leeway for the bias adjustment portion and allow most tube samples to be turned off, letting you control how 'on' they are at idle.
With more negative voltage on the cap than stock, the tail resistor for the splitter has to be increased in value. It basically withstands the full bias supply voltage,which the schematic shows as 17V or 19V - difficult to read. Anyway, the tail resistor has this plus 2V8 across it and is a pair of parallel 33ks - 16k5 total. Suppose you increase -Vb by 20%, then increase R-tail by 20% and the splitter is happy. Note that you can check the splitter voltages with the power tubes out.
You might also want to add a diode from the cathode end of the tail resistor to ground. Place the anode at ground and the cathode on the resistor. During power-up, the end of the resistor will be held at one diode-drop below ground, protecting the tube. Once the tube warms up, the diode will be reverse biased as the resistor is pulled to its 2V8 level noted on the schematic.
Have fun
If you revert the amp to stock you can measure the voltage across the bias filter cap, since this is used to feed both the output stage and the splitter. This gives a reference for sensible mods.
The cap is unprotected from over-voltage, say if the splitter tube is removed. It would be good to add a zener across the cap that allows there to be somewhat more negative voltage than is present now. This will give you leeway for the bias adjustment portion and allow most tube samples to be turned off, letting you control how 'on' they are at idle.
With more negative voltage on the cap than stock, the tail resistor for the splitter has to be increased in value. It basically withstands the full bias supply voltage,which the schematic shows as 17V or 19V - difficult to read. Anyway, the tail resistor has this plus 2V8 across it and is a pair of parallel 33ks - 16k5 total. Suppose you increase -Vb by 20%, then increase R-tail by 20% and the splitter is happy. Note that you can check the splitter voltages with the power tubes out.
You might also want to add a diode from the cathode end of the tail resistor to ground. Place the anode at ground and the cathode on the resistor. During power-up, the end of the resistor will be held at one diode-drop below ground, protecting the tube. Once the tube warms up, the diode will be reverse biased as the resistor is pulled to its 2V8 level noted on the schematic.
Have fun
