Re: 300B Amplifier Schematic
Posted: October 4th, 2019, 9:53 am
Shashi,
I really recommend NOT building the DRD circuit as your first SET project. I believe a cap coupled SET is much better suited for your application. The DRD is better for a Full range amplifier but you really need aware of alot of things that add to the complexity of this otherwise simple circuit.
1. The B+ power supply needs to be around 600-650 VDC since you need around 350-400 VDC across the 300B and around 150-175V across the driver, plus whatever drop you have through the choke feeding the plate of the driver. This means you need to use series-connected power supply capacitors (with voltage balancing resistors) since electrolytic caps are not available more than 450V rated. You also need to use large value capacitors since you half the capacitance when you stack the caps, and you need really good filtering for low noise, since the design feed AC to the cathode through the ultrapath capacitor from B+. This design has very poor PSRR (power supply rejection ratio). USe of filter chokes are a must.
2. Output transformer needs to have an operating voltage of at least 650VDC. High pot rating at 2000V is a typically based on insulation breakdown, not operating voltage. Just a warning.
3. Need a high voltage ultrapath capacitor between the B+ and the 300B cathode. I have used a 100uF/450VAC PP in oil (motor-run), Jack Elliano started recommending using a high value high voltage electrolytic, but I am not thrilled with that approach. Now days, you can probably use a 800V or 1200V PP DC-Link capacitor. Still, it will not be cheap for the value you need.
4. Balancing the circuit. You cannot rely on the resistor values in the cathode shown on the schematic. You need to balance these to the driver tube and 300B tube that you use. I end up using a tapped (adjustable) wirewound resistor as a cathode resistor. It is very tweeky to both set the 300B operating point and the driver operating point, since both change when you make the adjustment. It has to be redone when you change tubes.
It is SO MUCH EASIER to use a capacitor coupled (or even transformer coupled) circuit. The power supply voltage is much more reasonable using available components. You can use a fixed resistor in the 300B cathode (with a lower voltage rated cathode bypass capacitor, compared with the Ultrapath cap). The driver tube operating point is also a set and forget once you select the appropriate plate and cathode resistors (and cathode bypass capacitor). It is far more likely to work on first turn-on. I already recommended that a cap coupled circuit is better suited for your bandwidth limited application anyway. And the lower voltage is safer to work with, including output transformer rating.
Regarding the output transformers, they should work for your bandwidth limited application, but much cheaper output transformers would work fine as well as a start. If you were considering them for full range at all, the inductance is a little on the low side, and the weight at 2kg each is pretty light. I would not expect these to have very good low frequency response. 35kHz at the top-end seems low if this value is based on -3dB (if based on -1dB, then OK). But otherwise, should work. I would put these under a cover, though, for protection.
David
I really recommend NOT building the DRD circuit as your first SET project. I believe a cap coupled SET is much better suited for your application. The DRD is better for a Full range amplifier but you really need aware of alot of things that add to the complexity of this otherwise simple circuit.
1. The B+ power supply needs to be around 600-650 VDC since you need around 350-400 VDC across the 300B and around 150-175V across the driver, plus whatever drop you have through the choke feeding the plate of the driver. This means you need to use series-connected power supply capacitors (with voltage balancing resistors) since electrolytic caps are not available more than 450V rated. You also need to use large value capacitors since you half the capacitance when you stack the caps, and you need really good filtering for low noise, since the design feed AC to the cathode through the ultrapath capacitor from B+. This design has very poor PSRR (power supply rejection ratio). USe of filter chokes are a must.
2. Output transformer needs to have an operating voltage of at least 650VDC. High pot rating at 2000V is a typically based on insulation breakdown, not operating voltage. Just a warning.
3. Need a high voltage ultrapath capacitor between the B+ and the 300B cathode. I have used a 100uF/450VAC PP in oil (motor-run), Jack Elliano started recommending using a high value high voltage electrolytic, but I am not thrilled with that approach. Now days, you can probably use a 800V or 1200V PP DC-Link capacitor. Still, it will not be cheap for the value you need.
4. Balancing the circuit. You cannot rely on the resistor values in the cathode shown on the schematic. You need to balance these to the driver tube and 300B tube that you use. I end up using a tapped (adjustable) wirewound resistor as a cathode resistor. It is very tweeky to both set the 300B operating point and the driver operating point, since both change when you make the adjustment. It has to be redone when you change tubes.
It is SO MUCH EASIER to use a capacitor coupled (or even transformer coupled) circuit. The power supply voltage is much more reasonable using available components. You can use a fixed resistor in the 300B cathode (with a lower voltage rated cathode bypass capacitor, compared with the Ultrapath cap). The driver tube operating point is also a set and forget once you select the appropriate plate and cathode resistors (and cathode bypass capacitor). It is far more likely to work on first turn-on. I already recommended that a cap coupled circuit is better suited for your bandwidth limited application anyway. And the lower voltage is safer to work with, including output transformer rating.
Regarding the output transformers, they should work for your bandwidth limited application, but much cheaper output transformers would work fine as well as a start. If you were considering them for full range at all, the inductance is a little on the low side, and the weight at 2kg each is pretty light. I would not expect these to have very good low frequency response. 35kHz at the top-end seems low if this value is based on -3dB (if based on -1dB, then OK). But otherwise, should work. I would put these under a cover, though, for protection.
David