One of the reasons I normally stay out of threads with more than 8 or 10 replies, because by that time it normally has gone off the rails. ;-)
Jan
Ok, here's a redrawn Power Supply with changes discussed.
I've replaced the first cap with a 20uF film but 2nd and 3rd stages are ELs. I've replaced the first set of dropping resistors with a choke. Adjust R1 on test to reach b+ of 370v.
Hammond 270BX has 412 ohms of DC resistance across the HV secondary. 206 ohms per leg. Using originally spec'd C1 at 100uF requires ~250 ohms. Substituting a lower value C1 will lower the Series Impedance requirement, but it will also lower the b+. Adjusting the 100 ohm dropping resistor in the power supply may compensate for this loss, but a larger C1 may be required, along with additional series impedance in the form of resistors at pins 4&6.
Heater supply redesigned for 6.3vDC output.
I've replaced the first cap with a 20uF film but 2nd and 3rd stages are ELs. I've replaced the first set of dropping resistors with a choke. Adjust R1 on test to reach b+ of 370v.
Hammond 270BX has 412 ohms of DC resistance across the HV secondary. 206 ohms per leg. Using originally spec'd C1 at 100uF requires ~250 ohms. Substituting a lower value C1 will lower the Series Impedance requirement, but it will also lower the b+. Adjusting the 100 ohm dropping resistor in the power supply may compensate for this loss, but a larger C1 may be required, along with additional series impedance in the form of resistors at pins 4&6.
Heater supply redesigned for 6.3vDC output.
Check the plate voltage limits for the 6SN7. What are the cathode voltages for the biasing? Should there be 370v on the plates anyway? What is the actual supply B+ at the first cap?
RCA Datasheet shows an Absolute Maximum Voltage of 450v, while the Tung-Sol Datasheet puts the Maximum Plate Voltage at 300v.
We're still in theory over here. Guess I outta fire up PSUD2.
We're still in theory over here. Guess I outta fire up PSUD2.
One is a GTA, the other a GT. Not the same tube variant. The GTA is more powerful.
Since the sections are direct coupled, plate to grid, then the following grid is at high voltage so the cathode for that section has to be high also, hence the 18K resistor. So the plate/cathode voltage should be well under the 300v max. It's too bad the designer didn't add in the voltages for clarity. He also made the grounding symbols different for one in the same point, signal and power.
If we assume 9mA through the output section, it should compute out to 162v. on the K. The grid should be about 10v under that and over to the plate of the first section.
Let's not overcomplicate this, it's just a gain stage and a cathode follower. B+ doesn't need to be 'exactly' 370V, anything from 350 to 400V should be fine if the caps are suitably rated, but it would be a good idea to add a protection diode between grid and cathode of the follower.
Cap value will have minimal effect on the DC voltage.
With blocking cap a bit less than 18.
On my daily wrun, will give the precise numbers later.
On my daily wrun, will give the precise numbers later.
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More in line with what I would want in line stage. Are there any negatives to not using a bypass?
Not bypassing the cathode increases the stage output impedance. It is not a problem in this particular circuit, because you have a cathode follower after the first stage. The cathode follower presents a very high impedance to the first stage, and a low output impedance.
There are many ways to reduce the gain, the simplest one adding a passive voltage divider with resistors, or playing with the anode/cathode resistor of the first stage. But what kind of input signal do you expect? What input impedance do you require?
There's more than one way to skin a cat 🙂
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Well being a line stage I would expect signals between 1 and 2 volts like a CD player would present. As far as impedance goes everything feeding this will have a SS output stage so relatively low impedance from the source side.