Last night I built the stage based upon the schematic kindly provided by PRR. I found out that the seperate power supply has far too much ripple to work with the cathode follower. Measured at the output, the supply has around 330 mV of ripple! That’s about 1000 times the maximum amount I normally shoot for in a line stage. I should have measured it first. As it it stands, with the existing power supply, it hums loudly. I haven’t built much over the summer, so I’ll look at this experience as a warm up...
Hmmm. Or rather hummm. I had a look at the power supply for this circuit (it looks like a switching supply) and found that the supply provides 0-6.2 Volts DC for the heaters, and +35 and -35 Volts DC for the B+. I was expecting 0-70 Volts. I get +35 and -35 when I measure between the respective B+ terminals and the 0 Volt terminal of the 0-6.2 Volt heater supply. When I measure between the +6.2 Volt terminal of the heater supply and the negative B+ terminal, I get -41.2 Volts, and when I measure between the +35 Volt terminal and the +6.2 Volt heater supply terminal I get +28.8. The circuit ground is therefore connected to -35 Volts. I have only ever built regular linear power supplies with 0 Volts at ground, and all the voltage showing up at the positive B+ node. I am unfamiliar with supplies of this sort, especially in the context of a tube circuit, and the implications with respect to the electrical performance of the cathode follower I have put together based on PRR's schematic. What is the proper way to connect to the B+ of the supply I have at hand?
This goes with the original mu follower circuit. So think of it this way:- The 70 volts negative is effectively ground or 0 volt line. The 70 volt positive is B+. Then the heaters are at half B+ or as you measure about 35 volts positive with respect to ground / 0v. Think 'stacked' again.
With stacked or totem pole layouts the top triode cathode can be at a higher potential than the heater cathode breakdown voltage (the insulation between the heater and cathode has a voltage rating called Vhk) if the heaters are connected to ground. So the heater voltage is elevated to something that keeps both halves of the triode within that rating and often around half B+ volts. Hope that makes sense?
Should not matter here in your CF, the 6922 has a breakdown in excess of 50 volts, just remember not to connect any part of the heater circuit to ground!
Intrigued that you say ''it looks like a switching supply'' a picture might help.
Alan
With stacked or totem pole layouts the top triode cathode can be at a higher potential than the heater cathode breakdown voltage (the insulation between the heater and cathode has a voltage rating called Vhk) if the heaters are connected to ground. So the heater voltage is elevated to something that keeps both halves of the triode within that rating and often around half B+ volts. Hope that makes sense?
Should not matter here in your CF, the 6922 has a breakdown in excess of 50 volts, just remember not to connect any part of the heater circuit to ground!
Intrigued that you say ''it looks like a switching supply'' a picture might help.
Alan
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