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RE: cathode follower plate voltage varies with current

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Lew

In Reply to: RE: cathode follower plate voltage varies with current posted by Ralph on February 14, 2024 at 11:31:59

As mentioned, 5-6mA.
Way below max plate dissipation with 250V plate voltage, now that Paul pointed out I was incorrect in my assumption that max plate V for ECC99 is 200V. (It's actually 400V per section for ECC99.) So I don't need a plate R at all.
Incidentally, when I was playing around with adding the 12K ohm plate resistance, shown in my diagram B, I did try bypassing the R by placing a 10uF capacitor from the plate to ground. This did not sound good at all. That's how I came up with the idea to bypass the 12K resistor with a capacitor connected from the anode to the PS across the resistor, shown in my diagram C. That would preserve a low impedance connection to the PS while also lowering DC plate voltage. Don't know if this would work well, because I haven't tried it and don't need to worry about it.

Edits: 02/14/24 02/14/24

Follow Ups:

10uf isn't enough. - Ralph 09:17:09 02/15/24 (3)
As far as the bypass is concerned, the tube is in parallel with the 12k resistor. The more current you run the lower the impedance of the circuit; the larger the bypass needs to be. Otherwise it will have a cutoff well into the audio band so I'd expect that to be unpleasant.
- RE: 10uf isn't enough. - Lew 10:59:16 02/15/24 (2)
  What about the CCS? Would the CCS figure into the impedance seen at the plate of the CF, using the equation Z (looking down from the plate)= Rp+(mu+1)*Rk, where Rk is the very high Z of the CCS? In that case, 10uF bypass cap to ground would be overkill.
  
  If I am wrong, then Z (for bypass calculation) = the parallel sum of 12K ohms and 2300 ohms (Rp for the ECC99). Then the net Z that needs to be bypassed for audio frequencies is around 2K ohms. In that case, I agree; 10uF is not quite enough. 50 or 100uF would be more like it.
  
  At this point, it's all moot because I don't need to reduce plate voltage.
  - I forgot about that- it complicates things. - Ralph 11:09:00 02/15/24 (1)
    A good CCS has a nice high impedance, so you're probably right.
    
    Did you optimize the CCS to the operating point of the tube?
    
    I've found it useful to run a sine wave thru things just to verify that the circuit is actually running the way its intended. More than once I've found it a country mile off.
    
    Is this at the output of the MP-1 phono section?
    - RE: I forgot about that- it complicates things. - Lew 12:21:43 02/15/24 (0)
      To your last question, yes.
      And yes I ran a sine wave through the circuit before finalizing. In fact I swept it from 20Hz to 10kHz.
      Now I think about it, I need to make a small correction in the calculation. The parallel sum of the 12K resistor and the net impedance from the plate down is essentially 12K ohms, because the tube plus CCS Z is so high. Still, 10uF should be enough.
      
      Edits: 02/15/24 02/15/24

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