Tube DIY Asylum

Which discussion? Which schematic?

24.27.41.140

Triode_Kingdom

In Reply to: RE: Input signal with respect to cathode bias voltage posted by Chip647 on January 08, 2025 at 14:26:40

Is the cathode fixed with a bypass cap, or does it float?

Follow Ups:

RE: Which discussion? Which schematic? - Chip647 07:02:36 01/09/25 (4)
The balanced preamp
- Input Voltage - Triode_Kingdom 09:53:50 01/09/25 (3)
  "the specifications for the preamp says that it has a max input signal of 10.5 volts RMS"
  
  I don't know the source of this schematic, the tubes being used or the background of this measurement. But I can tell you that the input stage can be driven with a larger signal than is represented by the standing DC cathode voltage. The reason for this is that the cathode - which isn't bypassed for AC - will vary with the instantaneous value of the input signal. This prevents the grid going positive WRT the cathode, up to the point where the tube can no longer supply the necessary current through the cathode resistors.
  - RE: Input Voltage - Chip647 13:57:18 01/09/25 (2)
    Sorry, you posted the Decware schematic (linked above) in Tubes on the 6th. I thought it would be familiar.
    
    Looking at the load lines I was having trouble seeing that much signal range. Thanks for your help.
    - RE: Input Voltage - Triode_Kingdom 16:05:50 01/09/25 (1)
      I think it was Tre' who posted it. To tell the truth, I had forgotten about it because of the ensuing discussion that focussed on the other drawings. Anyway, this ability to handle larger signals is one reason I tend to use this technique myself. It can also reduce distortion by the local feedback it creates.
      - RE: Input Voltage - Chip647 07:44:36 01/10/25 (0)
        
        In Reply to: RE: Input Voltage posted by Triode_Kingdom on January 09, 2025 at 16:05:50
        
        Thanks Triode. It was Tre who posted it (distracted by my zoom meetings ;-)
        In guitar amp circuits, the rule of thumb is that the elimination of the bypass cap reduces gain and also reduces the blocking distortion from big input signals. I was having trouble visualizing the mechanism.
        
        Edits: 01/10/25

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