Tube DIY Asylum

Input signal with respect to cathode bias voltage

172.11.32.65

Chip647

over in the tubes section there is a discussion about a balanced circuit. Looking at the schematic I had a question. The input tube is cathode biased with a 910 ohm resistor to 1.8 volts, however the specifications for the preamp says that it has a max input signal of 10.5 volts RMS. This seems a bit irrational, am I missing something? A 4v signal would drive the grid positive and completely distort. Isn't around 3v RMS the max on a tube with a 1.8v cathode bias?

Follow Ups:

It doesn't work that way - Ralph 10:38:45 01/09/25 (1)
A 4v signal would drive the grid positive and completely distort. Isn't around 3v RMS the max on a tube with a 1.8v cathode bias?

When the grid Voltage rises, the tube conducts harder. The plate Voltage drops and the cathode Voltage increases. The 1.8V is when the circuit is at idle.

In my opinion that circuit is missing some opportunity. Usually on a balanced input you would want to use a differential amplifier, which could be done by joining the cathodes together. It would be more effective doing so if the cathode resistor were tied to a B- supply rather than ground and better still if a constant current source were used instead of a resistor.
- Thanks Ralph. That makes sense. - Chip647 07:55:43 01/10/25 (0)
  Edits: 01/10/25
Which discussion? Which schematic? - Triode_Kingdom 19:55:26 01/08/25 (5)
Is the cathode fixed with a bypass cap, or does it float?
- 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)
        
        In Reply to: RE: Input Voltage posted by Chip647 on January 09, 2025 at 13:57:18
        
        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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