diodes on tube cathode-theory?

[gorhrut]

I understand diode/LED biasing on the cathode but there are some other occasions where diodes are used on the cathode that i do not totally comprehend....



#2 ive seen something similar in marshall amps, the way they had it set up it is said to act as a compressor/limiter.

#3 is this even feasible on the cathode? ive seen this used in the actual signal path but.....


anyone know the theory behind these?

[Malcolm Irving]

A diode requires a small forward voltage drop Vd to conduct. When it is conducting the drop Vd remains approximately constant. Different types of diode have a different value of Vd, from about 0.7 volts for a signal diode up to around 4 volts for blue LEDs.

In circuit 1, the diode is contributing some 'fixed' bias voltage in addition to the bias voltage produced by the cathode resistor. This will give higher gain compared to providing the same overall bias from a (larger) resistor on its own.

In circuit 2, the diode will conduct when the cathode current is greater than Vd / Rk. Above this current level the bias will remain fixed (giving higher gain), below this current level the diode will 'switch off' and we are left with cathode resistor bias (lower gain). I think this would affect the transition into cut-off clipping.

In circuit 3, I share your doubt that the extra diode has any effect. I can't see any circumstances where the cathode would be more negative (by a margin of Vd) than ground.

I don't have any practical experience with these circuits, so the above is just (my idea of) the theory.

[gorhrut]

thanks a lot!!! for some reason i couldnt wrap my head around that....now i got it.

[R.G.]

I suspect the actual reason is more plebean. The cathode resistor is usually there in non-cathode-biased amps to sense cathode currents for biasing. As long as the current is normal, this is a 1R or 10R or similar value resistor, and the power dissipated is minimal. If the tube loses bias voltage or shorts, the current in the resistor skyrockets and the resistor either melts down, burns up, or opens like a fuze. All of these lead to a non-functional amp and a warranty repair when a tube replacement might have done the trick. The diode keeps the voltage across the resistor from burning the resistor out in such cases and may save an expensive warranty repair.

I don't think it's much help with the sound, as the resistor is usually sized to keep its voltage well below the diode conducting.

[Malcolm Irving]

... The cathode resistor is usually there in non-cathode-biased amps to sense cathode currents for biasing. As long as the current is normal, this is a 1R or 10R or similar value resistor, and the power dissipated is minimal. If the tube loses bias voltage or shorts, the current in the resistor skyrockets and the resistor either melts down, burns up, or opens like a fuze. All of these lead to a non-functional amp and a warranty repair when a tube replacement might have done the trick. The diode keeps the voltage across the resistor from burning the resistor out in such cases and may save an expensive warranty repair. ...

Interesting point, but that would apply to a power stage, where we would expect transformer loading and a pentode tube. The circuits in the drawings look more like cathode biased pre-amp stages.

[gorhrut]

yeah my question was general theory behind it, so thanks, both answers were helpful.

I am considering trying these in my rack preamp. (first two, anyways...)

[mooreamps]

you can use it on a power tube........


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yeah my question was general theory behind it, so thanks, both answers were helpful.

I am considering trying these in my rack preamp. (first two, anyways...)