Lowering input capacitance when using parallel tubes

[ChrioN]

And more importantly: what does the input capacitance do to the signal source?

 

[Svart]

Don't know how to lower it in tubes but in general, input capacitance limits slew rates unless you drive that capacitance with higher currents.

 

[Samuel Groner]

Input capacitance might cause the following effects:

* It limits bandwith if the driving source impedance is non-zero at HF.
* As noted in the previous reply it limits slew-rate if the driving source has unsufficient output current.
* It causes additional HF distortion if the driving source impedance is non-zero at HF; this is because the input capacitance shows a dependence on input voltage (although the effect for tubes is surely much lower than it is for transistors), which causes bandwidth modulation.

As you can see from the above statements it all depends on the application; the order of magnitude might be grossly different for various scenarios. I'm no tube designer but basically there are two possibilities to reduce input capacitance:

* cascoding
* bootstrapping

Both might lead to negative input capacitance and resulting parasitic oscillation in some cases. Care is required in implementation.

Samuel

 

[AndreasS]

Lowering the input capacitance is possible by using pentodes (or cascoded triodes) or negative voltage feedback (by reducing the Miller capacitance).

Regards Andreas

 

[ChrioN]

Sorry for the noobness, but whats the difference between cascoding and paralleling?

 

[AndreasS]

Pls. look here:

http://www.freewebs.com/valvewizard/cascode.html

Regards Andreas

 

[PRR]

> what does the input capacitance do to the signal source?

I think you should focus on answering that, -yourself-, first.

Samuel or I could answer the question for any specific situation.... "it all depends". But there are TOOO many situations to give any general answer about what matters in YOUR situations. And capacitive loading effects "should" be Basic Audio Knowledge.

(Just like snow-loading should be Basic Knowledge for a Maine carpenter..... hmmmm.)

Once you can rough-analyze the capacitive effects, you may realize that in audio, grid capacitance is often not a big deal. And when it is, how to change the situation.