Tube mic decoupling cap in between tube plate and xformer

[deuce42]

This may be a silly question, but what is the sonic effect of increasing of lowering the value of the cap that usually sits between a tube mic's transformer and the tube anode?

Done a search here but can't seem to come up with my answer.

 

[noulou]

the whole system acts like a high pass filter.

lowering the capacitor value has the effect of increasing the cut-off frequency of this high pass filter.

 

[rodabod]

Ignoring primary inductance, increasing the value lowers your cut-off. You get an extra octave for each doubling of the value.

Considering inductance, you gte a resonant LCR filter which will have a resonant frequency meaning an relative increase in response. The effect of this all depends on the circuit in question - some of the filters formed as less damped than others.

 

[MagnetoSound]

The value of the coupling cap (not decoupling cap) can sometimes be quite critical.

The resonant peak that Roddy is talking about needs to be calculated so as to fall outside the low end of the audio band.

Many often think that lowering the value of this cap will produce less bass response, only to find that what happens in practice is it moves the resonance UP into the low-bass area to where it actually causes a rise in bass response.

IMO, the best approach is to keep the resonant hump down below the point where it will cause an excess of unwanted infra-sonic noise such as rumble and traffic noise, and not try to tailor the low end with the coupling cap, but to address the need for low-cut before the head amplifier, where it will have the additional benefit of increasing overall headroom.

 

[lassoharp]

This may be a silly question, but what is the sonic effect of increasing of lowering the value of the cap that usually sits between a tube mic's transformer and the tube anode?

Done a search here but can't seem to come up with my answer.

Search 'parallel feed' or 'parafeed' to get an idea of the general circuit considerations at hand. 

Here's a couple net articles (their focus is a bit on choke loaded plates vs resistive loading but it should give you a good idea of what's going on with the output cap)

http://www.siteswithstyle.com/VoltSecond/Parafeed_fun/Parafeed_fun.html

http://www.bottlehead.com/et/adobespc/parafeed.pdf

All of the above given answers tie in with the reasons a particular value cap was chosen - also phase shift and distortion will be effected in some degree with a different value cap.

 

[deuce42]

You guys are awesome. thanks very much this info and yes I meant to say "coupling" rather than decoupling.