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In Reply to: RE: Determining the size of inter-stage coupling cap posted by Tre' on January 23, 2017 at 07:18:50
Just to tag onto the comment from Tre a bit, when you put a choke in you now have two things to consider - the DCR (DC resistance, which is very small - not sure, but probably a few hundred ohms tops) and the reactance - think of it as AC resistance, which is very large as Tre points out. So a choke makes a nice low DC presence on the grid of the 300b, but a very high AC presence, also a good thing.
A little more,
the "AC impedance" (known as reactance) of the choke falls with frequency. A 1000Hy choke has a reactance of 6.3 meg ohms at 1000Hz but only 125k ohms at 20Hz.
The other issue is this, when you have a cap feeding a choke you have a 'tank circuit' and that LC tank circuit will have a low frequency resonance.
Depending on the DC resistance of the choke, to keep the resonance frequency below the audio band (and low in amplitude) it takes a rather large coupling cap (3uf to 10uf just as a ballpark number).
The BCP-16 GC has 3850 ohms of resistance and that's not enough to allow a "normal" sized coupling cap (.1uf to 1uf just as a ballpark number).
When the coupling cap is too small you can get an audible increase in the level of the low frequencies. Some use this to "tune" a system.
Have Fun and Enjoy the Music
"Still Working the Problem"
Quote: When the coupling cap is too small you can get an audible increase in the level of the low frequencies. Some use this to "tune" a system.
I am using grid chokes.
I want to make a comment on this because I did tune it to have very low -3db point which looks real good on testing, it but in reality there is NO useful POWER in the added resonance! once I added a larger 5uf coupling, much more useful power in the lower ranges.
BTW there is nothing wrong with larger coupling capacitors as long as you have good ones :)
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