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In Reply to: RE: Determining the size of inter-stage coupling cap posted by EricSan on January 22, 2017 at 18:24:42
I seem to be having some trouble posing a reply for some reason...
>>>Why does replacing R10 with a choke have the impact of changing R10 to 250k? Damping resistor? I thought a significant benefit of using grid chokes was a dramatic reduction in the grid-to-ground resistance. <<<
Al reported good results replacing R10 with a MagneQuest BCP-16 GC, so I just assumed its impedance was somewhere close to 250k. Paul's original advice was to change R10 from 390k (from original Sound Practices article) to 250k to allow for greater grid leakage than the original WE91.
Given the design of R10= ~250k and that a substitution of R10 with a choke seemed not to cause problems, I was wondering how much latitude exists with the choice of C7 and if it could be increased from 0.1uF to 0.25uF without introducing problems. Or should I stick with 0.1uF for this part?
Please be patient with me, I'm new to tubes and trying to learn.
Thanks!
Follow Ups:
The BCP-16 GC is listed at greater than 1000Hy of inductance.
1000Hy has a reactance at 1kHz of 6.3 meg ohms.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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.
Tre'
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 :)
Lawrence
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