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In Reply to: Re: Dan, why on earth recommend a circuit that... posted by Tre' on March 27, 2007 at 08:03:51:
Hi.This is a triode I/P.
Yes, short lengths of coaxial or shielded hook-up wire is often used from the I/P jack to the triode grid though I never touch it. I don't believe in any shielding for signal paths, even for phonostage.
Let's take an example of a minature coaxial cable, Belden 8218 (R59/U) used as the I/P cable. It gets 20.5pf/ft. Cgp of 6N1P is 3.1pF & amp factor A=35.
So the Miller effect capacitance alone is Cgpx(A+1)=3.1x36=111.6pF.So the total I/P capacitance is sum of (grid I/P stray capacitance due to ideally no longer than 1/2 ft coaxial cable=10.25pF + Cgp)x (A-1) plus Cgk. Neglecting the very small Cgk (a couple pF?), we have total grid I/P capacitance not less than 480pF !!!!
Follow Ups:
I understood that the Cgp is multiplied by the gain of the tube. I didn't know the C of the coaxial cable would be multiplied by the gain of the tube?I would think, in your example, that the total shunt C would be 111.6pF plus 10.25pF or 121.85pF total.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi.RHD4 stated in "Input impedance & Miller Effect":-
"The total input capacitance also includes the capacitance from grid to cathode plus (A+1) times ANY stray capacitance from grid TO plate".
My interpretation is ANY stray capacitance from grid TO plate (not between grid and plate) also includes capacitance of the hook-up wires etc etc.
"Stray capacitance" is defined as "Undesirable capacitance btween circuit wires, btween wires & chassis, or between components and the chassis of the electronic equipment".
Correct me if I were incorrect.
The whole idea is to minimize any stray capacitance added to the grid circuit. Coaxial cable is a ready source of stray capacitance.
I would use only twisted pair withOUT any overall shield from the I/P jack to the triode grid. ALL my SS & tube phonostages never ever use any shielded signal cables.Technically, the inter-conductor capacitance of a tightly twisted pair of wires of EQUAL size & IDENTICAL material is around 5pF, but the conductor-to-shield capacitance is in the order of some 40pF for EACH conductor, depending on the distance btween conductor & the shield.
For a coaxial cable, C=7.36xe/(log10 D/d) where e is the dielectric constant of the conductor insulation, D is the overall diameter of the insulation & d is the diameter of the conductor.
As Mark said it right, too much shielding capactance can only ruin the sound. This is the very reason I step away from any wire shielding.
For really EMI/RFI noisy environment, use a 100% metal chassis.
c-J
Stray capacitance should be minimized, always. I agree. But..."The total input capacitance also includes the capacitance from grid to cathode plus (A+1) times ANY stray capacitance from grid TO plate".
"My interpretation is ANY stray capacitance from grid TO plate (not between grid and plate) also includes capacitance of the hook-up wires etc etc."
NO, the capacitance of the hook up wire would be grid to cathode.
The C of the hook up wire to the grid is not Cgp but it does add to the tubes Cgk which is not multiplied by the gain of the tube.Take a look at fig. 12.1 that shows the "stray capacitance" between the grid and the plate. I think this "stray capacitance" IS the interelectrode capacitance of the tube but even if the book is talking about outside stray capacitance it would still be stray capacitance between the grid and plate not between the grid and ground (cathode).
Total input capacitance of a tube is Cgk + (Cgp * (A+1)) not Cgk +Cgp * (A+1).
Cgk would include capacitance which is native to the tube plus any external stray capacitance in the grid to cathode circuit.
Cgp would include capacitance which is native to the tube plus any external stray capacitance in the grid to plate circuit.
Take care.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
my passive RIAA filter. The .034uf cap, that's shunting to ground, multiplied by the second tube's gain is gonna leave me with no highs at all.:-(
Tre'
P.S. R1 = 22000ohms (including output impedance of driving stage)
It seems funny that Gary Pimm would use these values?Please understand that this is all sarcasm. Please don't take it personal. Just having some fun.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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