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RE: "Tre i have no transformer in the phono stage"

Posted by Tre' (P) on August 29, 2024 at 16:35:25

In Reply to: RE: "Tre i have no transformer in the phono stage" posted by Best audiotransformers on August 28, 2024 at 14:22:48:

C7 will form a high pass filter with the total cathode impedance of the tube V1T. The total cathode impedance value is the cathode resistor value, R5, in parallel with the impedance of the cathode itself. This will not be a very large number. Without working all the math my educated guess is about 500 ohms. 160uf gives a -3db point of that filter of 2Hz which leaves 20Hz unmolested.

So yes, the 220uf that is there now could be made smaller but not much smaller.

Here's the equation,
The "total" cathode impedance Zk is cathode resistance of the triode Zk` in parallel with cathode resistor Rk.
Triode cathode resistance is Zk`= (Rl+rp)/(mu+1)
Where mu is amplification factor and rp is anode resistance in concrete operating point. Rl is load resitance for AC, and this is anode resistor (in preamp stage) or reflected primary load of the OPT (Za) in output stage. Note that actual Rl can be Ra//Rg, or anode resistor in parallel with second stage grid resistor.
The final formula for Zk (Zk`//Rk) is:
Zk = (rp + Rl) / ((mu + 1 + (rp + Rl)/Rk)
And Zk is what C7 is seeing.

I built it into a spreadsheet so I can just enter values.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 08/29/24 08/29/24 08/29/24 08/29/24

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