PRR,
Thank you for this very instructive and complete answer. :thumb: :shock:
[quote author="PRR"]> It's not only the resistance of the Primary, is it?
No. But pretend it is. And pretend the transformer is really a resistor. 1uFd and 16.5K comes out to, 9.65Hz. That is the -3dB frequency. -1dB will be near 19Hz, -0.5dB near 38Hz.
The full math will include the 100K in parallel with the plate resistance of the tube. Not indicated on your drawing. I'm going to WAG 40K. Then 16.5K+40K gives -0.5dB at 11Hz.
[/quote]
I'm very surprised. I've read that some people find the microphone to have more basse freq. with a 1.5 µF (6.43 Hz) and 2.2µF (4.38 Hz), that' s why I thought my calculations were wrong. The first schematics of the C12 had a C8=0.5µF (19.29 Hz).
Can a value of the cap change the sound (independently of the influence of its value in the circuit) ? I hope you understand what I mean. I'm just wondering how people can hear a difference... as we talking about frequencies under 20 Hz.
[quote author="PRR"]
Actually, the transformer is an inductor. And L-C networks resonate. I don't see an inductance number on the spec-sheet, but their 20K plot suggests 600H. And 600H against 1uFd is 6.5Hz. Is it up or down at 6.5Hz? Depends on the tube plate resistance again; also loading. But if Q is anywhere near "1", then whether up or down at 6.5Hz it will be dead-flat by 13Hz, or 19Hz even if Q is as high as 9. My SWAG for Q is 0.6, so I expect dead-flat by 13Hz.
[/quote]
Ok, so again with those C8 values, it's quite impossible to hear the roll off.
[quote author="PRR"]>
C7 shelves bass and, again, we wanna know what the tube and voltage is. However it is unlikely that Gm is higher than 1,000uMho, with 100K in plate and 2K7 at cathode it will reflect as more like 2K ohms. Response will drop a bit by 0.8Hz.
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Around 160 Volts at B+ and 6.3 Voltsat the heater.
The tube will be a GE6072A.
[quote author="PRR"]
The capsule capacitance in series with 1nFd, the 1Gig, and the 200Meg are a low-cut. Dunno what that capsule capacitance is. Can't readily change it. But probably 60pFd. In series with 1nFd or 1,000pFd, still 60pFd (maybe 57pFd but our uncertainty is larger than 57-60pFd). The two resistors sum to 167Meg. (Yeah, I moved the 1Gig over after the 1nFd because I "know" neither really matters.) Then 60pFd and 167Meg is -3dB at 16Hz. This is your main electrical roll-off. Electrically, you want the 200Meg bigger BUT the tube's grid current noise in the larger resistor gives a larger noise voltage and more hiss.
[/quote]
I don't no about the capacitance of the capsule. No data in the Peluso Web site. (I will probably use a CEK12)
Could you tell me what R7 does in this circuit ?
BTW, is there a good book or articles I could read ? I do like to understand and learn what every part does ?
[quote author="PRR"]>
The electronics, as-shown, are flat for any musical bass. You want more, turn a knob and boost it.
The tube is a fancy-number 12AY7 running 0.6mA and Rp is near 60K.[/quote]
Thanks a lot,
eD