Cathode bypass values

[xtian]

Let's talk about the Marshall plexi 50 watt, and cathode bypass values. Given a fixed cathode resistor value, reducing the value of the bypass capacitor can roll off bass frequencies.

Question 1: as you reduce the cap value to 0, and then remove the cap entirely, what happens to the EQ when there is no cap?

Question 2: For values where a 25uF cap is on the threshold of making a difference in bass, why does Marshall have a 330uF cap on V1a?

[roberto]

Look at my simulations in the sticky thread I opened some time ago.
You'll find all information. Without the Ck you'll obtain the unbypassed gain (dependingo on the Rk) and a flat response. A more gradual saturation of the stage (due to NFB due to unbypassed Rk) and something like a "scooped" sensation with more than 2k7.

Remember also that the bigger the cap, the "slower" the response of the stage.

[tubeswell]

To take Qn 1:

HT = 300V, Ra = 100k, Rk = 1k5, Ck = 0 F, Rl = 330k (assumed)

[img624]http://farm9.staticflickr.com/8174/7948 ... 2580_b.jpg[/img]

Looks pretty flattish around most of the geetar frequencies to me

[martin manning]

Fully bypassing the cathode resistor with a large cap is worth about 6dB gain across all frequencies for a typical 12AX7 stage. Partial bypass of the cathode resistor (reducing the cap value) produces a shelving response where the ~6dB boost slides up in frequency as the cap value decreases. The 330uF you see in the early Marshalls is about 10x more than necessary to cover the audible range, but it insures that all of the bass gets amplified through the stage. The output coupling cap (in conjunction with plate resistance, plate load, and the load following the coupling cap) is then controlling the low end of the response, where the typical 22n coupler with 100k Ra and 1M volume control load puts the corner frequency around 7Hz, which means no attenuation below about 30Hz. A very good treatment of this subject is here, beginning on pg. 25: http://www.freewebs.com/valvewizard1/gainstage.html

[xtian]

Wonderful. Thanks, chaps.

[pdf64]

Consider that the first Marshalls were a very close copy of the Fender 5F6A, a Bassman, ie designed for bass guitar.
Pete

[xtian]

A very good treatment of this subject is here, beginning on pg. 25: http://www.freewebs.com/valvewizard1/gainstage.html

Wow! No kidding. This really made the concept clear, after years of wondering. From p.25:

By placing a capacitor in parallel with the cathode bias resistor as in fig. 1.17, any instantaneous rise in cathode current will be diverted into charging the capacitor, and if cathode current falls, the capacitor will supply the deficit from its own charge. Another way of looking at it is to say that the capacitor shunts or ‘bypasses’ to ground any AC signals on the cathode so that signal current does not flow in the cathode resistor, while the DC bias voltage remains unchanged. With either explanation the result is the same: the cathode bypass capacitor ‘smoothes out’ changes in cathode voltage, helping to hold the cathode voltage constant, preventing cathode feedback and allowing full gain to be realised. The cathode bypass capacitor also affects the distortion characteristics of the stage. Without it, cathode-current feedback tends to slow down the onset of grid current, giving a smoother and more compressed sound. Adding the capacitor removes this feedback effect, result in somewhat harder clipping and a more aggressive overdrive tone.

[tubeswell]

A very good treatment of this subject is here, beginning on pg. 25: http://www.freewebs.com/valvewizard1/gainstage.html

Wow! No kidding. This really made the concept clear, after years of wondering. From p.25:

By placing a capacitor in parallel with the cathode bias resistor as in fig. 1.17, any instantaneous rise in cathode current will be diverted into charging the capacitor, and if cathode current falls, the capacitor will supply the deficit from its own charge. Another way of looking at it is to say that the capacitor shunts or ‘bypasses’ to ground any AC signals on the cathode so that signal current does not flow in the cathode resistor, while the DC bias voltage remains unchanged. With either explanation the result is the same: the cathode bypass capacitor ‘smoothes out’ changes in cathode voltage, helping to hold the cathode voltage constant, preventing cathode feedback and allowing full gain to be realised. The cathode bypass capacitor also affects the distortion characteristics of the stage. Without it, cathode-current feedback tends to slow down the onset of grid current, giving a smoother and more compressed sound. Adding the capacitor removes this feedback effect, result in somewhat harder clipping and a more aggressive overdrive tone.

What's more, a smaller bypass cap has a faster charge/discharge time so it will only boost the shorter-wavelength (i.e.: higher) frequencies.