[Question] Decrease cathode cap vs high pass filter

[Bombacaototal]

A bypass capacitor allows the AC guitar signal to bypass the cathode resistor and significantly boost gain. A 25uF bypass cap is large enough to bypass all guitar frequencies.

A bypass cap of 5uF would probably boost mid low frequencies while bypass cap as low as .68uF would boost boost only mid and high freqs.

My question is: how different is a 5uF cathode vs a 25uF cathode with a high pass filter (220k/500pF)?

Which would you choose between both?

[Malcolm Irving]

The bypass cap produces a ‘shelf filter’ – below a certain frequency the gain stays constant and above another (higher) frequency the gain is constant, but at a higher level. The difference between the two gains is only 6bB. You can find some curves in Fig 1.26 here:
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

An RC high pass filter, on the other hand, has constant gain above a certain frequency and a continuously falling gain below that (same) frequency. So for example if the ‘corner frequency’ is 1kHz the gain at 100Hz is down by 20dB and at 10Hz down by 40dB.

The bypass cap method is often used in high gain amps to reduce bass before distortion is introduced. It has also been used in some amps to give a slight ‘treble boost’ for added sparkle.

[Bombacaototal]

The bypass cap produces a ‘shelf filter’ – below a certain frequency the gain stays constant and above another (higher) frequency the gain is constant, but at a higher level. The difference between the two gains is only 6bB. You can find some curves in Fig 1.26 here:
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

An RC high pass filter, on the other hand, has constant gain above a certain frequency and a continuously falling gain below that (same) frequency. So for example if the ‘corner frequency’ is 1kHz the gain at 100Hz is down by 20dB and at 10Hz down by 40dB.

The bypass cap method is often used in high gain amps to reduce bass before distortion is introduced. It has also been used in some amps to give a slight ‘treble boost’ for added sparkle.

Thank you very much Malcolm. It is very well explained and it helps a lot