a switchable high pass filter in Fender build, sorta?

[pjd3]

Hey all there,

Would you think it feasible to form a faux select-able high pass filter by either having switchable couple caps values or, switchable cathode cap values?

If it was feasible without electronic issue, would you choose one over the other for a particular reason?

Thanks, in general I just find being able to high pass at different frequencies a nice thing in general for nearly anything.

I ought to mention this is primarily for a Baritone guitar with low DCR low output pickups and I don't know yet how much "boom" I will need to shave off, and to me, that doesn't always mean less bass, it means removing frequencies below a certain point without removing the energy of the frequencies too much above that.

Thank you,
Phil Donovan

[xtian]

Switchable cathode-bypass caps are easy, and a good choice to trim bass early in the circuit (first or second gain stage, or both). But coupling caps can have 200-300vDC on them, making it impractical to switch during operation. But you've always got a Bass control, right?

[Stevem]

My first thought to implement something simple would be to use a rotary switch to short out coupling caps off the plate that are in series .

As you place a short across more caps the girth of the low end would increase.
The first cap off of the plate that is blocking the DC should not get the shorting out treatment.

The issue I see with swapping out cathode bypass caps is while they will effect low end cut off, they will also effect gain which is something you may or may not want to take place.

[pdf64]

Partial cathode bypass is a shelving filter, the bottom end can’t be cut be more than 6dB, maybe less depending on the circuit detail.
Whereas a HPF rolls off bottom end at 6dB per octave
Maybe that will help your decision making.
But why not adjust the bass control appropriately?

[pjd3]

Thanks all for all the obviously worthy feedback on this.

If simply turning down the bass does the trick (if it ends up needed a trick) then that would be great. I figured not knowing if or where boom would be present might benefit from being able to more specifically target a cut off where frequencies below that would be more quickly rolled off, and the frequencies above that might be kept present moreso than the bass control but, if its say, all a 6 dB butterworth filter no matter where it happens then it would probably be a waste of added control.

I suppose after the build phase and into testing the amp I could just try a few different values of coupling caps until it sounded like there was a good range of bass knob control.

This amp is for a baritone 28.625" scale that will primarily be used for electric fingerstyle, celtic type songs and such. I want a nice definition on the lower strings, full without muck or boom. Thats why I'm using kinda low inductance, low DCR pickups on the guitar, try to keep a stringy piano like quality to the guitars lower end.


thanks all
Phil Donovan

[roberto]

Partial cathode bypass is a shelving filter, the bottom end can’t be cut be more than 6dB, maybe less depending on the circuit detail.

You can cut more, it just depends on the gain of the unbypassed vs bypassed stage. A trick to increase the range of the shelving filter is to change the reference of the cathode or the grid. I use it for last stages to have more eq freedom EG using an high value cathode resistor referenced to a negative value (or the grid to a positive one, as you prefer): more eq capability and a warm stage with an high value Rk (10-15k).

[Mattallica]

I have found that if you like the voicing as is and just want to cut some low end, then vary the coupling cap.
If you want to add a bit of "brightness" to the sound, then try smaller cathode caps.
How much gain will you be using? this is something to consider also.

[pjd3]

Thank you all for your feedback on this:

I can't as of yet speak of the gain in terms of units but that this is going to be a twin reverb amp as the initial tone stage, and will remain in the totally clean realm for probably its entire use. I think some of the recommendations may be above my experience but basically I"ve been under the impression that a bass control was a set frequency that was let through to ground more or less, but something like a selectable coupling capacitance would be an actual selectable high pass of sorts.

Since this is for a baritone guitar, often I would expect to have a capo up as high as the 7th or 9th fret, and sometimes it could be an open (no capo) with a drop A low string. On a drop A open song I might have expected too much "Boom" in the lower notes that might rob the notes of clarity or distinction. For some reason I imagined a few high pass frequency selections to address that potential issue than being at the mercy of set frequencies. All through the years many have stated that different size coupling caps would be the solution for amps that were either too Boomy or, to lean in the bottom. But, I've been given some good guidlines based on experience here and a little experimenting will probably be called for to see how these actually play out.

thank you all!

Best,
Phil D.

[thetragichero]

sorta related, i'm finishing up a real simple build that has a 3-way rotary switch as a "bass" control that selects between 330nf (hardwired to cathode so empty switch position) with 1uf or 22uf added in parallel to the cathode resistor of the single pentode preamp stage. just enough difference between all them to make the control useful. kind of a combo bass/gain control

[pdf64]

Partial bypass on early stage cathodes can make the circuit fussy about the valve, to avoid hum it needs to have low heater to cathode leakage, or DC heaters.