LPF placement

[hylaphone]

Hi folks, I have BF Fender style preamp here and the customer wants a switchable LPF around 4kHz. What is the best approach to add a simple passive filter?
Switchable cap across a plate resistor? Or inserting an RC somewhere, perhaps just before the second gain stage?
I'm used to doing HPF with coupling caps, this is new to me.
Thanks!

[Helmholtz]

You might wire a 1nF cap across the plate resistor of the input triode.
Effective R for the frequency calculation is 39k (being the output impedance at the plate) with a typical input stage.
To avoid switching pop you should wire a 1M resistor in series with the cap and use the switch to short the resistor.

What I don't like with this method is having to deal with high voltage at the switch.

There might be a better solution e.g. by making some component of the tone stack switchable. Would require some playing with the TSC.

What is the reasoning behind the mod? How did the customer arrive at 4kHz? Is 6dB/octave steep enough?

[uneumann]

You might wire a 1nF cap across the plate resistor of the input triode.
Effective R for the frequency calculation is 39k (being the output impedance at the plate) with a typical input stage.
To avoid switching pop you should wire a 1M resistor in series with the cap and use the switch to short the resistor.

What I don't like with this method is having to deal with high voltage at the switch.

There might be a better solution e.g. by making some component of the tone stack switchable. Would require some playing with the TSC.

What is the reasoning behind the mod? How did the customer arrive at 4kHz? Is 6dB/octave steep enough?

To avoid high voltage, move the R-C-sw network to the grid leak resistor after the coupling cap.

[Helmholtz]

To avoid high voltage, move the R-C-sw network to the grid leak resistor after the coupling cap.

I understand what you mean. Better to have the cap ground-referenced.
But the "BF" preamp circuit I had in mind has the tone stack between the first and second triode.
And the source impedance of the tone stack is variable and frequency dependent.
Actually there's more than one "BF" preamp circuit. Hoping the OP will post a schematic.

BTW, a cap to ground right at the input will result in a 12dB/octave LC-LPF.
But corner frequency will depend on PU inductance.
Also doesn't allow for pedals between guitar and amp.

[uneumann]

But the "BF" preamp circuit I had in mind has the tone stack between the first and second triode.
And the source impedance of the tone stack is variable and frequency dependent.
Actually there's more than one "BF" preamp circuit. Hoping the OP will post a schematic.

A coupling cap is approx a short to AC at high freq's (like 4k) so putting the LPF before or after a coupling cap makes no difference.
The impedances before and after the coupling cap at HFs are the same.

yes, a schematic would be helpful

[Helmholtz]

A coupling cap is approx a short to AC at high freq's (like 4k) so putting the LPF before or after a coupling cap makes no difference.
The impedances before and after the coupling cap at HFs are the same.

Sure, but where's the relation to my point about the tonestack?
A tonestack doesn't have a constant source impedance.

A possibility would be to add a "coupling cap" and 1M ground reference resistor in parallel with LPF cap before the tonestack.

[uneumann]

Sure, but where's the relation to my point about the tonestack?

Simply to say that if the LFP is OK to put on the plate resistor, it is also OK to put on the grid leak resistor. The effect on the tone stack (if any) is the same.

[Helmholtz]

The effect on the tone stack (if any) is the same.

After the tone stack the effect of a say 1nF cap to ground is not easily predictable as the source impedance varies with frequency and the tone stack settings.
See my edit above.

[uneumann]

.... A possibility would be to add a "coupling cap" and 1M ground reference resistor in parallel with LPF cap before the tonestack.

Yes - that's a scenario that fits what I'm thinking of. Lack of schematic makes the discussion ambiguous.
Yes - I'm using "grid leak" as a generic term to represent the load past a simple coupling cap.
A tone stack doesn't have a single simple coupling cap - so you're right - it's not the case I'm thinking of.

[hylaphone]

Thanks for the suggestions. I thought of bypassing the plate resistor but am iffy about having B+ at the switch. Here is the schematic, just the Bass Instrument section.
bassman_100_schem.pdf
As a standalone preamp, this will also have a DI output after V1B. The guy wants some high end tamed so it doesn't blast him in the stage monitors via DI->FOH.

[Helmholtz]

Why not just tame the highs in the DI output?
How is it wired?

BTW, a good speaker emulation filter needs to be far more sophisticated than a simple high cut filter.
Another possibility would be a line level equalizer in the DI path.

[hylaphone]

Why not just tame the highs in the DI output?
How is it wired?

hmm I hadn't thought of that. Don't have a full schem drawn up but basically after the second triode the signal splits to two volume controls, each feeding a source follower. One goes to a 1/4" line out jack, the other goes to a 4:1 transformer to XLR

[Helmholtz]

A speaker/cab emulation filter: https://www.valvewizard.co.uk/cabsim.html

[The Dude]

Why not just tame the highs in the DI output?......

^^^^^^Yes, that. If you cut in the preamp, it's likely to make the amp sound dull on stage. The inherent problem here is that the amp doesn't have high frequency drivers like a P.A. system does. As has been suggested, a schematic would help. I'd look at how some of the other guitar DI circuits are built. Peavey had some rigs with a frequency compensated XLR direct out- I think the Rockmaster preamp is one, but there are certainly others.

[hylaphone]

Sorry guys, I should have clarified. This is for bass guitar, dude just wants to knock off the highs coming through the monitors.
I think an RC or LC would be adequate, just not sure he best spot to place it...