Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: passaloutre on April 06, 2023, 02:23:20 pm

Title: Omit Presence knob, but keep it in the circuit?
Post by: passaloutre on April 06, 2023, 02:23:20 pm

I have an amp which is essentially a Bassman preamp, Blackface reverb circuit, and Tweed Deluxe-ish power amp (schematic attached).

I pretty much leave the Presence control at halfway all the time and never change it. I'd also like to add a Master Volume. My thought it to fix and internalize the "Presence" control and put the MV pot in its place.

Now the Presence control, being a pot and a capacitor, could be replaced with two resistors and a capacitor obviously. But I'm trying to save some board space, since my layout never counted on putting three more components in there, and I'm maybe a little anal about piling up components where they weren't supposed to be (I like a neat board).

Anyways, would it be possible to replace that 5k pot and 100n cap with a single resistor and capacitor? Like, would a 5k resistor with a parallel 47n cap be equivalent to the pot turned halfway up? I hope this is making sense.

I know I could just drill another hole for another knob, or even put it on the back, but I never move the Presence knob, so I'd like to eliminate it if possible.

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: sluckey on April 06, 2023, 02:46:09 pm

Experiment. Replace the pot with a 4.7K or 5.1K resistor. Replace the .1 cap with different values until your ears are happy.

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: passaloutre on April 06, 2023, 02:52:49 pm

You mean I actually have to listen to the thing?  :l2:

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: tubeswell on April 06, 2023, 03:17:26 pm

Quote from: passaloutre on April 06, 2023, 02:23:20 pm

would a 5k resistor with a parallel 47n cap be equivalent to the pot turned halfway up?

Only if it’s replacing a 10k linear pot (I’m assuming you’re meaning a 5k/5k voltage divider)

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: passaloutre on April 06, 2023, 03:26:11 pm

The existing Presence pot is 5k, and it's set halfway, so the divider is 2k5/2k5.

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: sluckey on April 06, 2023, 03:32:25 pm

Quote from: passaloutre on April 06, 2023, 02:52:49 pm

You mean I actually have to listen to the thing?  :l2:

You could use the same method as before...

Quote

I pretty much leave the Presence control at halfway all the time and never change it.

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: tubeswell on April 06, 2023, 06:31:23 pm

Quote from: passaloutre on April 06, 2023, 03:26:11 pm

The existing Presence pot is 5k, and it's set halfway, so the divider is 2k5/2k5.

Now you’re getting there :-)

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: jeff on April 07, 2023, 11:46:13 pm

Edit: Post a picture of your board/layout

Title: Re: Omit Presence knob, but keep it in the circuit?
Post by: jeff on April 07, 2023, 11:46:35 pm

I don't want to give bad advice if your board is laid out differently