Presence Control - Working Range

[jborders5]

I'm finishing up a modified Bluesmaster build and although the presence control does work, it's range is limited to the last quarter of rotation. There is little to no change from zero to 3/4 and a huge change 3/4 to max. This is the standard Marshall/Bluesmaster PI & presence circuit. The build is a 50W and I've tried 68K - 100K feedback resistors. The rest of the circuit is a 25K linear pot, .1uf and 4.7K tail. Any thoughts?

[Structo]

Which tap of the OT are you connecting to?

Oh and the schematic I have shows a 15K pot.

[dogears]

BM uses 25K pot. I had a real one here.

Which tap of the OT are you connecting to?

Oh and the schematic I have shows a 15K pot.

[ds1000]

[jborders5]

Which tap of the OT are you connecting to?

I'm using the 4ohm tap.

[dimitris]

jborders5 as you have find out the useful range of the marshal presence circuit is when the pot is under 10k resistance.If you don't like that you can parallel a 25-27k resistor to your presence pot to halve it's value.Another option that was suggest by Gil Ayan and works great is to use a reverse audio 50k pot.
Both solutions will expand the useful range from 9-10 o clock to full.
Also with your 50 watt amp try the 8 ohm tap with a 100k feedback resistor.
You may like that better.

[Structo]

My error.
I have three or four schematics and the first showed a 15K pot.

The others show a 100K, 25K pot with a 4K7 tail, .1uF cap.

[swt]

BM uses 25K pot. I had a real one here.

Which tap of the OT are you connecting to?

Oh and the schematic I have shows a 15K pot.

any other useful info from there??. Boost? maybe a clip? thanks!! you are really lucky to get those real ds in your hands...

[bluesfendermanblues]

I'm finishing up a modified Bluesmaster build and although the presence control does work, it's range is limited to the last quarter of rotation. There is little to no change from zero to 3/4 and a huge change 3/4 to max. This is the standard Marshall/Bluesmaster PI & presence circuit. The build is a 50W and I've tried 68K - 100K feedback resistors. The rest of the circuit is a 25K linear pot, .1uf and 4.7K tail. Any thoughts?

To answer your question, whether your presence control is in good working order?, the answer is 'yes' your presence control works fine.

If you want a more even distribution of the presence 'effect' across the pots range, you could add a resistor fixed resistor in the 'bottom' of the pot, covering the range 1-8 after which the pot will kick inn. So what you get is the current 8-10 range, spread out more even.

My guess is that a 1.5 Kohm resistor with a 500 Ohm pot would do the trick for you.

Alternatively you could wire your presence control on '10' and put a TwoRock/Vox AC30 'cut' control in instead.

[ayan]

I'm finishing up a modified Bluesmaster build and although the presence control does work, it's range is limited to the last quarter of rotation. There is little to no change from zero to 3/4 and a huge change 3/4 to max. This is the standard Marshall/Bluesmaster PI & presence circuit. The build is a 50W and I've tried 68K - 100K feedback resistors. The rest of the circuit is a 25K linear pot, .1uf and 4.7K tail. Any thoughts?

To answer your question, whether your presence control is in good working order?, the answer is 'yes' your presence control works fine.

If you want a more even distribution of the presence 'effect' across the pots range, you could add a resistor fixed resistor in the 'bottom' of the pot, covering the range 1-8 after which the pot will kick inn. So what you get is the current 8-10 range, spread out more even.

My guess is that a 1.5 Kohm resistor with a 500 Ohm pot would do the trick for you.

Alternatively you could wire your presence control on '10' and put a TwoRock/Vox AC30 'cut' control in instead.

In my experience, the reverse log approach works great, as already mentioned above. Now, if you're going to do a "cut" type control, then you mgiht as well use an audio pot and wire the presence control backwards. It will have an even taper and it will "cut" the highs as you turn up.

In the end, I got used to the rather horrible feel of the normal Dumble control (using a linear pot, whether it be the classic style or Blues Master style). Most of the time I keep the control well within its useless range (so say just below 2 o'clock); some times when playing live I turn it up some more.

Cheers,

Gil

[talbany]

Another cut type control that works well is...a .002/250k pot off the grid couplers.. It's a kind of NFB to the grids since they are out of phase.. It also works well.. Many EL-84 type amps use this to help with the buzzy top end the 84's sometimes give you.. I've used this on several D-type amps with some success.. Thought I would throw that out as well since it acts like a presence control w/more range (Komet pic Attatched)..Good Luck

Tony

[BobW]

Good tip, thanks Tony.

[mlp-mx6]

I believe that's a picture of a Komet.