Would this circuit work

[J Luth]

I have a guy that wants to remove the presence control on his Bassman 5F6-A and replace it with an adjustable negative feeback circuit. I drew up a circuit but not sure if it will work.

[Enzo]

Why did you reduce the pot value of 5k to 1500 ohms as a resistor? Remember, the NFB is a voltage divider from the signal voltage at the speaker.

27k over 5k is a roughly 6/1 divider. A 27k over 1.5k is more like a 20/1 divider. SO that change alone makes a large difference. BY adding the pot, you can then reduce the NFB even further. meaning it will have a much smaller effect.

[J Luth]

Enzo

My mistake, there was supposed to be a question mark in the text as to the value of R1 which is labeled as 1500R.

The question on the resistor, R1, was when removing the 5K presence pot doesn't the circuit loose it's ground?

Correct me if I'm wrong and I probably am, doesn't the presence pot just remove highs rather than change the amount negative feedback voltage?

What he would like to do is be able to increase and decrease the amount of negative voltage coming back to the PI. I thought that possibly putting in a 50KL pot along with reducing the 47K to 22K resistor would allow the voltage to be adjusted below and above what it has now.

Still learning as I go.

[Enzo]

Don't lose the pot, just disconnect its wiper. OR replace the pot with a 5k resistor (4.7k actually) Yes, a resistor needs to be there.

For high frequencies, that presence control cap looks like a short circuit, so with the wiper to the bottom, it is out of the circuit. The more you turn it up, the more it shunts the high freqs around the resistor. meaning you get less NFB of the highs. Changing the amount of NFB is HOW it removes highs.

[Chuck H]

The easiest way to do what you want would be to replace the existing "presence" capacitor (100n) with something like a 22uf elecrolytic cap (observe polarity with negative (-) to ground).

The more complicated way would look like the modified layout below. This is what most people do either because they don't entirely understand how the circuit works (?) or because the series pot for feedback resistance has a better control taper/feel.?. I've never done it myself. If you do it like the attachment you should follow the layout. You don't want those feedback leads close to the preamp and you don't want to share the preamp ground for this circuit.

[J Luth]

Enzo

If I did remove the pot and put in a 5K resistor, could I then use the 50K pot and resistor to vary the feedback voltage?

[J Luth]

Thanks Chuck H

I'll talk to the owner and see which way he wants to do it.

[Chuck H]

FWIW you could just tack a 22uf (- to ground) parallel to the existing 100n cap to see how it operates for taper. That way it could be undone easily if you decide on the other way.

[J Luth]

Getting ready to try the mod but have one more question. Enzo stated that the there should be a 4.7K resistor installed once the persence pot is removed and the circuit installed. I am not quite sure where that resistor should be installed.

Circuit below

[Enzo]

4.7k = 5k = same thing. You removed the rest of the circuit in your drawing. Look at the original, it had a 5k pot at the bottom of the cathode circuit - the presence control. Replace the control with the resistor. Ditch the cap on the control. SOMETHING has to complete the circuit to ground from the cathodes.

[Chuck H]

I'm providing a modification of your schematic showing your circuit (left) and the one I proposed (right) with the 4.7k resistor in place.

I think you should consider the ratios for these circuits. If the amp is currently using a 47k resistor that would be a feedback ratio of 9.4:1. The stock ratio for that amp would be 5.4:1. Your circuit would allow adjustment from 4.6:1 to 15.3:1. 15.3:1 isn't even an uncommon stock value for many amplifiers and I don't think it would be a very "gratifying" feature. Especially considering that the full CCW adjustment for your circuit would be MORE than the normal amount of feedback for that amp (not usually desirable) and the presence control literally eliminated ALL feedback at it's pass frequency. This is why my circuit uses the stock 27k feedback resistor value and a 250k pot. With my circuit the amp would have the stock feedback ratio with the control set cull CCW (this is more "intuitive" from a users perspective I think) and allow for adjustment to 54:1 feedback ratio. That's a more gratifying number IMHE with circuits like these. But that's just my opinion as a guy who designs circuits. Some other things to consider...

Your circuit, using the most convenient rewire, would require that you either relocate the PI tail ground or run a longer than typical lead to the stock ground point. I can't say how this might affect noise or stability but these complications are possible. My circuit uses the stock ground point for the PI tail. Also, the most convenient rewire for your circuit requires that you "float" a resistor on a lead whereas my circuit has all components firmly anchored on all terminations. Neither of these two things are likely to be a big deal in reality, but I do try to keep these things in mind when I design. And I designed this modification just for you and this thread.

Below are the schematics. Below that is a modified layout showing YOUR circuit on the board using what I think is the most convenient method. But, again, if this were my project I would use the layout and circuit shown in post #5.

[J Luth]

Thanks Enzo.

Probably going to leave the stock presence control, it's up to him.

The amp is here now it turns out to be a reissue amp that has had the circuit boards removed, a hand wired eyelet board and Mercury Magnetics output trans installed.

Took a quick look and some of the resistors are not correct as shown the 59 schematic. Two in particular are the feedback resistor which is 82K and the resistor connected to the choke which is 20K instead of 4.7K.

[J Luth]

Thanks Chuck

Told him to leave the stock presence If he still wants to change it I'll show him the the options you show but to leave it up to him

Finding a few things on this amp that are weird, listed above.

[Chuck H]

Thanks Chuck

Told him to leave the stock presence If he still wants to change it I'll show him the the options you show but to leave it up to him

Finding a few things on this amp that are weird, listed above.

Well there's another option...

You could use a 25k push/pull pot as the presence control (they don't come in 5k). Wiring like the later DC blocked Marshall design. Then use the pull switch to parallel a 22uf capacitor with the stock presence cap. Effectively making the presence control act on ALL frequencies and work as a variable feedback control. So the amp is stock, with a presence control when the switch is in and the pot acts as a variable feedback control with the switch pulled. Have your cake and eat it too

[J Luth]

Thanks Chuck H

Now the fun begins.