trying to add a presence pot to a AA864 bassman?

[jamme61]

http://ampwares.com/schematics/bassman_aa864.pdf
I've tried a 25k linear pot off the feedback resitor, to ground ( different values .1, .68,) . My problem is, I only hear the presence come on, at the very end of the pots taper. It is more or less like, an on of switch, with no in-between sound? I also tried running the pot with cap, in parralel with the 100 ohm resistor off the PI, same result. any info help, would be great. Thanks

what does the 100 ohm resitor do? I don't see it on other fender amps that are running a presence pot? -

http://ampwares.com/schematics/bassman_6g6.pdf

[Structo]

First of all the resistor is a 100K resistor.

Do you have the 4K7 and .1uF cap to ground from the pot?

[Alexo]

Try an audio taper pot.

[alvarezh]

Jamme, I have no knowledge of the theory involved, but if I were to copy the circuit I would do it completely:

Edit- Tom beat me to it!

1) Replace the 800 ohm feedback resistor with the shown 100k.

2) Replace the 100 ohm with the shown 4.7k

3) install your pot and cap as shown on the schematic.

Your AA864 probably has a little less feedback energy because it is running at a lower voltage. I would use the original values and depending on what you are looking for (more presence control vs. amp stabilization) I would lower the 100k feedback resistor to maybe to 82K.

Maybe someone with more experience could chip in!

All the best.

[jamme61]

first I like to thank you guys for giving me your time.
I went with 4.7k instead of the 100 ohm9 and changed the FBR from 820 ohms to100k. This made the presence pot work but killed the tone and volume of the amp(volume cut in half). zi'll go over my work again and make sure I have it right. thanks again appreciate it.

[alvarezh]

Glad you're getting closer.

Put temporarily another 100k resistor parallel to the one you have already installed. That will bring the total resistance down to 50K (or close to).

Leave the rest as it is, if its working you should have done it right. It's all a matter of getting the right values because the amps have different feedback signal strengths.

Try it and see (hear) what happens.

All the best.

[martin manning]

Here's what I would do: Change the 820 ohm feedback resistor to 8k2, and the 100 ohm to 1k. That will keep the feedback ratio the same as it was. Use a 5k pot in series with a 1uF cap for the presence. That should put the frequency break in abot the right place. You might also consider changing the 22k tail resistor to 20k, but it might not be necessary.

[alvarezh]

Jamme, Martin is one of the most knowledgeable guys on the forum, I strongly recommend you follow his advise.

All the best.

[ToneMerc]

first I like to thank you guys for giving me your time.
I went with 4.7k instead of the 100 ohm9 and changed the FBR from 820 ohms to100k. This made the presence pot work but killed the tone and volume of the amp(volume cut in half). zi'll go over my work again and make sure I have it right. thanks again appreciate it.

Jamme, those changes would make the amp seem dead as you went from lot's of FB to hardly any. If read you correctly you just want to keep the amp relatively as is and add a pres control. Try these

1.8K FB, 220 ohm tail, .1uf and a 5K pot

8.2K FB, 1K tail, .1 and 5K pot

TM

[alvarezh]

Ah…… and so is ToneMerc!

Now you have two great guys helping you. I'll just be reading and learning too.

I am sure you'll end up with a fine working presence circuit.

All the best.

[martin manning]

Thanks for the vote of confidence, Horacio! Jamme, you can certainly play with the cap value to taste... 0.1 to 1.0 is a good range to try.

[jamme61]

hey you guys are great! going to bed now but, can't wait to try your other sugestions. right now the stk aa864 sounds a lot better but, we'll see what tomorrow brings. Thanks again

[jamme61]

first I like to thank you guys for giving me your time.
I went with 4.7k instead of the 100 ohm9 and changed the FBR from 820 ohms to100k. This made the presence pot work but killed the tone and volume of the amp(volume cut in half). zi'll go over my work again and make sure I have it right. thanks again appreciate it.

Jamme, those changes would make the amp seem dead as you went from lot's of FB to hardly any. If read you correctly you just want to keep the amp relatively as is and add a pres control. Try these

1.8K FB, 220 ohm tail, .1uf and a 5K pot

8.2K FB, 1K tail, .1 and 5K pot

TM

yes that is what I want to do. My question is, do I keep the 4.7k resistor on the pot the same value? or do I remove the 4.7k off the pot and go back to the original 100 ohm off the tail resistor to ground and run the 5k presence pot in parallel with the 100ohm resistor?

[martin manning]

Wire it like the 6G6. Actually the only thing wrong with your first attempt (as in the original post) is that the pot is way too big. It should be about five times the value of the tail resistor. I suggested increasing the sizes of the feedback and tail resistors proportionally to keep the feedback ratio constant, but with a 1k tail you can then use a 5k pot. The cap size goes with the tail resistor value to control the frequency range affected by the presence control. TM, I think you have it backwards- excessive feedback will reduce the gain and deaden the sound.

[jamme61]

does this make sense? I tried leaving everything stk aa864. I put a 1k linear pot in series with a 3.3uf cap. I ran the pot and cap in parallel with the 100 ohm resistor, that runs off the tail resistor (see on aa864 schematic) This seems to give me the presence that I wanted and leaves the amps original sound and volume alone. My ears say it works but just checking to see if this will cause a problem with the amp? You guys have been great! I have posted this question on other forums and have got little to no response. Thanks again for helping and not chastising me on my limited tech knowledge.