# Thread: Adding a presence control to a Deluxe AB763 type circuit that didn't have one

1. ## Adding a presence control to a Deluxe AB763 type circuit that didn't have one

If I wanted to mimic a bassman 5F6a presence, they used a 5k pot at the bottom of the feedback voltage divider.

Bassman nfb ckt:
and the NFB ckt consisted of a 27k resistor followed by a 5k pot.

Deluxe AB763 nfb
NFB consisted of 820 resistor followed by a 47R resistor.

The bassman's presence control fed the center lug on the presence pot to a cap, that would
get more or less signal passed through the cap, depending on the pot rotation.

We can't get a 47R pot. But we can get, say, a 5k pot, like used on the bassman. I can find a 2k pot, but its like 4x the price, which is stil only around 11.00.

If I wanted to use about the same relative voltage divider that was in the Deluxe ckt, then solve
47/(47+820) = 2k/(2k+x)

x = (47+820)*2k/(47) - 2k = 34893.61702
or about 34.8 kOhms which I can find.

so far so good. Replace the 820R resistor with 34.8k, replace the 47R resistor with a 2k pot, then hook the center lug to a cap.

What I don't know how to calculate, is the effect of changing the voltage divider on the phase inverter circuit, and the effect on tone of the changes (besides the obvious change by bleeding some of the feedback signal through teh cap to ground depending on the pot setting).

Could this add instability to the amp since the feedback wiring would now be routed across the inside of the chassis to the pot?

Would this cause, say, oceans to rise, or damage to the ozone layer?

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2. Originally Posted by mikepukmel
If I wanted to mimic a bassman 5F6a presence, they used a 5k pot at the bottom of the feedback voltage divider.

Bassman nfb ckt:
and the NFB ckt consisted of a 27k resistor followed by a 5k pot.

Deluxe AB763 nfb
NFB consisted of 820 resistor followed by a 47R resistor.
Don't forget the Bassman OT is 2 ohms so it has half the voltage output of the 8 ohm AB763 OT making that 16% look more like 8% compared with the Deluxe.

I'd try the AB763 with a 5k presence pot and a 100k to 47k feedback resistor. It should be in the ballpark (5%-10%)

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3. Thanks Dave!!

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4. I'll guess by your response to Dave's post that you'll be going with the 5k NFB load below the tail, so this may not matter, but... If you DID decide to use different/lower resistance values in the loop it should be recognized that this changes the criteria for capacitor value if the same frequencies as the Bassman type presence control are the goal. 100R pots really aren't hard to get. You could use a 100R pot and a 1.8k NFB resistor, but the cap value would need to be on the order of 6uf to target the same frequency as the Bassman circuit.

Dave's point about the 2 ohm secondary on the Bassman is an important one. The NFB loop is a voltage, not current dependent circuit. That 2 ohm load produces a low-ish voltage and that's why the NFB resistor is only 27k on the Bassman. 27k/5k would be 5.4:1 (from the 2 ohm tap) where the Deluxe at 820/47 would be 17.4:1 (from the 8 ohm tap) Big difference! So, using a 5k pot (load) to keep with the stock feedback ratio for the Deluxe you would use an 87k NFB resistor.

The last amp I did had a 1k load and an 18k NFB resistor from the 8R tap. This would be about the same as the Deluxe. It requires a cap value of roughly .47uf to target the same frequencies as the Bassman circuit.

I would offer formulas, but I don't know them off the top of my head and I use spice and calculators for most of my stuff anyway.

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5. Thanks Chuck. I think I have a vague understanding of the voltage divider part, but not so much the whole phase inverter.

I don't have any idea how to size the cap. Im assuming that the cap would be sized based on the guitar's frequency curve, rather than the bass, although my Brother (who is a fine guitarist) says that lots of his friends use the Bassman as a guitar amp.

First to tackle was how to setup the voltage divider to work on the Deluxe circuit. I tried to think of it as two circuits: one with the pot fully CW and one CCW. The voltage divider is the same since the lower resistor is across the pot. But at one point, the FB signal is fed about directly into the cap, and the other rotation, the signal is first fed through the approx 5k resistor, then to the cap. One is like a RC in parallel, the other way is like the cap is almost shorted. I tried to draw it but didn't come out very good, will try again and post a picture to get around my poor wording.

Formulas would be great if you happen to find them in a file someplace, someday. I would love to learn to use Spice someday. (long list of things to learn!). Specific formulas are sometimes hard to find.

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6. something like this. (equivalence for RC analysis)

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7. Oh man, I just noticed this: "...less ear-friendly but handy for jazz." Leo_Gnardo" Heh, thanks Leo.

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8. If you look at the NFB loop, it's just a voltage divider added to the bottom of the LTP PI tail. The way the presence control is wired very effectively preserves the intended ratio for all but the capacitor passed frequencies. When the cap is shorted at it's leads (full CCW on the pot) it is not in the circuit and all frequencies are elevated by the pot value. When the cap is across the pots resistance (full CW on the pot) all the frequencies passed by the cap are grounded out of the feedback signal being fed to the PI tail.

To some degree the LTP PI doesn't sweat being elevated, within reason. But it's true that the more you elevate, the more "working voltage" is subtracted from the swing because it's now on the cathode instead of the plate. For an amp like the Bassman, with it's 10k tail that extra 5k elevation (now a total of 15k) was determined to be an acceptable loss of working voltage. But the BF type amps use the substantially lower 47R additional elevation. The designers had their reasons I'm sure and how much difference another 5k elevation added to the 22k to 47k tail of the BF circuits is probably no big deal, but I chose the 1k NFB shunt value just to play it safe, tonally. There is no safety issue with the 47r, 1k or 5k values. Anyway... A 1k shunt only changes a 22k tail elevation to 23k. Mice nuts. So that's what I did. The appropriate cap value would then be .47u for the same response as the Bassman (lots of upper mids) but .33u is nice too and might sound sweeter and less aggressive for a BF type amp (IMHE).

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9. Originally Posted by mikepukmel
Oh man, I just noticed this: "...less ear-friendly but handy for jazz." Leo_Gnardo" Heh, thanks Leo.
Some context will help. We were discussing that day overtone series, especially the upper partials, which I said were "less ear-friendly but handy for jazz."

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10. Yes... I took it out of it's context because it's funnier that way During that thread I was thinking about some of the atrocious distortion tones used by various jazz musicians too, when they dare to use distortion. I'm sure they are often intentionally discordant, but they still sound awful sometimes. I'm not talking about players like McLaughlin. Think Di Meola or Skunk.

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11. Originally Posted by Chuck H
Yes... I took it out of it's context because it's funnier that way During that thread I was thinking about some of the atrocious distortion tones used by various jazz musicians too, when they dare to use distortion. I'm sure they are often intentionally discordant, but they still sound awful sometimes. I'm not talking about players like McLaughlin. Think Di Meola or Skunk.
Yeh, or Larry Coryell... but I luv that stuff! In any case I'm honored and delighted you fly the quote, keep up the good work, thanx!!!

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12. If it is simple voltage divider you can easily go for the calculations from a calc like this. But for the complex cases you have to hook over the manuals one by one.

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13. So from the discussion Sarah, do you think this is a simple or complex case?
Can you recommend any of those hooking manuals?
I find it interesting that for your first post you have chosen to link to an external website.

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14. When I want to protect my loads "on the go," I use.... (desperately fights urge to hijack any further...)

Justin

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15. Originally Posted by Justin Thomas
When I want to protect my loads "on the go," I use.... (desperately fights urge to hijack any further...) Justin
(in Inspector Clouseau voice) I .... smell a bot! Do you smell a bot? A smelly, smelly bot, pairhaps? Hm????? I have nevair smell thees bot befo' . . . but I am sure it is a bot!

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16. Originally Posted by Leo_Gnardo
Some context will help. We were discussing that day overtone series, especially the upper partials, which I said were "less ear-friendly but handy for jazz."
Heh, yeah, better. Knowing jazz musicians, and knowing that over the years you must have done audio for many, at first I thought it was a tongue in cheek type joke, like ahh yes those jazz guitar players, with their oven mitts, drop D tuning, and heavy fuzz guitar, ...

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17. Originally Posted by Sarah
If it is simple voltage divider you can easily go for the calculations from a calc like this. But for the complex cases you have to hook over the manuals one by one.
Thanks for the link and comment!!

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18. I have a circuits book, pretty good one, just have to get my lazy carcass to open it and read the words and forumlas. I just have to finish this beer first.

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19. Hey! Leave Sarah alone. She's only trying to lend her expertise on how to handle loads. Incidentally... Hooking through all the Manuals could be a hell of a task south of the border. I (and the Manuals of the world) wish her luck.

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20. Originally Posted by Chuck H
To some degree the LTP PI doesn't sweat being elevated, within reason. But it's true that the more you elevate, the more "working voltage" is subtracted from the swing because it's now on the cathode instead of the plate. For an amp like the Bassman, with it's 10k tail that extra 5k elevation (now a total of 15k) was determined to be an acceptable loss of working voltage. But the BF type amps use the substantially lower 47R additional elevation. The designers had their reasons I'm sure and how much difference another 5k elevation added to the 22k to 47k tail of the BF circuits is probably no big deal, but I chose the 1k NFB shunt value just to play it safe, tonally. There is no safety issue with the 47r, 1k or 5k values. Anyway... A 1k shunt only changes a 22k tail elevation to 23k. Mice nuts. So that's what I did. The appropriate cap value would then be .47u for the same response as the Bassman (lots of upper mids) but .33u is nice too and might sound sweeter and less aggressive for a BF type amp (IMHE).
I did some number crunching last night but didn't get around to posting. What's interesting about this is that an AB763 DR uses 470R bias on the PI, 22k for the tail, and 47R for the shunt. Adding these up you get a total of 22k517. If you change the 47R shunt resistor to 5k as was recommended earlier, that brings the total cathode resistance up to 470R + 22k + 5k= 27k470. What surprised me was that the AA763 DR used 470R bias + 27k tail + 47R shunt = 27k517. So the AB763 with the 5K shunt mod ends up having about the same total elevation as the AA763. As it turns out, if you crunch the numbers through a PI gain calculator there's no real change to write home about when it comes to PI balance or output. This makes me wonder why Fender went from Rk=27k in the AA763 to 22k in the AB763. Is there a signficant difference? I've never compared them.

I'm thinking that if you resize the presence cap properly then the amp would sound great / not that different from before.

As fun as an exercise like this is from a design standpoint, I have to ask -- why is a presence mod necessary for an AB763 Fender circuit? They're already bright enough because of that bright switch. If anything, they're too bright and benefit from a smaller bypass cap on the volume pot, and flipping that bright switch on/off gives a lot of tonal change. I never thought of a presence as necessary in the circuit.

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21. Originally Posted by Chuck H
Hey! Leave Sarah alone. She's only trying to lend her expertise on how to handle loads. Incidentally... Hooking through all the Manuals could be a hell of a task south of the border. I (and the Manuals of the world) wish her luck.
Clouseau again? "Pairhaps thees Sarahbot has confus-ed MEF with her crocheting club chatroom, eh?"

I wonder if that link leads to a special offer on sports team jerseys. Bet you half a peach it does.

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22. Originally Posted by Chuck H
Hey! Leave Sarah alone. She's only trying to lend her expertise on how to handle loads. Incidentally... Hooking through all the Manuals could be a hell of a task south of the border. I (and the Manuals of the world) wish her luck.
Clouseau again? "Pairhaps thees Sarahbot has confus-ed MEF with her crocheting club chatroom, eh?"

I wonder if that link leads to a special offer on sports team jerseys. Bet you half a peach it does.

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23. Originally Posted by Leo_Gnardo
I wonder if that link leads to a special offer on sports team jerseys. Bet you half a peach it does.
I clicked it but didn't stick around long once I saw it was just another on line calculator.?. But the bots are becoming more strategic. They play it cool and then try to subtly slip in their wares. It never works here because:

A) No chicks post here (ok, I suspect one or two, but they never came off like a bot)
2) That calculator is only vaguely related to the subject by virtue of being electronics related. Clearly just an insertion maneuver.

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24. > Clearly just an insertion maneuver.

... this is one case where I'd gladly accept some insertion loss.

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25. Originally Posted by bob p
> Clearly just an insertion maneuver.

... this is one case where I'd gladly accept some insertion loss.
Yeah, well, I've been suffering insertion loss since I got married. YMMV

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26. How can a bot sign up, react to the signup, and post? Don't they have to go through those "Im not a robot" photo clickey things?

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27. Originally Posted by mikepukmel
How can a bot sign up, react to the signup, and post? Don't they have to go through those "Im not a robot" photo clickey things?
With a little human assistance? Time will tell - if "Sarah" writes back, in a huff, and says "I am certainly NOT a bot, and I resent the implication, blah blah blah woof woof how could you etc etc... " then we'll know, won't we.

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28. Originally Posted by bob p
I did some number crunching last night but didn't get around to posting. What's interesting about this is that an AB763 DR uses 470R bias on the PI, 22k for the tail, and 47R for the shunt. Adding these up you get a total of 22k517. If you change the 47R shunt resistor to 5k as was recommended earlier, that brings the total cathode resistance up to 470R + 22k + 5k= 27k470. What surprised me was that the AA763 DR used 470R bias + 27k tail + 47R shunt = 27k517. So the AB763 with the 5K shunt mod ends up having about the same total elevation as the AA763. As it turns out, if you crunch the numbers through a PI gain calculator there's no real change to write home about when it comes to PI balance or output. This makes me wonder why Fender went from Rk=27k in the AA763 to 22k in the AB763. Is there a signficant difference? I've never compared them.

I'm thinking that if you resize the presence cap properly then the amp would sound great / not that different from before.

As fun as an exercise like this is from a design standpoint, I have to ask -- why is a presence mod necessary for an AB763 Fender circuit? They're already bright enough because of that bright switch. If anything, they're too bright and benefit from a smaller bypass cap on the volume pot, and flipping that bright switch on/off gives a lot of tonal change. I never thought of a presence as necessary in the circuit.
Thanks Bob, would love to see the gain calculations, if you ever have time to sketch them out. Absolutely no reason to do this mod other than forcing myself to learn more than I do now. Well, ok, maybe one more reason: I got the chassis cheap, like 1/2 price what the couple of big amp parts sites sell them for, but its actually a Deluxe Reverb chassis, so it has 3 more holes on the front panel than it should have (being no trem, no reverb). I put two Mid pots in, will see how that turns out, which leaves one more hole, so I thought to fiddle with what effect presence would have on the sound of this franken-amp.

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29. gain calculations -- just google for LTPI gain calculator and you'll find what you need. just enter the numbers and click calculate. sorry, but i can't post the actual numbers, as i did them on a piece of scrap paper and we've already had our garbage day.

here's one:

https://www.ampbooks.com/mobile/ampl...g-tailed-pair/

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30. Originally Posted by bob p
gain calculations -- just google for LTPI gain calculator and you'll find what you need. just enter the numbers and click calculate. sorry, but i can't post the actual numbers, as i did them on a piece of scrap paper and we've already had our garbage day.

here's one:

https://www.ampbooks.com/mobile/ampl...g-tailed-pair/
I love on line calculators, but I've never been convinced that the ampbooks PI calculator is accurate, I think it may be missing something because if you plug in a tail of just 100R, while the balance is skewed horribly, of course, you get an average gain of about 25. Then if you plug in a 100k tail you get about the same gain!?! There's just no way that you can float the cathode to the same resistance as the plate without a change in gain over a virtually grounded cathode. Both the Valve Wizard and Aiken's articles on the LTP suggest that the tail value for balance needs to be weighed against the available voltage and how much swing you need. I trust those guys.

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31. Originally Posted by Chuck H
I love on line calculators, but I've never been convinced that the ampbooks PI calculator is accurate, I think it may be missing something because if you plug in a tail of just 100R, while the balance is skewed horribly, of course, you get an average gain of about 25. Then if you plug in a 100k tail you get about the same gain!?!
With a 12AX7 and the range of tail/bias resistor values usually used the gain is 25 to 30. Ideally the current in the tail is constant so the voltage across the bias and tail resistor doesn't change and therefore they can have no effect on the voltage gain (I think).

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32. Originally Posted by mikepukmel
I think I have a vague understanding of the voltage divider part, but not so much the whole phase inverter.

I don't have any idea how to size the cap. Im assuming that the cap would be sized based on the guitar's frequency curve, rather than the bass, although my Brother (who is a fine guitarist) says that lots of his friends use the Bassman as a guitar amp.

First to tackle was how to setup the voltage divider to work on the Deluxe circuit. I tried to think of it as two circuits: one with the pot fully CW and one CCW. The voltage divider is the same since the lower resistor is across the pot. But at one point, the FB signal is fed about directly into the cap, and the other rotation, the signal is first fed through the approx 5k resistor, then to the cap. One is like a RC in parallel, the other way is like the cap is almost shorted. I tried to draw it but didn't come out very good, will try again and post a picture to get around my poor wording.

Formulas would be great if you happen to find them in a file someplace, someday. I would love to learn to use Spice someday. (long list of things to learn!). Specific formulas are sometimes hard to find.
By itself the PI has a voltage gain of about 25 but it is inside the output stage negative feedback loop. It's the voltage divider which sets the gain of the output stage (from PI input to speaker)
When the Presence is fully CCW the cap is out of circuit and the (closed loop) gain is set by the divider ratio. When the Presence is fully CW the cap shorts the higher frequencies to ground so there is no HF negative feedback and the gain is the open loop gain of the output stage which is usually about 2 to 3 times the closed loop gain.

A cap value of 0.1u is used with a 5k pot because that value affects the 3kHz 'Presence' frequency range. If you change the resistor from 5k to 1k the cap has to be changed to 0.5u to keep the RC time constant and frequency response the same.

I've Spiced it up below. It shows how the frequency response changes as the Presence pot is rotated from CCW to CW. You can see that at low frequencies the gain is 17dB and at higher frequencies (3kHz) with the Presence fully CW (green trace) the gain is 25dB i.e. it has a maximum Presence boost of 8dB.

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33. I suspect that you may have used a resistive load in the sim?
With a real speaker inductive load, the degree of available presence boost may be a fair bit more than that.

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34. Originally Posted by pdf64
I suspect that you may have used a resistive load in the sim?
With a real speaker inductive load, the degree of available presence boost may be a fair bit more than that.
OK here you go

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35. Originally Posted by Dave H
With a 12AX7 and the range of tail/bias resistor values usually used the gain is 25 to 30. Ideally the current in the tail is constant so the voltage across the bias and tail resistor doesn't change and therefore they can have no effect on the voltage gain (I think).
I'd think so too, that the gain isn't affected by changes to B+ (in a reasonable range of voltages). However,the available HR is going to be different. The load line plot will have the same slope, just shifted R-L based on the B+ to tail voltage (I think ).

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