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

[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:
5k/(5k+27k) = 0.156 about 16%
and the NFB ckt consisted of a 27k resistor followed by a 5k pot.

Deluxe AB763 nfb
47R/(47R+820R) = 0.054 about 5%
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?

[Dave H]

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:
5k/(5k+27k) = 0.156 about 16%
and the NFB ckt consisted of a 27k resistor followed by a 5k pot.

Deluxe AB763 nfb
47R/(47R+820R) = 0.054 about 5%
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%)

[mikepukmel]

Thanks Dave!!

[Chuck H]

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.

[mikepukmel]

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.

[mikepukmel]

something like this. (equivalence for RC analysis)

[mikepukmel]

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

[Chuck H]

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).

[Leo_Gnardo]

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."

[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.

[Leo_Gnardo]

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!!!

[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.

[g1]

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.

[Justin Thomas]

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

Justin

[Leo_Gnardo]

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!