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

[mikepukmel]

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, ...

[mikepukmel]

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!!

[mikepukmel]

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.

[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.

[bob p]

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.

[Leo_Gnardo]

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.

[Leo_Gnardo]

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.

[Chuck H]

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.

[bob p]

> Clearly just an insertion maneuver.

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

[Chuck H]

> 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

[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?

[Leo_Gnardo]

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.

[mikepukmel]

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.

[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/

[Chuck H]

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.