Prresence control wirh 12AT7?

[phydauex]

General design question for everyone.
Has anyone built a presence control into a typical Blackface circuit and stuck with the 12AT7, instead of switching to a 12AX?

It seems that all the circuits I've looked at that used a presence control also use a 12AX7. I'm reworking a later version Bassman 50 head to "Blackface" specs (AA864), which is straight forward enough. The person I'm doing it for also wanted to put in a Presence control in place of the master vol, that he wanted taken out. My concern is that he wants a more polite amp, than if I put in the typical 12Ax in PI plus a presence control. I'm sure someone has done this before, but looking for experiences anyone has had doing this.

Thanks, Matt M

[Chuck H]

How do you plan to implement the presence control? The AA864 has an 1:8 feedback ratio. That's pretty high. The presence control with this ratio will be very aggressive. Not only that but the standard cap value for the presence control won't work with this loop. Well, it will "work" but it will only affect very very high frequencies. The impedance of the circuit affects the knee frequency of the cap. Can you post a diagram of your plan?

The circuit doesn't care if you use a 12at7 in the PI.

[redelephant]

You could include a series resistance with the presence cap so the effect is 'normaliezed'. Alternatively use a relatively large potentiometer so the presence can span from nill to extreme.
Since feedback is less with presence, the amp will be more aggressive than without, so using a 12AX7 might make it overly so?

[Chuck H]

Sticking with the stock resistor values you would need to use roughly a 4.7uf capacitor to affect the same frequencies with the presence control as the more typical .1uf cap affects in a Bassman/Twin circuit. I think it would be best to change circuit values to get that down to where you could use a film cap in the circuit instead of a NP electrolytic. I'd use a 1k pot (this would replace the 100 ohm resistor) a 10k feedback resistor and a .47uf 200V film cap. Trying to go all the way to Bassman/Twin type NFB loop values to use a .1 cap would mean using a 47k feedback resistor and tacking another 5k to the tail of the already 22k PI tail.

[MWJB]

As Chuck points out, sticking with the BF 100ohm load, the presence cap needs to be much bigger than 0.1uf, 1uf works OK, easy to find as a film cap.

You can leave the BF PI largely intact & just change the NFB load & dropping resistors to 4.7K & 39K (to retain stock AA864 ratio) respectively, add the 5F6A late/6G12A 25KL pot & 0.1uf cap?

[phydauex]

This is what I was after, some actual design parameters. I have a idea of what is going on, but I didn't know about the feedback ratio. How is that calculated? I was going to start with the 6GC-A PI values and work back from there (820--6800,@43K NFB, 25kl with .1uf with 4.7k in parallel). I knew this would be more aggressive, so I would walk from there to reduce the gain of that layout. I didn't know about making the cap bigger, I've got a bunch of 1uf flim caps in my parts.

Is there a place to get the formulas to design it proper w/o wingin-it? It's nice to know what I'm doing, although I get a good feel for what values do, I kon't know why specifically. Thanks for all the suggestions, I need to reread Aiken's page on phase inverters.
Matt

[MWJB]

In the BF/SF amps Fender used a 9.2:1 NFB ratio for 2 & 4ohm outputs, 18.4:1 for 8ohm outputs, Champs & Princetons typically had a bigger ratio/less NFB.

To determine NFB ratio, add load resistor (to ground) to the NFB dropping resistor value (to speaker jack), then divide by load resistor value.

E.g. 100+820 = 920
920/100 = 9.2:1

Actual values don't matter too much, the ratio is key, once Fender dropped the 25K presence control & 4.7K load resistor (presence pot is typically 5 times the size of the NFB load resistor), they obviously realised that they already had part lines on order that gave the desired ratios & went with the 100/47ohm NFB loads and the 820 NFB droppers. There's no "correct" ratio, limiting factor might be that too high a ratio (too little NFB) makes an amp unstable in extreme cases, maybe just brittle in lesser cases. Too low a ratio (too much NFB) will kill your tone.

Here's some Fender values....

V@spkr/Dropper R/Load R/NFB Ratio/V fed back/Amp
14.1 / 56,000/ 4,700/ 12.9/ 1.095/ Fender 6G6A Bassman
15.49/ 10,000/ 1,500/ 7.7/ 2.020/ Fender 6G7 Bandmaster
12.65/ 56,000/ 1,500/ 38.3/ 0.330/ Fender 6G9A Tremolux
4.47/ 2,700/ 47/ 58.4/ 0.076/ Fender AA764 Champ
10.95/ 2,700/ 47/ 58.4/ 0.187/ Fender AA964 Princeton
14.14/ 820/ 47/ 18.4/ 0.767/ Fender AB763 Deluxe
14.14/ 4,700/ 47/ 101.0/ 0.140/ Fender AB763 Deluxe, Hi gain mod
12.65/ 820/ 100/ 9.2/ 1.375/ Fender AB763 Pro/Bandmaster
10.00/ 820/ 100/ 9.2/ 1.087/ Fender AB763 Super Reverb/Concert
17.89/ 820/ 47/ 18.4/ 0.970/ Fender AB763 Vibroverb/Vibrolux
17.89/ 68,000/ 4,700/ 15.5/ 1.157/ Fender Blues Deluxe/De Ville
10.95/ 75,000/ 4,700/ 17.0/ 0.646/ Fender Blues Junior
17.89/ 100,000/ 4,700/ 22.3/ 0.803/ Fender Pro 6G5A; Vibrasonic 6G13A
14.14/820/100/9.2/1.537/Fender Pro Reverb; AA165 Bassman
12.65/820/100/9.2/1.375/Fender SF Pro/Bandmaster/Vibrolux
28.28/820/47/18.4/1.533/Fender SF/BF Showman (single speaker).
20.00/820/100/9.2/2.174/Fender SF/BF Twin Reverb/Dual Showman
20.00/820/100/9.2/2.174/Fender SF/BF Twin Reverb/Dual Showman
28.28/100,000/4,700/22.3/1.269/Fender Showman 6G14A
20.00/56,000/4,700/12.9/1.549/Fender Twin 6G8A

[phydauex]

Thanks to everyone who posted. I'm getting a better handle on dialing this amp in. I got it wired up and turned on for the first time tonight, and so far so good. My initial response is it sounds right! I used a 470/22k tail with 39k nfb/with 25k pot with a 4.7k resistor across it and a 1uf cap. It sounds about right, it might be have a touch more gain than a straight up AA864, I want to swap in an 820/100 combo with a 1k pot and still use the 1uf cap, to see how that changes things. If it's true that it's the ratio that's important, then it shouldn't change much other than possibly the freq of the presence control. I'm still not sure what fomula I should be looking at for that calc. This is turning out great and I know the owner will be stoked when he hears it. Besides, the amp also had a bunch of problems like broken screen resistors, a bug nest inside, old leaky filter caps , etc. Now new filters, all new plate resistors (2w) new grid/screen resistors reworked PI vlaues, new power tubes, and a good cleaning. It makes me feel pretty good to spend some time figuring things out before I start tearing it apart and have it fire up without smoke or problems on the first try.

Thanks again for the sharing of knowledge that happens around here.
Matt

[Chuck H]

FWIW that 1uf cap is diggin into the audio range about ten times further than the standard .1 cap. Not to say it's wrong but as it is it's more a NFB control than a presence control.

If you do swap it the standard 820/100 combo and stick with the 1uf you will only be "presence"ing the very very high end.

It's cool to experiment but this is all over the map without a plan. You could just mess around until you land on something you like or finally settle but it would be much better to get your head wrapped around how the circuit values are affecting the frequency.

Personally I don't like using a 5k dropper on a 22k tail. I know that technically it shouldn't matter but I try to keep the PI tail overall resistance lower when I'm already using a 22k tail. As far as the feedback circuit you aren't limited to the 5k dropper or the 100 ohm dropper. There are a whole bunch of options in between. Using a 5k dropper a .1 cap will have a knee frequency right where a tweed Bassman or Twin would be. If you go with the 100 ohm dropper you would need a 4.7uf cap to achieve the same result. THE DROPPING RESISTOR SETS THE FREQUENCY OF THE PRESENCE CAP RELATIVE TO IT'S IMPEDANCE. 100 ohm = low impedance, 5k = high impedance. A capacitors frequency knee is relative to the impedance of the circuit it is used in.

Sorry I can't tell you the math. I'm not as slick as I should be in that regard. I use modded simulation programs often to key in on values. But the above criteria are absolutely true. So you can goof around and flip flop between an overall feedback control and an ultra high presence control as much as you like. I think experimenting is part of the point. But it's not a "presence" control yet or either way.

Your feedback ratio is about eight or nine to one. This "ratio" can be simulated from any starting value. So, you could leave your 1uf in place and try a 470 dropper and a 4.7k feedback resistor (yes that's ten to one but it's close enough). This combination would give you roughly the same effect as a typical presence control. Then you can hear what it "should" sound like. At that point you could decide if you want to change it. One small trouble is that to do the noiseless presence type circuit you would want to use a 2.5k pot with this dropper and FB resistor combo. And 2.5k pots aren't common. But you could still do a standard type presence control using a 500 ohm pot and a 4.7 k feedback resistor. Still with the 1uf cap.