Which OT tap to use for presence?

[Midtex]

I'm scratch building an amp based loosely on the '59 Bassman 5F6. The OT I'm using has 4,8, and 16 ohm taps. Which tap would be best to take my signal from for the presence circuit and what value resistor should I use to change the 27K used on the Fender circuit off of it's 2 ohm secondary? Thanks in advance for any advice!

[phsyconoodler]

The presence and negative feedback are all in one on the bassman.I would say use the 4 ohm tap,but someone else might have other insight.

[Midtex]

Yes, I assume the "presence" circuit is doing something with negative feedback although it feeds the signal back into the PI circuit rather than farther back into a gain stage like the hifi amps circuits I'm more familiar with.

[Midtex]

Looks to me that the presence control is varying the frequency and amplitude of the negative feedback signal.

[sluckey]

Use whichever tap you like. Here's a chart I had for a very similar NFB loop. I had an equation to calculate but don't recall what it was. Anyhow, it's not that critical. You can probably tune it by ear. There's no wrong or right here.

2Ω tap = 22K
4Ω tap = 29K
8Ω tap = 39K
16Ω tap = 56K

[Midtex]

I had a feeling it wasn't real critical because Fender used 27K and Weber uses 56K on the same 2 ohm tap. That seems to be the range of vaulues I am seeing. I don't have much experience though and I think I remember reading an article on a hifi amp that they thought something was improved by moving the NFB loop from the 8 to the 16 ohm tap, so I didn't know if there was some inherant advantave to having it on the higher impedance winding. I will just experiment with the sound. Thanks!

[DummyLoad]

experiment. let your ears decide.   

--ISO

[jojokeo]

It seems that the most common tap utilized for a presence control comes from an 8ohm tap when you look at a great many schematics and it's tied to the most commonly used pot value of 5k. What's of most importance is the 20:1 ratio here. Use too small of a dropping resistor (100k) and your sound will suffer from too much signal feeding back into the circuit. Use too little and you wont get quite the amount of response expected/wanted. As ISO says, your ears will guide you once you understand the principles involved.

[stingray_65]

Midtex,

Now I don't have any scientific proof for my thoughts on this, but here's my thinking on this subject.

For NFB and presence I always use the lowest  inductance tap, even if I have to modify the values in my components.

On a different forum I was working out a balanced output that I put on most of my builds. It was suggested that I use the highest inductance to tap because it was felt that the higher the inductance, the more windings and therefore a more complete and fuller signal to use.

Conversely, fewer windings lower impedance and PERHAPS a less complex signal would be ideal for NFB or presence. My reasoning is less "dampening" of the subtlies in the signal.

I know this sounds like a bunch of HiFi hocus pokus, but it works for me.

I had done an experiment once on a 5F2A circuit and added a 1 knob tone stack in the NFB loop. As the pot was turned up and more highs were passed in the loop, the amps tone darkened, A LOT! As the bass was passed , the amp brightened into a HARSH tin can / AM transistor radio sort of noise.

Where it sounded best was in the range where a presence control would be, go figure!

I've not experimented by performing A/B comparisons on my taps, I've always wired them like I described and have been very pleased.

But I do know that subtle changes in values in the NFB and Presence do have a BIG impact on the final tone shaping. It's fun cheap and easy to mod. Like suggested before, use your ears and when ever possible use different guitars too. what sounds great with a single coil often sounds VERY different with a humbucker. It's easy to wire in a switch and change  cap and resistor values, or even cut the loop for other voicing options.

Ray

[Geezer]

You can probably tune it by ear. There's no wrong or right here.

That's what I do in my amps.....tune it by ear.

I use a close (slightly higher) value pot (in your case, say a 100k linear) as a variable resistor temp wired in with alligator clips.
I start with the pot @ max resistance (least amount of NFB), strum a chord, then turn the pot until I hear a slight reduction in output/volume.
I play the amp a little (chords, lead stuf, favorite songs, etc) & see how the amp "feels" or reacts.
If it's a bit too "raw", I turn the pot a little to reduce resistance (add more NFB) & try it some more.
If it's too "compressed" or lifeless, I adjust the pot to increase resistance (less NFB).
I keep adjusting until I sounds "right to my ears, then remove the pot from the circuit, measure the resistance & replace it with the closest value fixed resistor.

G

[LooseChange]

Hey Geezer, I like the way you explained that.  I think that's what I listen for but you put it in words.
Dan

[jojokeo]

Nice job of explaining how to dial in NFB Geezer. However, initially his question was about "presence" control. Can this method work the same when you're adding this instead of NFB only?

[HotBluePlates]

I had a feeling it wasn't real critical because Fender used 27K and Weber uses 56K on the same 2 ohm tap.

If you have a feedback loop, and double the series resistor as you've noted, you get less feedback.

I don't have much experience though and I think I remember reading an article on a hifi amp that they thought something was improved by moving the NFB loop from the 8 to the 16 ohm tap, so I didn't know if there was some inherant advantave to having it on the higher impedance winding.

Be careful noting things from hi-fi and applying to guitar amps!  If they made no other changes, they boosted the amount of feedback, which might be desirable for clean reproduction in a hi-fi amp.

The output transformer takes a voltage applied to the primary and induces a similar voltage in the secondary, in a ratio determined by the amount of turns of the primary to the secondary. Different secondary impedances are accomplished by having more turns for the higher impedance tap.

For the same primary voltage, if you move to a higher impednace secondary tap, you get a higher voltage output. The total power output is the same, as impedance went up, current went down, and voltage went up. If you doble the impedance (move from 8 ohm to 16 ohm tap), you will get SqRt 2 more voltage, or V * 1.414. If you quadruple the impedance (4 ohm tap to 16 ohm tap), you get 2x the voltage output.

Which output tap to use really depends on where the feedback will be injected back into the circuit, and how much voltage you need to develop, as well as what tap(s) might be available. Much of the initial voltage available at the secondary is thrown away in the voltage divider forming the feedback loop (because a smaller amount of voltage is needed).

Use whichever tap you like. Here's a chart I had for a very similar NFB loop. I had an equation to calculate but don't recall what it was. Anyhow, it's not that critical. You can probably tune it by ear. There's no wrong or right here.

2Ω tap = 22K
4Ω tap = 29K
8Ω tap = 39K
16Ω tap = 56K

See, Sluckey basically told you this with his chart. It doesn't matter which tap is used, but higher impedance taps have higher voltage outputs, so a bigger series resistor is used to maintain the stock level of feedback.

[bobmegantz]

You can use any tap you want.  The resistor values (there are two forming a divider) are chosen to give the desired feedback factor.

If you want the same feedback factor as in the Fender circuit, there is an easy way to calculate the series feedback resistor value needed, assuming you want to retain the existing shunt resistor.  You can find it in a number of places, including Merlin's preamp book.

As stated by others, you may also want to use a different feedback factor (usually less feedback) or even none at all.  There are ways to make the feedback factor adjustable as well.

Bob

[jojokeo]

Do people use this method for a presence control or only for nfb?

[PRR]

> presence control or only for nfb?

This specific presence network IS a NFB network.

The amount of NFB you like depends on your speaker.

Fender open-back cabs and stiff cones worked well with loose NFB.

Marshall copied the 5F6A but his looser cones favored closed-cabs and more NFB. He simply moved Fender's NFB network up to the higher tap. This also gave a "different" overdrive to go with the different brand, knobs, etc.

Build. Connect anywhere. Play. Modify.

High NFB came into fashion 1950s-1960s. Then transistors did the high-NFB thing much better than any tube. The fashion now is less NFB, more raw tube sound, and pick your speakers to handle the rawness. Increase the value of the resistor from (whatever) speaker tap back to the driver. Gain goes up, color goes up, clipping is less abrupt, bass-notes cone-slap more. Tinker until happy.

[jojokeo]

Thanks for clearing that up nicely PRR. I know that they both inserted neg feedback but didn't think the presence control was exactly the same because of it's use w/ the capacitor is all - not trying to be difficult. This hobby is difficult enough as it is and tiny nuances from someone's descriptive writing can mean all the difference sometimes with another's understanding or interpretation. 

[PRR]

> same because of it's use w/ the capacitor

Turn the knob to ground, the cap does nothing, it is a simple NFB network (except in 5F6-A the NFB goes both to the off-side grid and the common tail).

Bring the knob up, less treble gets NFB-ed away, gain rises in treble.

The tricky part of the _5F6-A_ connection is that injection into the common tail resistor. That appears to shift the distortion balance between middles and highs. Analysis is messy; results suggest this oddity has musical uses.

[Midtex]

Thanks for all the discussion. I used a 56k series resistor off the 16 ohm tap and seem to have nailed it first try. I just finished the amp a few days ago and finally had an opportunity to take it to a friends house today who is an excellent player and has a good ear for subtle tone and frequency changes. He was quite complimentary of the amp and said the frequencies tweaked by the presence control were right on. He used a 2 x 12 sealed cabinet with a pair of Celestion Vintage 30's and it sounded awesome.