Marshall 'presence' control, why not on similar era Fenders?

[mikepukmel]

In the marshalls, e.g. 1968 50 watt, there is a 'presence' control at one corner of the long tail pair:

http://drtube.com/schematics/marshall/1986u.gif

its been too long to remember what adjusting this control sounds like, but anyone know why Fender never used these? Has anyone tried adding one to a Fender,e.g. Deluxe or Pro?

Thanks!
MP

[Leo_Gnardo]

Fender DID use these. See the 5F6A '59 Bassman 4x10, that's where Marshall nicked their circuit from.

Presence works in the feedback circuit of the power amp. You could say it tilts the frequency response "up" as you dial it from 1 to 10. Puts a little more sizzle into the high frequencies. Or rolls 'em off a bit if dialed down.

[mikepukmel]

Thanks Leo.
Also, thanks, didn't know the Bassman had this control.
Wondering what would happen ( or not) If I fiddled with this on my Deluxe build. I pulled off the reverb and tremolo circuits (never had much use for either) so I have 3 holes in my chassis for pots. Would be nice to put something useful in there.

[mikepukmel]

So its going to dump some of the high freq (depending on the value of the capacitor on the wiper of the pot) from the nfb to ground? This affects the frequencies of the nfb that get back into the phase inverter? So, it will cancel out more or less HF?

[Chuck H]

Since your Deluxe is a build it's probably a 5e3? if so then unless you added a feedback loop, it doesn't have one. If you have a different Deluxe that does have feedback loop then you'll need to design around the circuit values for the presence control to operate properly. Simply kludging in a .1uf cap and a 5k pot won't do and some people have tried this with BF type amps without the desired result.

[mikepukmel]

Since your Deluxe is a build it's probably a 5e3? if so then unless you added a feedback loop, it doesn't have one. If you have a different Deluxe that does have feedback loop then you'll need to design around the circuit values for the presence control to operate properly. Simply kludging in a .1uf cap and a 5k pot won't do and some people have tried this with BF type amps without the desired result.

Thanks Chuck!

its an AB763:

http://www.thetubestore.com/lib/thet...-Schematic.pdf

it does have a feedback loop, very similar to other Fender two output bottle amps, 820 ohm resistor in line with OT, and 47 ohm resistor to ground. I was surprised that almost all Fender amps have the same 820 ohm resistor in the feedback line, but only change the resistor to ground to 100 ohm for the bigger output amps like twin and bassman. (I had thought that the size of the resistor in the 820 ohm position depended on the output of the amp, but must be wrong).

I don't have the engineering chops at this point to figure out the pot and cap to use, or if I need to put another resistor in there around the pot someplace. (or band aids, and neo sporin, and a fire extinguisher, or a credit card to order another power or output trans that I burn up...)

[Dave H]

I don't have the engineering chops at this point to figure out the pot and cap to use, or if I need to put another resistor in there around the pot someplace. (or band aids, and neo sporin, and a fire extinguisher, or a credit card to order another power or output trans that I burn up...)

Try changing the the 47 ohm resistor to a 5k pot with a 0.1u cap and change the 820 ohm feedback resistor to 82k. That should give the stock deluxe feedback ratio (and sound) with the presence pot fully ccw.

[mikepukmel]

I sorta, kinda, sorta got a cartoon view of what's going on. The 820 and 47 resistors form a voltage divider. For the fender its like 47/(820+47)th or about 5% of the signal gets fed back into the LPT. For teh Marshall I looked at (1986 plexi) it was about 10% I think (5k/(47k+5k) or about 10%, but some of that is bled off through a cap from the wiper of the 'presence' pot. Im a guessin that 47 ohm pots are not very common, so one of the reasons why marshall chose those components in the feedback circuit was to be able to get a pot that was available? (or Im all wrong, guessing).

Off topic question, ok not so far off: I looked around on the web to figure out what resistance to expect if the wiper is at either extreme of the motion for a given rating for the pot. E.g. if its a 5k pot, and the wiper is at the extreme, then one end to the wiper is about 5k. But if you turn the wiper all the way back, the resistance one lead to wiper lead isn't 0, right? I tried to find it in data sheets, but couldn't.

[mikepukmel]

Try changing the the 47 ohm resistor to a 5k pot with a 0.1u cap and change the 820 ohm feedback resistor to 82k. That should give the stock deluxe feedback ratio (and sound) with the presence pot fully ccw.

Thanks Dave, I will order the parts!

[Dave H]

I looked around on the web to figure out what resistance to expect if the wiper is at either extreme of the motion for a given rating for the pot. E.g. if its a 5k pot, and the wiper is at the extreme, then one end to the wiper is about 5k. But if you turn the wiper all the way back, the resistance one lead to wiper lead isn't 0, right? I tried to find it in data sheets, but couldn't.

There may be some end resistance but it won't matter as it will be much less than the 82k feedback resistor.

I think Fender changed the 47 ohm to 100 ohm on the Twin because the feedback is taken from a lower impedance tap than it is on the Deluxe. The 8 ohm tap has 1.414 times the voltage of the 4 ohm tap.

[guitman321]

Look at Concert 6G12A.

[mikepukmel]

There may be some end resistance but it won't matter as it will be much less than the 82k feedback resistor.

I think Fender changed the 47 ohm to 100 ohm on the Twin because the feedback is taken from a lower impedance tap than it is on the Deluxe. The 8 ohm tap has 1.414 times the voltage of the 4 ohm tap.

Thanks Dave.

[mikepukmel]

Look at Concert 6G12A.

Thanks Guitman. Looking at the Concert schematic:
http://www.thevintagesound.com/ffg/s...12-a_schem.gif
I see how the voltage divider is setup and the pot/cap for the presence control. The components on the output tube side of the long tail pair are comparable to the Deluxe. But on the Preamp side, the two resistors leading from the divider point of the feedback to the long tail pair cathode: on the deluxe its 22k and 470 and on the Concert its 6800 and 820. It looks like those two resistors affect how feedback is introduced into the long tail pair, is that correct?

[guitman321]

It looks like those two resistors affect how feedback is introduced into the long tail pair, is that correct?[/QUOTE]

Not how but how much. I only intended to show the Fender circuit for reference.

[robrob]

"I think Fender changed the 47 ohm to 100 ohm on the Twin because the feedback is taken from a lower impedance tap than it is on the Deluxe."

Yes, the AB763 amps with 2 or 4 ohm secondaries got the 100 ohm NFB resistor to compensate for their lower voltage output. The Super Reverb's 2 ohm secondary gives it the least NFB of all the blackface amps.

https://robrobinette.com/AB763_Model_Differences.htm