Adding switchable PRESENCE control to a 1974 Fender SF Twin Reverb

[TelRay]

Hi all... you might remember this beauty from the "Repair and Restoration" section

http://ampwares.com/schematics/100W_...F_Twin_Rev.pdf

[you can skip this section if not interested in the backgroung of this specific amp, but some things could be of use for people dealing with Fender Twins]

This amp is pretty much to standard with the exception of:

- PULL SWITCH distortion function has been removed
. all long wires going from the MASTER VOLUME to several sections of the board
. 12M + .o1 uF + 12 K associated to the distortion function (see post #455 for historical details details: LINK)
- REVERB signal feed increased (see post #359 point #4: LINK)
- Added an ADJUSTABLE BIAS VOLTAGE TRIMPOT (LINK)
- just in case, also DEATH CAPACITORS removed and a three prong cable added

So, the idea is to see if I can use the (now unused) GROUND SWITCH at the back of the amp to have three levels of PRESENCE:

- NONE
- MID
- MAX

I've been "cultivating myself" and understood that I need to play with

- NEGATIVE FEEDBACK Resistor (820 Ohm in my Twin)
- PHASE INVERTER tail resistor (33 KOhm in my Twin)
- NEGATIVE FEEDBACK tail resistor (100 Ohm in my Twin)



What I've understood from the 5F6A Fender Bassman circuit is that:

- 100 Ohm resistor needs to be replaced by an RC arrangement

- the value of the 820 resistor needs to be recalculated to maintain the same voltage divider ratio
originally 820/100=8.2 and now X/4.7K=8.2, then X=39 KOhm
- the value of the 33K resistor needs to be decreased by 4.7 K (R value with the potentiometer at 0), then 28 K

Now, from the three settings (NO PRESENCE, MID and MAX) the two fixed R values at the extremes are clear:
- NO PRESENCE: 4.7 K
- MAX PRESENCE: 4.7 K + 25 K = 30 K

QUESTION #1:
what is the MIDDLE setting based on?

a) Half way of the the potentiometer turn (12.5K if it is linear)
in that case it's 4.7 K + 12.5 K = 27 K

b) the RESISTANCE that provides the middle value of the voltage divider ratio between the NO PRESENCE and MAX PRESENCE values?
RATIO= 8.2 at NO PRESENCE setting
RATIO= 39K/(4.7K+25K)= 1.3 at MAX PRESENCE setting
Middle value (8.2+1.3)/2= 4.75
So the resistance value 8.2 K

QUESTION #2:
do you guys see a way to use the 3 position GROUND SWITCH to do this? (I have not gotten my head around this yet for some reason)



thanks!

[Leo_Gnardo]

I really have to ask - isn't a Twin Reverb bright enough without having to add more presence? I s'pose... if you think you really need that.

If not, I'd propose a simpler mod that follows what a Vox "cut" control does. You could set it up as a switch or pot. Just one cap say 0.022 uF, a 250K pot, and a resistor to limit the filtering of the pot so your signal doesn't get totally schwamped when you dial away from the bright end of the pot's travel. Or, a switch, a cap and a couple resistors, selected to do what makes your ears happy.

FWIW I have on a couple occasions added a presence control to Fenders without having to revamp the output drive tube circuitry to resemble Fender's 5F6A Bassman. Swap out the 100 ohm resistor for a 100 ohm pot, and have a comparatively large value film cap as the filtering element. I've used 4.7 uF ordinarily used in a crossover. If you can locate a non polarized electrolytic that would work too. You may want to try a different values & find which satisfies your ear.

[TelRay]

thank you for jumping in!
i guess the first reason is "curiosity"
I have the Magnatone M10, Ampeg Jet and this Twin... (all amps you guys helped me get in shape) none of them have a presence control
I have been reading about the differences in TONE SHAPING at the preamp (subtracting) and power section (boosting) and got me curious
It also presented a challenge to try to figure out the mod by myself (supported by some research of course) and present it to you guys for validation.
To come back to your very pertinent question, I don't think I need it. I'm perfectly happy with setting the mod up and leaving the SWITCH in the OFF position forever after trying the PRESENCE sound for a few weeks.
Thank's for the suggestion on the alternative mod too

[pdf64]

H...
I've been "cultivating myself" and understood that I need to play with

- NEGATIVE FEEDBACK Resistor (820 Ohm in my Twin)
- PHASE INVERTER tail resistor (33 KOhm in my Twin)
- NEGATIVE FEEDBACK tail resistor (100 Ohm in my Twin)
...

No, the LTP tail resistor is not part of the global NFB network.
The 820 and 100 form a potential divider, the output of which is the actual feedback signal applied to the LTP input. Hence the smaller the output, the lower the degree of feedback, and the closer the closed loop gain gets to the open loop gain. As Leo suggests, bypassing the 100 resistor with a 4u7F cap would achieve a presence boost, a small resistor (eg 47ohms) in series with the cap would achieve a 'half presence' option.

[TelRay]

No, the LTP tail resistor is not part of the global NFB network.

you are right, I was not suggesting it was. however in my reasoning that 33K R sees the currently existing 100R before it goes to ground. I thought I had to reduce it's value so that the total resistance was still equivalent to 33K when introducing the 4.7K R.
the value of the 33K resistor needs to be decreased by 4.7 K (R value with the potentiometer at 0), then 28 K
I guess I was wrong, thanks for pointing that out and the mod suggestion.
In any case I sensing from the posts that you don't really see this as an interesting mod for this amp. So I might look for something else

[Helmholtz]

In any case I sensing from the posts that you don't really see this as an interesting mod for this amp. So I might look for something else

Just tack a 4.7µ ecap across the 100R and see how you like it.

[TelRay]

i happen to have a 4.7uF 25V ecap... voltage rating too low for this?

[Helmholtz]

i happen to have a 4.7uF 25V ecap... voltage rating too low for this?

25V rating is fine. Cap polarity: negative to ground. Should boost frequencies above 340Hz.

[TelRay]

vielen dank! the "negative to ground" warning is very pertinent

[TelRay]

the e-cap I had was finally a UES Bi-polarized Nichicon (so i couldn't have blown it even without Helmholz help)
I've measured 5.6 uF on the cap (just on the edge of the +20% tolerance) so the boost will start a little lower at 284 Hz.
I've found a more elegant solution than using the GROUND SWITCH at the back of the amp and hooked it to the unused PULL feature of the MASTER VOLUME pot at the front
Will put the amp back together and try it for some days
thanks!

[TelRay]

Hi all,
I've tried this for a few days and liked it.
Overall the effect is subbtle and I think (we went over the twin master volume NFB in another post) it is more noticeable at higher master volume settings.
I cannot use very high volume settings at home (even less a fender twin insane high volume) so I will need to wait until I can take the amp to the studio for a more serious test.
Does that make sense (higher PRESENCE effect at higher MASTER VOLUME SETTINGS)?

[Leo_Gnardo]

Does that make sense (higher PRESENCE effect at higher MASTER VOLUME SETTINGS)?

Fletcher-Munson effect? The ear is less sensitive to extremes of the audio spectrum at low volumes.

[pdf64]

At max, perhaps power amp overdrive is masking presence boost.
At low settings, perhaps the reference to the global NFB, as opposed to 0V, is causing an unexpected effect?