how to add presence to fender twin reverb

[kwm488]

hi
 i just need help about presence problem. i follow this link to add a  presence pot, but it doesnt work. can anyone tell me how to solve the problem?

http://forums.birdsandmoons.com/forum/showthread.php?p=385301

[LooseChange]

Look at any 5F6A schematic and copy that.

[sluckey]

And what did you do to fix the bias problem in your other thread?

[kwm488]

hi
i can't get the desire bias range. i change the resistor which solder on the bias pot from 27k to 15k. problem solved. i also change the resistor which connect between bias tap and diode from 1.2k to 470ohm like original blackface.

in this movement, i still don't understand why people use 100uf bias cap instead of original 50uf?

[tubeswell]

hi
i can't get the desire bias range. i change the resistor which solder on the bias pot from 27k to 15k. problem solved. i also change the resistor which connect between bias tap and diode from 1.2k to 470ohm like original blackface.

in this movement, i still don't understand why people use 100uf bias cap instead of original 50uf?

100uF takes longer to charge up, and more capacitance = more ripple cancellation, but 100uF is not a heck of a lot more effective than 50uF when it comes to setting the bias voltage. In fact, for bias voltage, less capacitance is 'better' from the point of view of ensuring that the bias voltage stabilises as quickly as possible at start-up.

[kwm488]

Hi
Tubewell, is it mean that I should replace 100uf bias cap to 50uf?

Also, I remember that 5f6a presence is not work in twin reverb. I see this information in other forum. So I need help for my amp

Last, I plan to change my silverface ot to mm blackface design ot. Mm told me blackface ot sound more sweet, have anyone try it?

[LooseChange]

The 5F6A presence will work fine in the Twin as long as you use the correct components. You need to change the NFB resistor as well as the tail resistor that becomes the presence pot. There are other ways to do it but all will involve those two components.

Are you asking about changing the Output Transformer from Silverface to Blackface using Mercury Magnetics iron??

[kwm488]

Hi
Is it mean I need to change 820 nfb resistor to 27k like bassman? And twin reverb have 100ohm resistor which connect to nfb too. I remember other people said bassman presence not work because of this resistor.

My twin reverb is 1976 silverface with master volume. The reverb ot is blow , my bad of connect error. I feel very unhappy, because i cant buy it and my amp will not vintage anymore. Finally, I change it to mm reverb ot, sound much more better than old reverb ot. So I hope to ask everyone option, should I change to mm blackface twin reverb ot? Mm tell me blackface ot better than silverface ot, sound more sweet

[vince]

Maybe cut voxy-style control will be better option?

[LooseChange]

Yes, like the Bassman or like a Showman 6G14, a Twin 6G8A or maybe a Vibrasonic 5G13. Check them all.

You blew up your OT in the 76? Did this one have Ultralinear taps? Replace with anything good. If MM says the Blackface is warmer, I'll believe them.

[kwm488]

Sorry for my bad English.my meaning is that the reverb transformer(it is small ), not output transformer.

So I consider should I buy mm twin reverb ot? Because sound upgrade even just replace reverb transformer.
My silverface is not ultalinear ot.

[OldHouseScott]

Try this and see if it works for you.

[kwm488]

sorry, my bad, i don't know how to read schematic. is it mean i should use  1K reverse pot

[tubeswell]

Hi
Tubewell, is it mean that I should replace 100uf bias cap to 50uf?

Well if it was me I'd have probably built it with between 10-50uF, but since you've got 100uF in there and if it works, you may as well just leave the part where it is.

I see the others have responded to you other questions about NFB/presence component values.  Nevertheless I would say that what you have to understand about NFB, is that it takes a signal from later-on in the signal path, and puts it back to a point that is earlier-on in the signal path, in a way that 'cancels out' some of the earlier signal in order to 'clean it up' a bit. But not all presence control circuits are the same. The strength of the voltage divider's output signal depends on the strength of the input signal, so you need to work out how much output signal you want to be fed back to the pre-amp based on how strong the signal is at the OT secondary. A simple thing to do is copy a circuit that already works and tweak the NFB and/or tail resistor values and the cap value until you get an effective range of presence control.

And yes - the 1k rev-log pot shown in the schematic posted earlier is 'anti-log' by another name, but you could use a linear pot at a pinch.

[Fresh_Start]

This article on negative feedback from Aiken Amps may be helpful.
http://aikenamps.com/GlobalNegativeFeedback.htm

Chip

[robrob]

Here's one way to add a presence control to a blackface/silverface amp. It uses the 5F6A Bassman presence control and the amp's NFB & PI Tail resistors are adjusted to compensate for the presence circuit's added resistance.

https://robrobinette.com/AB763_Modifications.htm#Presence_Control