Express without presence control

[harryk]

After finishing one Express and one Liverpool I am aiming to do one two channel tube amp with a Express lead preamp channel and a Fenderish clean preamp channel. Because I am lacking space in my chassis I thought that i could leave presence control away of this amp. How does it affect to Express sound not to have it? I never use presence control actively but I do not want to lose Express magic if it has a remarkable impact to it`s sound. Regards Harry

[Structo]

Could you put the presence pot on the back of the chassis?

[peesinstew]

if you always keep the presence at the same position, you could just measure what value you have it set to and wire in some fixed resistors in it's place. Of course, this would only free up space on the front panel, not inside the chassis.

[jaysg]

Go one step further -- internal trimpot, back of the chassis or pointed up like a Fender bias control.

[Fischerman]

I originally built mine with a Presence control but immediately yanked it out since it never went above 0. The last thing my amp needed was any extra brightness. I put a Cut control of sorts in it's place (so I can remove high end).

[harryk]

I could put it somewhere back or inside of the chassis but I am also intending to test somekind of VVR/PS-mastervolume to this amp. I heard that negative feedback/presence control affects to that soundwise. I only want to know if presence is essential to capture Express magic or not. Anyhow I can always test it in real life, but before drilling too many holes etc. it would be nice to know other memebers opinions of this matter. Harry

[paulster]

Presence should still work fine with any form of power scaling, so long as you are scaling the PI as well.

Where it falls down is when you use a post-phase inverter master volume (PPIMV) which makes the presence control ineffective once you turn it down a reasonable amount as you're not getting the expected level of NFB.

For a power scaled type solution you'd want a pre-phase inverter master volume (assuming you scale from the PI onwards) so that as you turn down the B+ and reduce the available headroom, you track this reduction at the PI input as well with the master volume (known as the 'drive' control by one vendor in particular).

[Firestorm]

After finishing one Express and one Liverpool I am aiming to do one two channel tube amp with a Express lead preamp channel and a Fenderish clean preamp channel. Because I am lacking space in my chassis I thought that i could leave presence control away of this amp. How does it affect to Express sound not to have it? I never use presence control actively but I do not want to lose Express magic if it has a remarkable impact to it`s sound. Regards Harry

Harry--
Does it have to be two separate channels (with separate EQ and the option of channel bridging)? Or would it be enough to be able to dial in a Fender clean sound on a single channel?
The stock TW tone stack is not especially versatile: it's "midrange dip" is centered at about 110Hz (not very midrangey); all the bass gets rolled off by the .002 coupling cap going into V2; so pretty much what you have left is treble boost/cut.
If you look at V1a and V1b (ignoring the tone stack) it's already very close to a Fender preamp.
So something to consider: rework the tone stack to be closer to Fender values (maybe 100k slope resistor, 250pf, .02uF, .02 uF, 250k treble pot, 250k bass pot, 25k mid pot). That will get you a more Fendery sound. Then make the slope resistor switchable between 100k and roughly 33k; at 33k, you're back very close to a traditional TW EQ.
Then wire up the unused triode on V2 like the mix stage on a Fender reverb amp and rig a switch to shift between V2a (TW) and V2b (Fender). Feed the signal into both sides and let the two triodes share a 100K plate resistor (so you only need one lead to the PI) and do the switching by lifting the cathode on one tube or the other. If you switch it some other way (or want to have both at once -- probably unstable), make the anode resistor 50K (two tubes, roughly twice the current).

[harryk]

Firestorm your proposal is interesting. I have done many two channel preamps with relay switching without any problems and that is why I thought that I do it also this time that way and with separate tone controls. I have to think which way to go with this project. I still have not got any answers if leaving presence away will affect substantially to Express sound. Still waiting. Harry

[peesinstew]

I guess I'm not sure whether you're asking if you can get rid of just the presence control or if you're asking if an express will sound right without any NFB.

I can't answer the second question with any sort of authority, but my initial thought is that the express is on the verge of instability and you might need the NFB to keep it stable and working properly.
I know that you don't need a presence control to implement NFB in the circuit, but you'd have to figure out how much you want.

When you say you never use the presence control actively, does that mean you always keep it at zero, or always full up, or somewhere in between?

[harryk]

When I say that I never use presence control actively it means that it is somewhere in the middle position. I have never felt presence to be a tool to taylor my guitar tone and this thinking and question started when I asked could I leave it away without sacrificing Express magic sound. Harry

[Firestorm]

Just to complicate things further Kevin O'Connor has a drawing of a "Resonance" circuit (TUT 1, I think) that works like a presence control, but on lower frequencies. It basically reduces the speaker damping so the speaker can flop around a little more. I never liked it in amps that already had strong bottom end, but I wonder what it would do in a TW?