Linear preamp step with gain 1

[Pedro Vecino]

After unsuccessful attempts to add a presence circuit to a circuit similar to that of a Princeton Reverb I had thought of adding a preamp step with gain 1 and unbypassed cathode to apply feedback there with an effective presence circuit. It is the added gray area (not yet determined). It is taken from the Super Amp 5F4.
https://el34world.com/charts/Schemat..._super_5f4.pdf

How could it be built? The interest is in obtaining a preamp step with gain 1 and that does not alter the contours of the previous circuit. It can be a useful resource for this and other purposes.

In orange, the Princeton circuit.
In gray, the preamp step added.



One of the resources to attenuate the extra amplification starting from a traditional 100K plate / 1K5 cathode circuit could be a split anode resistor but I don't know how I could compose the previous voltage divider to get closer to that idea. I have put values by eye in the voltage divider (2M2 and 470K) but I do not know how to calculate the set mathematically to at least find an approximation.



Thanks to all!

[vintagekiki]

Princeton Reverb add a presence similar as
https://el34world.com/charts/Schematics/files/Orange/Orange_mkii1972.pdf

[Pedro Vecino]

Thanks. It's essentially the 5F4 circuit, but with that I lose the effect of the cathode capacitor. It is one of the things that I try to preserve.

[Helmholtz]

After unsuccessful attempts to add a presence circuit to a circuit similar to that of a Princeton Reverb I had thought of adding a preamp step with gain 1 ..

The problem with your original presence wiring is the low NFB input impedance of (less than) 47R. The corner frequency of the 0.1µ presence cap and the 47R is above 30kHz!
So you need to scale your presence circuit vaues to the lower impedance.
I would start with 4.7µ and a 100R to 220R presence pot. For less presence at minimum pot setting use a higher pot value.

Your new proposal won't work satisfactorily, because you are losing the gain of the first triode within the NFB loop.
Consequently the loop gain will be lowered by maybe a factor of 50.
So the the presence effect will be extremely weak.

[Pedro Vecino]

Thanks. Yes, I understood that the values had to be scaled. Chuck H proposed values very similar to his (250R / 6.8uF) theoretically on a topic that I recovered, but he did a simulation and confirmed to me (thanks again) that the actual effect of presence was negligible.
I understand what you are saying, but the effect of the original NFB on this amp is very small. I think it could be absorbed in the final tuning of the gain on the new triode without problems.

I have tested the 5F4's presence circuit (applied directly to the cathode and with the cathode capacitor removed) and it works great. It's just what this amp needs.

My proposal is probably like killing a fly with cannon shots, but it wouldn't be the first time

[vintagekiki]

Presence control in principle raises (boost) the frequency range above 2.5 kHz and therefore in relation to the bass range.

[Helmholtz]

Presence control in principle raises (boost) the frequency range above 2.5 kHz and therefore in relation to the bass range]

...or the range above e.g. 600Hz (https://www.ampbooks.com/mobile/amp-.../slo-feedback/).

There's a lot of variation between different amps. In the end the presence is just an additional (active) treble control in the power stage.