Dumble SSS with Vibrato? And other questions

[caphead]

Hi everybody, I am new to this forum. I feel lucky that such a forum like this exists; I have found it to be a priceless treasure-trove of information already.
I have seen the schematic for the SSS, and I am quite motivated to build a variation of it, but with a Fender style Vibrato. I am not normally a fan of this effect, but I got to play a 1961 Fender Super recently and I fell in love with it's vibrato. Problem is that it uses a completely different circuit that involves 2 1/2 12ax7's and effects the preamp signal and not the output section as it does in the latter models (I have included the schematic in this post). The question I have is whether I should place the vibrato before the reverb or do it the other way around?


Second question I have is about the feedback loop that connects the cathode of the first stage to the plate of the second stage in the SSS. Would this be considered a positive feedback loop?

[Structo]

Generally speaking, reverb is usually the last effect in a chain.

Feedback on preamp stages is called local negative feedback.

Helps to keep the signal clean and stable.

[martin manning]

Fender harmonic vibrato is a cool effect for sure, but it will take up a lot of space in your chassis. There is no reason that you couldn't do a stand-alone version and put it in the loop just ahead of the PI, same as in the Fender Super.

[caphead]

The dilemma I come across when thinking about the position of the vibrato is that if it comes before the reverb, the verb pan will become overwhelmed by the pulsating of the vibro effect; if the vibrato comes after the reverb then the architecture of the original SSS circuit will be compromised, leading likely to unwanted distortion. Is my logic on this correct? I have been looking through various schematics trying to find an example of vibrato/tremolo coming before reverb, and I have not yet found one. As for the issue on the feedback loop: I am familiar with negative feedback loops and their effects, but it looks to me from the way the SSS schematic is drawn, is that first loop is a positive feedback loop. I've attached the well known schematic from another post to aid in further discussion. Thanks guys thus far for your input and knowledgeable opinion.

[martin manning]

I am familiar with negative feedback loops and their effects, but it looks to me from the way the SSS schematic is drawn, is that first loop is a positive feedback loop.

Notice where the feedback is applied- to a non-inverting input.

[caphead]

I am familiar with negative feedback loops and their effects, but it looks to me from the way the SSS schematic is drawn, is that first loop is a positive feedback loop.

Notice where the feedback is applied- to a non-inverting input.

I suppose I am missing something then, could you perhaps elaborate? The way I see it is that the cathode of the first stage is in-phase to the input, the signal is then inverted when it comes off the plate, it goes through the eq/gain section, then it is inverted again from the second stage plate, making it in-phase with the input stage grid/cathode.

[martin manning]

A signal applied to the cathode isn't inverted at the plate, where a signal applied to the grid is. Therefore a FB signal that is in-phase with the grid input signal is negative feedback when it's applied to the cathode.

[caphead]

A signal applied to the cathode isn't inverted at the plate, where a signal applied to the grid is. Therefore a FB signal that is in-phase with the grid input signal is negative feedback when it's applied to the cathode.

Oh, I get it now! Thank you, I never thought of that. I suppose this is just one more example of how brilliant Alexander Dumble really is.

[10thTx]

IF you did not have to have the 100w and you were willing to compromise on the LTPI to poweramp section, you could "borrow" from Fender and perhaps match this to the SSS preamp design that you posted.

With respect, 10thtx

[talbany]

A signal applied to the cathode isn't inverted at the plate, where a signal applied to the grid is. Therefore a FB signal that is in-phase with the grid input signal is negative feedback when it's applied to the cathode.

That's why they call it a cathode follower,, same phase,,

Tony

[Lachlan C.]

Just a thought...

Have you considered bias modulated trem/vib architecture?

- Lach