Last preamp stage in a Peavey 5150

[BJosephs]

V5A looks like it has a feedback path wrapped around it through 1Meg resistor. What does everyone think they were trying to do here? Attenuate the signal? Reduce output impedance to the tone stack? Does this stage generate any distortion or is it just a clever way to drive the stack???

I am looking into a making a new amp and I like the gain and tone of the raw 5150 preamp but I was interested in implementing a different FX loop (the Mesa rectifier/ SLO100 type) followed by a fender-ish reverb circuit.

I modified my egnater tourmaster into something more like an SLO100 but, believe it or not, I wanted more gain.

I guess I am wondering if I could drop the funky stage and couple the TS with a cathode follower or source follower mosfet stage.

Any thoughts?

PS - I couldn't get my scematic file to upload but I figure most of you guys have discussed this amp to death...

[daz]

Looks like what a dumble has, or i think it was a dumble. But they use a lot more resistance. like 22 megs. It gives a sort of compression. i tried it and it was kinda neat for some things, but i wouldn't want it in my amp permanently. Might be good on a switch. But 1 meg seems awfully low because 22m was almost too much. But it also was different in that it came before the coupling cap and used a cap of it's own, a .047 if i recall.

Anyways, heres the schematic....

http://www.schematicheaven.com/newam...ey_evh5150.pdf

[HTH]

just looks like a single-triode cathode follower stage. there was a thread the other day on this board with four examples of such CFs.

[Enzo]

HTH, he is looking at V5A, not V3B.

[BJosephs]

Ok, so maybe this stage was designed with a few things in mind. I doubt it was about impedance because of the 470K resistor that follows it. Maybe what they were trying to avoid was the distortion that a collector follower adds to big signals?

[Enzo]

Collector follower?

I don't know, it's just a little local NFB, perhaps it wants to tame a little of the excess gain of this monster amp, Or smooth it out a touch.

[pdf64]

This is a inverting stage with a gain of 1, input impedance 1M ohm, output impedance ? (a lot less than a regular common cathode stage, see http://www.aikenamps.com/FeedbackAmp.htm
to work it out yourself).
As pointed out, as the load it's feeding is 470k, it's not really doing much, so V4b plate could be coupled to the 470k with minimal loss, compared to the 1M load that the V5a circuit was presenting to it. Peter.

[Wakculloch]

I would have thought it's there as an extra stage simply to ensure the effects loop is in phase with the input, so it can also be used as a line in/out.....Maybe.

[Steve Conner]

I've used such devices as reverb mixers and parallel FX loop returns in my homebrew experiments. It's called an anode follower, and the feedback path makes the grid into a virtual ground, so you can feed several signals in there via other resistors.

So maybe PV are using it as the FX loop return mixer.

[Enzo]

But it is before the loop, the stage after that is a CF to drive the loop, the loop return is into a stage just before the PI.

[Steve Conner]

Oh, well I don't know what it does then

[Tubis]

That stage has lower headroom than the previous so it adds distortion and compression though it has unity gain.