Question about gain staging

[daz]

I recently stumbled upon a change in my homebrew that made me realize i may have had the gain at the various stages less than optimal for the sound i'm going for. Thats not to say it didn't sound good before, but i changed something and the tone changed in a few ways, some good some not so good. Like a trade off, but i felt the trade off was a worthy one. I lowered the gain at V1B with a split load plate and raised it at a point after that. The change that i liked was a pretty considerable gain in playing dynamics and a wider tone that seems to have more highs but in a good way. The trad off was the low end isn't quite as tight.

Anyways, it caused me to start thinking about where the gain should take place in a 3 stage + cathode follower preamp. What do you think.....lower gain in the first stage and building later, or cleaner at the earlier stage and gradually building later on? When i say building later i'm not talking about gain getting higher as it goes because of course it does, thats what a cascaded pre is for. I mean designing later stages for more gain than earlier. Is that a good idea? Seems like it. I'm now wondering if i should ad that gain back to V1B and instead cut gain at V1A. By the way, a while back i did another thing that also seems to point to later stages having higher gain being better, and this seems unorthodox compared to all the marshall style MV amp schematics i've seen. I increased the typically 100k plate and 820R cathode of the stage immediately before the CF to 220k/1.5k and it has a 4.7uf cathode bypass cap. Seriously improved the tone. I tried to go back to the common values a few times and each time it was a jaw dropper how much fuller and better it souds with the higher gain there.

So In short, is there a way that is considered more correct when it comes to which stages should be gainier in a cascaded preamp, early or later? I just posted this to see what you guys think on the subject.

[jazbo8]

It might be worth reading both parts of this article at TGP. They provide a good overview of the issues involved.

There is really no right or wrong way to do it, and just comes down to what kind of tone and sensitivity that you are looking for. But judging by the "crowd favorites", certain classic designs from Fender and Marshall are the tone that most players sought after, so that should tell you something about how to handle the gain structure and choose the circuit topology. It's not rocket science, so don't over-think it

[loudthud]

Daz, you didn't fully describe your amp. The two main three stage preamps I think of are the Marshall 2203 and the T-Wreck Express. Both have a cold clipper stage, stage 2 in the 2203 and stage 3 in the Express. There are other possibilities for gain structure. The cold clipper stage only has a voltage gain of about 6 so the gain could be cranked 10X and attenuated back down with added tone shaping. Where are the volume pots, tone controls and tone shaping attenuators in your amp?

The tone stack driver with cathode follower has pretty soft clipping on the positive side and not as much gain as I originally thought with an unbypassed 820 in the cathode. I recently did a little breadboard of that stage to look at it in detail. It seems like if you have too much hard clipping before the tone stack, the controls don't do much, you might as well just dime them. I've used up to 470K as the plate resistor, but that was in a bass amp, not for lead tone distortion. You need to tweek the gain stage cathode reisistor to get the headroom right.

[daz]

Well, whats funny about the stage before the CF is that i changed it to a 220k plate and 1.5k cathode ! And I've always bypassed the cathode R with a cap. I never thought to do that and wouldn't have because i wouldn't want more gain, but since i did this quite a while back i have tried going back to 100k/820R several time and it sounds worlds better with the 220k/1.5k ! I don't know why it sounds so much better because i never see that in other similar amps, but i can't go back because it always sounds way thinner and just plain tinny.

Anyways, the preamp is sorta similar to a 2204 but no cold clipper. It's 3 stages, 100k/1.5k/.68uf, 100k/820R/.68uf, and as described the 3rd is 220k/1.5k/.68uf. I know you're going to say the .68uf bypasses on all 3 stage are why the 3rd sounds so much better with the 220k/1.5k, but it was a 4.7uf when the 3rd was 100k/820. I just put the .68uf there because once i changed the 3rd stage the 4.7uf just was too much. This amp somehow has always needed a lot of top end in the pre or the lows get flabby, and yes i have tried very small couplers. It just needs a ton of top. But it sounds killer right now, not at all thin/tinny unless i want it to via the tone and presence controls. My tone controls are very effective too by the way, so thats not an issue. I now have the gain dropped via a split load and was then able to increase it in a place where i realized it was too low an impedance and choking the high end in a way. Did several tweaks to compensate for the changes and now i'm loving it. pretty much like it's always been except a considerable improvement in the *type* of hi end it has.