Just why IS the Express so gainy?

[nee]

Sorry for the potentially stoopid question! I'm still new at this! Why is the Express so gainy?

The preamp of the Express is nothing too far out of the ordinary, is it? The plate resistor values of 100K, and the cathode resistors of 1.5K bypassed with a 25uf cap is pretty much the "classic" gain stage. It doesn't look too different from the Bassman AB165, and certainly has none of the gain-boosting techniques of something like a Matchless DC30 which has a 220K plate resistor.

So, a little theory, if you please!

[geetarpicker]

Probably most of "the difference" is that it has THREE gain stages. Also there is not much between the 3 stages to load it all down short of the tone stack. Not to mention there is NO input resistance, the guitar drives the 1st stage directly. From the looks of it the 3rd stage is toned down a tad from what it could be, thank god!

GK

www.myspace.com/glenkuykendall
www.youtube.com/glenkuykendall

[funkmeblue]

don't forget it runs at near tweed voltages

[nee]

don't forget it runs at near tweed voltages

And is that lower or higher than average?

[Structo]

Higher

Leo was infamous for exceeding specs on tubes like the 6V6.

[funkmeblue]

I would say lower. Most amps run el34's at a higher voltage than 400 volts. Leo pumped up the juices in the blackface amps.

[ChrisM]

Ya, I always thought the Tweed sound had to do with the lowish voltages. While at least that is a portion of it...

[funkmeblue]

ya more like a low power tweed twin than a marshall running at 475 volts or higher. The lower voltages on v1 make the amp more dynamic.

[billyz]

bypass the 3rd stage and you will know why it has such gain.

[nee]

bypass the 3rd stage and you will know why it has such gain.

So the 3rd stage adds a fair bit of oomph?

[billyz]

I built an express clone with a third stage bypass switch. I need to turn the volume up to 12 oclock to approach the unbypassed wreck on 3, or 8-9 oclock . The sound is much cleaner with it bypassed, more jtm45 sounding. I like the sound both ways for different things.

[JamesHealey]

the 3rd stage is the cleanest of them all, but it pushes the PI hard.

The reason the trainwreck is so gainy is the fact the tonestack is so early on in the circuit so it doesn't reduce gain built up after it, none of the gain stages have any grid stoppers before them so they hit eachother very hard unlike a plexi for instance which has 470k grid stopper between the two stages this reduces the gain a lot, plus the amp running on lower voltages allows the tubes to clip harder.

[fishy]

This link might be interesting to some and answer some questions and probably raise more....
Scope plots from an Express based build.

http://home.polstra.com/amps/wreck1/scope/

[PaisleyTube]

Marchless SHARED 220k plateR's: terrible way to up the gain imho...

Gain Express:
See it as souped up Fender Bassman AB165 (same topology in the right/normal channel).

Express:
1st two stages are pretty "normal" but the output from the 2nd stage can hit the (lower gain) 3rd stage pretty hard and, then, the output from the 3rd stage can slam the (high gain) PI very hard.

Most gain/overdrive is generated in the PI, then the 3rd stage.

[Roe]

the usual grid resistors (e.g. 33k) wont reduce gain, it just removes high treble