Voltage Gain is Voltage Gain is Voltage Gain?

[Bill Mountain]

After playing a gig with my Turbo Rat on Saturday, I'm now obsessed with building my own booteek Rat-ish pedal.  My first idea was to experiment with alternate gain stages such at BJT's or FET's.  Quick math tells me that that max gain on a standard Rat is about 65dB.  I can get that easily with a few transistors, I can experiment with different bypass caps to get the high boosts, I can use some feedback caps or a LPF on the input to limit the high frequency gain, etc.

Ignoring the peculiarities of the 308 that makes the rat so special, if talking purely about gain does it really matter how it's achieved?  I plan to breadboard this regardless but it has sparked a genuine curiosity about gain stages and clipping diodes.

Will the diodes clip differently based on the source of the gain?

[teemuk]

They clip differently if the current flowing through them is different. With "shunt" -style diode clipping (as in RAT) this can happen if the gain stages have different output impedances or different headrooms.

But mostly it will be a rather subtle effect.

[Mark Hammer]

There's voltage gain, there's current, and then there's bandwidth.  The "peculiarity" of the 308 lies in the limited gain-bandwidth product, given the amount of voltage gain asked of the chip in that circuit.  Stated differently, it changes WHAT is getting clipped by the diodes.

[vigilante397]

I've built half a dozen RAT clones with half a dozen mods, and the only one I really consider an improvement on the stock RAT is germanium diodes for the clipping. 1N34's if I remember right. There's a slight cut in volume over the 1N4148's but the distortion is delicious.

[Bill Mountain]

There's voltage gain, there's current, and then there's bandwidth.  The "peculiarity" of the 308 lies in the limited gain-bandwidth product, given the amount of voltage gain asked of the chip in that circuit.  Stated differently, it changes WHAT is getting clipped by the diodes.

Agreed.  As an exercise I plan to look at ways to limit the bandwidth.  All I got so far are LPF's on the input and a feedback cap from C-B or D-G.  This of course will limit the bandwidth at all gain levels so I'm thinking of ways to increase the high end roll off as the gain increases.

I can't guarantee that any of this will sound any better than a stock Rat but stock is boring!!!

[karbomusic]

I can't guarantee that any of this will sound any better than a stock Rat but stock is boring!!!

If the guts of your RAT is anything like the guts of my recently disassembled RAT, you'll want to build your own anyway. Some of the worst build quality I have ever seen and that's being nice. Sorry, not knocking the sound but I was fairly annoyed knowing I spent near 100.00 for it after I opened it up and saw how it was made.

[ashcat_lt]

The effect of GBP can be approximated with a lowpass filter set really (!) low.  It also affects the opamp clipping, which I guess does also change what gets clipped by the diodes.  The slew rate distortion...IDK how to fake that in analog.

[deafbutpicky]

so I'm thinking of ways to increase the high end roll off as the gain increases.

I can't guarantee that any of this will sound any better than a stock Rat but stock is boring!!!

You can put a variable HPF/gain in the feebackloop (C to B) of a transistor [Cap parallel to Rf]but it's unlikely to be like a 308 without a lot of tinkering.
A rat is a rat as long as it's build like one and if you like the basic sound you'd be better of changing different parts of the circuit, so
may I ask where you want to go from there? It might be a lot easier not going discrete on this OPA and still achieve your desired sound.

[Transmogrifox]

I think generally the idea of matching the gain and the salient frequency response characteristics using a 3-stage discreet BJT amplifier is perfectly sound reasoning.

It won't sound exactly like a stock RAT, but it will sound "Rat-ish".  The discreet copy will have quirks of its own to add into the mix.

I think the main thing is to apply the frequency shaping within the first stage and let the following high gain stages do the gain thing. 

This will totally change the mechanics of the clipping mechanism -- you may even find it sounds good without diodes for clipping.

If you match the gain, faithfully emulate the pre-emphasis frequency response, nearly emulate the clipping threshold, there is a high probability it will sound Rat-ish. 

I sense the OP isn't going for a desired sound, but looking forward to a surprise by experimenting with something different inspired by the "same old".