gain and amp's ability to take a clean boost

[iknowjohnny]

One thing i have noticed as i try many different circuit changes to an amp with a fairly high gain preamp is that sometimes it will be more sensitive to a clean boost used to drive a average amount of gain over the top. Sometimes the amp will not seem all that sensitive to it and will have more gain of course, but it doesn't seem to become intense or very different, and the extra gain seems almost to sound no different aside from being much more noisy. And it doesn't give you a ton more sustain like it should. While other times with a different circuit configuration i hit the clean boost and the gain has an intensity to it that is great and a held note very quickly morphs into a singing higher harmonic.

I however have never seemed to figure out what it was in the circuit design i was using at the time when it worked that well which may have been responsible. Does anyone have any idea of what parts of the gain stage would be responsible for how well it takes a clean boost and how to maximize that effect?

[tubeswell]

Does anyone have any idea of what parts of the gain stage would be responsible for how well it takes a clean boost and how to maximize that effect?

Many things can affect the impact of a booster. the gain structure of the amp, the attenuation between stages, the number of stages (and gain of each), the way they are biased etc

All the stages in the signal path 'add-up' to to make the total overall gain of the amp. Say we start with a typical 12AX7 triode gain of about 30 for the first stage and assuming an impedance-bridge of at least 1:5 for the source impedance:input impedance ratio between the two stages followed by a gain of 30 in an identical second stage, the net result will equal an overall signal gain of 30 x 30 x whatever you put in at the start. So for a single coil geetar putting out 4mV PP from its p'up; (4mV x 30) in V1 gives .12V P-P hitting the second stage and .12V P-P x 30, will mean 3.6V P-P hitting the next stage. But if you stick a big signal in at V1 to begin with (say 1V P-P, then under the same conditions you'd end up with 30V P-P from the first stage, which would make the signal clip quickly in the next stage (assuming it was a typical pre-amp triode stage).

On the other hand, it could just as easily be affected by something as simple as the bias point of each stage. Too cold (bigger cathode resistor) and you get plate cutoff clipping - too hot and you get grid current limiting clipping. Having a signal boost could tip this over the edge one way or another. etc

[iknowjohnny]

Thanks for that. From the mind of a simpleton, I figured it's probably a result of how hot the first stage is. If it's already very hot the pedal wouldn't seem to make that much of a difference because the first stage it sees isn't going to have much room to change in the way it reacts. on the other hand, if the stage isn't very hot it does. Thats what i was surmising, but again i can only guess with my limited understanding. I hate to change anything because i'm happy as it is, tho there have been times where the circuit wasn't as good as this overall, but it DID take a clean boost a bit better.