Postby KevinOConnor » 03 Feb 2011, 09:13
Hi Guys
"Why is the difference in tone so little when i turn up the JVM 410H to 10 but so big when i turn up my Marshall plexi reissue to 10?
Why does a high gain amp like the JVM tend to distort the output stage less than a 1959 plexi wich has less gain?"
Amps designed with a master volume have preamps with higher gain than amps designed without an MV. The purpose is for the preamp to generate distortion and the MV to regulate how loud a sound you have through the "clean" power amp.
As I pointed out above, if you dial the MV above a certain point, the PA limits then hard-clips the already distorted preamp signal. Both effects change the sound.
A clean amp like a plexi is not designed to distort except when driven to clipping. It will limit first, providing a compression effect, then hard-clip. The sound difference is more dramatic because the preamp is still operating clean the whole time.
Most Marshall power amps are set up for a gain of about 20x. For maximum output of a 100W amp into 8-ohms, the output voltage is 28Vrms, requiring 28V/20=1.4V at the input of the PA. Even a 2-stage preamp can produce waaaay more than this - something on the order of 80V peak! A plexi preamp driving an EQ loses a good portion of this 80V but there are still tens of volts available where only single volts are needed to get full clean output.
In a MV amp, the preamp has three or more stages typically. Most often the EQ is after all that gain and ahead of the MV. because the signal at the final gain stage can be so hot, the designer pads the signal to the MV using a resistive divider. The resistors in this divider reduce the maximum voltage from tens of volts to singles of volts, resulting in less "over-kill" of excess drive voltage that can be sent to the PA input.
In any event, the fact the MV amp has a distorted signal coming from the preamp already swamps the tone changes caused by the PA - inasmuch as the resulting PA-tone changes seem to be less deep.
Presence of a MV does not keep the player from over-driving the PA.
Have fun
Kevin O'Connor