Power Scaling?

[Mats A]

Will all modern amps benefit from installing the Power scaling? I mean many modern high gain amps tend to get very little difference in sound when you turn them up loud or dime them.

[KevinOConnor]

Hi Guys

Power Scaling is of benefit to anyone who wants power tube compression or distortion as a component of their sound.

As discussed in another thread on this forum, and on my site, you know if you use such effects by adjusting the MV from a very low setting to a higher one. At the low setting, the sound is dominated by preamp distortion, so this is the "quiet reference".

Dial the MV up until you hear a tone change. This is where the transients of the signal are being lopped off as the power amp runs out of head-room. This will sound a bit "fat" or "round". It is "limiting" in compression lingo.

If you dial the MV up a bit further, the sound gets brighter again but not in the same way as the quiet-reference sound. This is the PA clipping the whole signal and putting out a squarewave.

If you prefer either of the MV-related tone changes, then you will benefit from having Power Scaling in the amp.

If when you got the the first tone change you dialed the MV back down, then you are not using output stage effects and won't benefit from having Power scaling.

Have fun
Kevin O'Connor

[Mats A]

Well i most often prefer the tone with the amp turned up loud.

But i ment i got a JVM 410H and a 2266 50 Watt Vintage Modern and when i use my Marshall Power Brake on the JVM and turn it up there is very little change in the tone. On the VM turned up with the Power Brake you get less fizz and maybe a little fatter tone than you get with the master at a lower setting like on 3 or 4.

-- Thu Jan 27, 2011 4:23 pm --

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?

[SteveD]

The drive signal into the JVM power amp is a lot lower than the Vintage Modern or a Plexi so it remains in a more linear region of its transfer characteristic. Nearly all of the distortion in a JVM is generated in the preamp so the sound will not change as much as your other amps when driving the power amps.

[Mats A]

Thank you Steve!
What determines the amount of drive signal that gets to the power amp since an amp with lots of gain can put less power on the output stage than an amp with less gain?

[SteveD]

The amplitude of drive signal into the power amp is a design choice based on how much of an agenda on the sound is required from the power amp, especially when the amp is running at higher volumes. Gain levels in terms of distortion has little to do with it.

[KevinOConnor]

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

[Mats A]

Beeing a swede :Embaresed You mean you can still distort the output stage even if you have a master volume but because of the gain you wont get the tonal changes by this "Presence of a MV does not keep the player from over-driving the PA."

Still i got a 1959 amp with a PPIMV fitted and if i turn that master down i almost get as much distortion as when i play it with the master on 10 with my Marshall Power Brake but the tone is not as fat and fizzier. So you must be able to get distortion from the preamp even if it´s designed to be a clean preamp.