Master Vol for my JTM45? Question for the tech-guys here

[Jimi75]

Folks,

I have a quick question. I have the option to install a master volume in my JTM45. So I could dime the channels and play at "survivable" level. There's this pretty well known amph theck here who would do it. He could undo the mod without destroying the look of the amph. Question is if this really makes sense and brings me to break up sound? Is this a good substitute for an attenuator?

What are your opinions and experiences? Would this change the overall sound?

Thanks for your input.

[tunghaichuan]

There are two types of master volume (MV) circuits you can have installed.

The first one is a pre-phase inverter MV, which is the preamp signal into the phase inverter (last 12AX7 before the power tubes) is dialed down. I believe this type was used on some of the 80s JCM800 Marshalls, which had much higher gain preamps than your JTM.

The second kind is a post-phase inverter MV. This kind is placed after the phase inverter tube. It is considered to be better sounding than the pre-phase inverter type because the phase inverter tube is overdriven with the post phase inverter type.

More information here:

http://www.people.cornell.edu/pages/mt24/Amp/mm/mvols.html

The problem with both of the above is that they do not drive the power tubes into distortion. Any distortion coming out of the amp is generated in the preamp. The classic Marshall sound is power amp distortion.

An attenuator will allow your power tubes to work hard, but will dial down the over all volume. An attenuator will compromise your tone.

Another option is to look into power scaling, which is a circuit that is installed into the amp to dial down the high voltage in the amplifier. Running the tubes on lower voltages allows you to drive the power tubes into distortion at lower volumes. However, since this does not push the speakers hard, some say that this type of circuit does not sound as good as a fully cranked amp.

IMHO, all of the above are compromises. The only thing that gets the cranked Marshall tone is to actually crank the amp up all the way.

tung





Folks,

I have a quick question. I have the option to install a master volume in my JTM45. So I could dime the channels and play at "survivable" level. There's this pretty well known amph theck here who would do it. He could undo the mod without destroying the look of the amph. Question is if this really makes sense and brings me to break up sound? Is this a good substitute for an attenuator?

What are your opinions and experiences? Would this change the overall sound?

Thanks for your input.

[Jimi75]

Inspector doctor TUNG, thanks so much for your answer, I hoped you jump in on this thread! All much clearer now for me after reading your explanation!

Jimi75
:AOK:

[markb]

IMHO, all of the above are compromises. The only thing that gets the cranked Marshall tone is to actually crank the amp up all the way.

This statement should be made a sticky and referred to in all questions about valve amps. :master:

[Plank_Spanker]

IMHO, all of the above are compromises. The only thing that gets the cranked Marshall tone is to actually crank the amp up all the way.

tung

Tung, you RULE! :dude: