Master Of The Volume

admin · Post by **admin** » Mon Oct 31, 2005 2:32 am

Charlie: I have read a number of discussions online over the years about the merit verus drawback of having a master volume knob.

Further, I note that in recent years Fender has built a number of amplifiers with a 1/4 power and full power switch, claiming the best of both studio and performance venues, respectively. I recall that Fender added a master volume with the advent of the Silverfaced models in the early 1970s.

What is wrong with having a master volume? Is it something to be avoided? Should it have its limitations, is it worth having a Silverfaced amplifier modified to remove this feature?

rkbsound · Post by **rkbsound** » Thu Nov 03, 2005 4:01 pm

I, too, would love to hear input on this topic. I'm not a Vox expert at all, but I think the AC-30 HW was the first AC-30 with a master volume, and that is considered their finest AC-30 re-issue. Now the CC's have them as well. Without knowing first hand how the new ones compare in tone to ones w/o a master, is there a loss or change in "tone"?

soundmasterg · Post by **soundmasterg** » Thu Nov 03, 2005 6:47 pm

There are different ways to incorporate a master volume, and some of the ways change the sound for the worse, with the Marshall JMP and JCM 800 series style master volume being one of the worst. I don't know what style Vox used in the new amps or in the AC30HW, but I am generally skeptical of master volumes myself until I've tried them out. There are ways to reduce the tone sucking effect of the Marshall type master volume, but it never sounds as good when the amp is dimed as when there is no master volume in the circuit in my experience. As far as making a loud amp like the AC30 useable in smaller venues, and master volume could go a long way towards doing this. Another option would be an attenuator, or a smaller amp.

In regards to the Silverface...it depends on which era of Silverface it is. Some of the Silverface master volume amps can be modded to sound much better, and disconnecting the master volume can help towards that goal. But other ones are a lost cause tone-wise and are a waste of time trying to make them sound like the older Fenders due to other changes in the amp.

admin · Post by **admin** » Fri Nov 04, 2005 2:58 am

Greg: I use an attenuator with my Vox AC30 that works very well. Just in passing, would you mind discussing the ways in which a master volume is used in different amplifiers.

soundmasterg · Post by **soundmasterg** » Fri Nov 04, 2005 2:57 pm

Peter, there are tons of different ways it can be used, but they mainly break down to either being used in the preamp, or in the power amp. Marshall used one of the preamp styles in the JCM800 and JMP, and they used one of the power amp styles in the Marshall Major. I'm not well versed in them enough to lay out the pros and cons in detail, but I've done some reading, and your fellow Canadian Kevin O' Connor goes into great detail about all the types in his excellent amp series of books know as the TUT (The Ultimate Tone)series. They're expensive, and hard to read for a novice due to the math involved in some places, but are very good books with a wealth of knowledge about amp building and repair. TUT1 has the most master volume info of the books so far. (TUT1, TUT2, TUT3, TUT5)

I don't like the preamp style that Marshall used on the JCM800/JMP's much myself as it seems to suck tone no matter what you do. I haven't experimented with the other styles myself yet so have no first hand knowledge on them.

Which attenuator do you use Peter? I just turn my AC30 up until it sounds good and damn the consequences!

kevin · Post by **kevin** » Fri Nov 04, 2005 5:08 pm

Greg,

I don't know what was used in the Marshalls, but the MV used in the AC30CC (and the HW and the 90's-era AC15-TB) is in the Phase Inverter stage. It's actually a modification of the Cut control. The Phase Inverter takes the signal, splits it in two, and flips one of the two signals 180 degrees out of phase. This, I believe, is so the power tubes can work in push-pull fashion. Anyway, this type of MV takes the two signals post PI tube, and blends them back together. Being that they're 180 degrees out of phase with each other, they perfectly cancel each other out.

I'm guessing this probably sounds best with a PI tube that has perfectly matched halves. Then again, maybe it makes no difference at all...

soundmasterg · Post by **soundmasterg** » Fri Nov 04, 2005 7:05 pm

Even with a PI tube that is perfectly matched between the triodes, you'll never get a balanced signal going into the power tubes at all frequencies, no matter the style of PI circuit. Some of the best amps sound as nice as they do because of a PI circuit that isn't all that well matched. For hi-fi, if you match the triode sections on some amps it can improve the sound, but for a guitar amp most people like it with mismatching to a point.

I haven't heard of a MV that does what you describe but like I said, there are LOTS of different designs. I'll have to think about what you said regarding that circuit to see how well it would work. The Silvertone 1484 uses a standby setup that shorts the grids of the power tubes together, and doesn't work 100% in cancelling out the signal. It does absolutely nothing to spare the tubes the shock of high voltage hitting them when the amp is turned on, but I suppose a MV could be hooked up to do something similar, but as a gradual approach since you would be using a pot. Have to think about that one though...

admin · Post by **admin** » Sat Nov 05, 2005 12:48 am

Greg: I use the THD Hotplate and it is a great bit of gear. The consequences for me, potential hearing loss and family fallout, are more than I can live with and so the attenuator was the solution.

soundmasterg · Post by **soundmasterg** » Sat Nov 05, 2005 1:58 am

Those are nice Peter. I've been to THD and met Andy Marshall and seen the operation there and its top notch.

kevin · Post by **kevin** » Sat Nov 05, 2005 3:39 pm

Greg,

If you'd rather see exactly what's going on, Vox has circuit diagrams available on their web site - www.voxamps.co.uk - in the "support" area.

soundmasterg · Post by **soundmasterg** » Sat Nov 05, 2005 3:58 pm

Thanks Kevin. I'll take a look see.