'72 or earlier Marshalls....w/Master Volume

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1/27/1997 1:21 PM
Bryan Griffin	'72 or earlier Marshalls....w/Master Volume Would someone please explain what happens when a "Master Volume" is installed in a Vintage Marshall. Does it split the preamp and power section? I've got a '72 100 watt Super Lead....the tone is great, but am I getting preamp or power amp distortion at low volumes? Thanks, Bryan

 

1/27/1997 2:01 PM

John Martin

Most master volume controls go between the last preamp stage and the driver stage. The only way to get poweramp distortion is to load down the output with some type of dummy load made out of some really heavy duty resitors. Better yet would be to purchase a comercially available unit such as the one made by THD, which I am told provides a resistive/inductive and capacitive load which comes much closer to what the amp sees from a speaker cabinet. Marshall and others make similar devices with names like "Power Soak" etc. So to answer your question: it is preamp distortion you are hearing.     JM out

 

1/27/1997 8:03 PM

Chuck

O.K. I've got a good answer.   The distortion is preamp. Some   "techs" or amp shops may want   to sell you a "new" type master   volume that is installed in   between the driver and the   power tubes. I tried this mod   and I don't recommend it. The   power amp is too linear in tone   at lower volumes. So even   though you'll get a bit more   distortion, it will sound   thoomy and buzzy when the   control is set low. I had a   very similar amp and it too   sounded great. When I wanted   more overdrive I would plug in   a Boss GE-7 pedal eq, push   up the midrange and roll off   some bottom and top. Then I   set the gain on the pedal to   full 15db boost. You can   control the character of the   overdrive by fine tuning the   freq bands on the pedal.   I would plug this into the top   jack of the normal channel then   use a patch cable to jumper from   the bottom jack of the normal   channel to the top jack of the   high treble channel. I ran the   normal channel on 10 and the   high treble on 6. Then I unplugged   the two outer power tubes so I   could push the power amp a little.   The tone was perfect. I added a   line in/out (not really an fx loop,   but it works with most effects) so   I could use my quadraverb. It's a   real easy mod. Chuck

 

2/17/1997 3:54 PM
Matt	I heard if you add a 100k ohm in series to the wiper of the added master volume pot the tone of the amp is consistent through most of the sweep of the master volume. Again, this is heresay.

 

2/18/1997 8:11 PM

Chuck

That would actually work very well. But if you add a line   in/out just post the master (which is where most are) it   effects the resistances and relative impedance of the   signal. My Qverb only works to about 6 on the master and   then starts to make a "crackle" even with the input set   below clip. I believe this is due to a poor impedance match   beyond that point. Whether your mod would effect the   relative loads and impedances in a negative way, I'm not   sure.

 

1/29/1997 10:29 PM
Steve Brewer	Check out the Trainwreck pages in Gerald Webers book "A Desktop Reference of Hip Vintage Tube Amps". He illustrates 4 ways to put a master volume in a amp.

 

1/29/1997 11:15 PM

Chuck

I think he already has a master volume.   He just wants to know about his amp. You   can't get an owners manual for modified   Marshalls. Its probably a standard "factory   type". Thats a 500 to 1M pot from the center   tab of your treble pot. This was the best   sounding one on a similar amp for my taste.   The Trainwreck claim to fame master is   specifically the one I didn't like. It did   have a bit more gain. But overdriving the   driver made the tone controls a-tonal and   the power tubes (still not pushing hard)   remained linear. The tone was flabby and   buzzy. I hated it. Just my opinion, don't   get mad at me. Chuck

 

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