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'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
 
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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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