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Re: Marshall master volume preamp

4/14/1998 1:38 AM
Re: Marshall master volume preamp
Thankyou, let me see how  
this works (excuse me if  
I goof).  
Thanks for explaining mic  
caps. Maybe sometime I  
can try them in my Super  
to tame the excessive spikiness  
in the very high end.  
4/1/1998 8:31 AM
I built one of these, with the 50w power amp  
too. It sounds great but it is kinda trebly,  
especially with the 1st volume control turned  
I would recommend making at least one of the  
treble-boosting caps switchable. If you  
compare the circuit with a plexi for example,  
the plexi has less treble boost on its 'bright' channel than this thing has.  
If you build one of these, you need to be  
careful about layout, grounding, and power supply bypassing, because the gain at high  
frequencies is very high, so it might  
oscillate (mine didn't).  
I also find the gain is so high that ANY  
tube I put in for the first stages is  
slightly microphonic. You might want to  
shock-mount the first tube. I plan to retrofit this.  
Another thing one might consider to make it  
more versatile is to make the channel switching footswitchable, rather than  
the way it is on the schematic.  
best o'luck,  
4/1/1998 3:12 PM
Michael Tousek
My overall impression of the Marshall master volume preamp is that it's an older design and sounds like it. It won't do the highly compressed, hairtrigger note thing like a Boogie or newer Marshall will, and at low volumes it sounds unrefined and even kind of funky. But when you pair it with a cranked output section and good speakers, this preamp sounds absolutely majestic. My experience with this preamp is with a silverface Bassman head which I modded into a 2204 clone.  
I didn't keep the preamp stock. The first thing I did was remove the .oo1 cap which goes around the first volume control. Actually, I had it on a switch for a while but found that I never used it because it made the lower gain settings painfully bright, so I yanked it to free up the switch.  
I also removed the second 470 (pico?) cap which goes around the second 470K resistor. I had this one switchable, but found that I liked it better switched out, too. I tweaked this amp to work with a strat equipped with stock single coils, so I could afford to reduce some of the amp's top end (to me, the 2203/2204 amps are a little bright even with a Les Paul).  
My final change was to increase the first plate load resistor to 220K, and the second to 150K. This makes the amp sound as crunchy with a Strat as it did with a Les Paul when it was in stock form.  
I didn't use the second input jack for a low gain input, either. I just hardwired the output of the first stage directly into the second stage, with no option of plugging in between the two. For cleaner sounds, I've got push/pull switch which selects in a Plexi preamp circuit.  
Compared to the Plexi circuit, the master volume circuit is brighter and can get quite a bit more distorted, but they seem to have a similar fundamental character. When I reduce the gain of the master-volume preamp and turn down the treble and presence controls slightly, it can sound nearly identical to the Plexi circuit.  
If you're looking for a preamp which can range from classic crunchy Marshall through something more aggressive and distorted, but you don't need super high gain, then the master volume preamp would probably be a good choice. This preamp likes to be paired with a loud output section, too, so that might be something to consider -- it probably wouldn't work as a straight-to-the board kind of thing.  
How do you plan on using it?  
Michael Tousek
4/1/1998 11:59 PM

Yeah, I built one of these critters also. I agree with most everyone else on the fact that it is far too "bright" for my tastes. I found a list of mods to tone the highs down a bit and fatten up the sound a bit on alt.guitar.amps, i can email it to anyone who is interested, i have yet to implement any of them on my amp due to time restraints (and the fact that i have another amp that i use primarily) But will be trying them in time.  
BTW, i found the best sound i get from this amp at the moment is by sticking a Boss metal zone, or turbo overdrive pedal in front of the clean input and leaving the gain maxed and the volume as loud as i can get away with.  
Oh yeah, to give you an idea on the brightness of this design, i play it through an old (60's) 4x12 Bass cab (Vase) and its _still_ too bright.  
4/2/1998 6:50 AM

I would be interested in the schematics!... Please e-mail me w/them... Thanx Frank.
4/2/1998 1:04 PM
Michael Tousek
How does the Marshall master volume amp you built sound at low volumes? I've had a chance to play several JCM 800's with this circuit and all but one of them were pretty underwhelming at low volumes. The amp which sounded different was smoother and had more gain than the others. I was hoping that mine would turn out like that one, but it definitely sounds like the other ones (maybe that one was modded or something). At low volumes the tone is kind of boxy and harsh with pronounced intermodulation distortion on the low notes. As I turn up the master volume, though, the tone blooms and the amp sounds really nice.  
I'm curious, is yours (or anyone out there with a JMP or JCM 800 2203/2204) like this?  
Michael Tousek
4/2/1998 3:56 PM

Ok well mine too sounds better when driven fairly hard, but it does a fairly good job at low volume, quite usable. But.....I have to mention one important thing, i'm using 6L6's as outputs with a plate voltage of approx 395VDC. So mine is probably not going to give me the same sound as a marshall-made 2004, which to me is fine, because i didnt intend to replicate the original sound, just to make a kewl sounding backup amp.  
One point though, this IM distortion on low notes. Does it occur on both inputs? and does the effect sound kinda "farty" (for want of a better word) ? Becuase mine is still suffering from this ailment, most noticable if you play through the low gain input because the distortion doesn't mask it. I was going to try rubber mounting the preamp tubes because i thought it might be the tubes acting microphonic due to vibration, but now you have me wondering.......Anyone else have an idea what is happening to us?

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